]> git.proxmox.com Git - mirror_zfs.git/blobdiff - module/zfs/dsl_scan.c
Verify block pointers before writing them out
[mirror_zfs.git] / module / zfs / dsl_scan.c
index 5b52681d8d8e13d59c0f3c625b406b8793e562a2..d398b6705551575918be7f6f9759047fe66422ad 100644 (file)
@@ -6,7 +6,7 @@
  * You may not use this file except in compliance with the License.
  *
  * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
- * or http://www.opensolaris.org/os/licensing.
+ * or https://opensource.org/licenses/CDDL-1.0.
  * See the License for the specific language governing permissions
  * and limitations under the License.
  *
  */
 /*
  * Copyright (c) 2008, 2010, Oracle and/or its affiliates. All rights reserved.
- * Copyright (c) 2011, 2016 by Delphix. All rights reserved.
+ * Copyright (c) 2011, 2021 by Delphix. All rights reserved.
  * Copyright 2016 Gary Mills
+ * Copyright (c) 2017, 2019, Datto Inc. All rights reserved.
+ * Copyright (c) 2015, Nexenta Systems, Inc. All rights reserved.
+ * Copyright 2019 Joyent, Inc.
  */
 
 #include <sys/dsl_scan.h>
@@ -34,6 +37,7 @@
 #include <sys/dmu_tx.h>
 #include <sys/dmu_objset.h>
 #include <sys/arc.h>
+#include <sys/arc_impl.h>
 #include <sys/zap.h>
 #include <sys/zio.h>
 #include <sys/zfs_context.h>
 #include <sys/vdev_impl.h>
 #include <sys/zil_impl.h>
 #include <sys/zio_checksum.h>
+#include <sys/brt.h>
 #include <sys/ddt.h>
 #include <sys/sa.h>
 #include <sys/sa_impl.h>
 #include <sys/zfeature.h>
 #include <sys/abd.h>
+#include <sys/range_tree.h>
 #ifdef _KERNEL
 #include <sys/zfs_vfsops.h>
 #endif
 
+/*
+ * Grand theory statement on scan queue sorting
+ *
+ * Scanning is implemented by recursively traversing all indirection levels
+ * in an object and reading all blocks referenced from said objects. This
+ * results in us approximately traversing the object from lowest logical
+ * offset to the highest. For best performance, we would want the logical
+ * blocks to be physically contiguous. However, this is frequently not the
+ * case with pools given the allocation patterns of copy-on-write filesystems.
+ * So instead, we put the I/Os into a reordering queue and issue them in a
+ * way that will most benefit physical disks (LBA-order).
+ *
+ * Queue management:
+ *
+ * Ideally, we would want to scan all metadata and queue up all block I/O
+ * prior to starting to issue it, because that allows us to do an optimal
+ * sorting job. This can however consume large amounts of memory. Therefore
+ * we continuously monitor the size of the queues and constrain them to 5%
+ * (zfs_scan_mem_lim_fact) of physmem. If the queues grow larger than this
+ * limit, we clear out a few of the largest extents at the head of the queues
+ * to make room for more scanning. Hopefully, these extents will be fairly
+ * large and contiguous, allowing us to approach sequential I/O throughput
+ * even without a fully sorted tree.
+ *
+ * Metadata scanning takes place in dsl_scan_visit(), which is called from
+ * dsl_scan_sync() every spa_sync(). If we have either fully scanned all
+ * metadata on the pool, or we need to make room in memory because our
+ * queues are too large, dsl_scan_visit() is postponed and
+ * scan_io_queues_run() is called from dsl_scan_sync() instead. This implies
+ * that metadata scanning and queued I/O issuing are mutually exclusive. This
+ * allows us to provide maximum sequential I/O throughput for the majority of
+ * I/O's issued since sequential I/O performance is significantly negatively
+ * impacted if it is interleaved with random I/O.
+ *
+ * Implementation Notes
+ *
+ * One side effect of the queued scanning algorithm is that the scanning code
+ * needs to be notified whenever a block is freed. This is needed to allow
+ * the scanning code to remove these I/Os from the issuing queue. Additionally,
+ * we do not attempt to queue gang blocks to be issued sequentially since this
+ * is very hard to do and would have an extremely limited performance benefit.
+ * Instead, we simply issue gang I/Os as soon as we find them using the legacy
+ * algorithm.
+ *
+ * Backwards compatibility
+ *
+ * This new algorithm is backwards compatible with the legacy on-disk data
+ * structures (and therefore does not require a new feature flag).
+ * Periodically during scanning (see zfs_scan_checkpoint_intval), the scan
+ * will stop scanning metadata (in logical order) and wait for all outstanding
+ * sorted I/O to complete. Once this is done, we write out a checkpoint
+ * bookmark, indicating that we have scanned everything logically before it.
+ * If the pool is imported on a machine without the new sorting algorithm,
+ * the scan simply resumes from the last checkpoint using the legacy algorithm.
+ */
+
 typedef int (scan_cb_t)(dsl_pool_t *, const blkptr_t *,
     const zbookmark_phys_t *);
 
 static scan_cb_t dsl_scan_scrub_cb;
-static void dsl_scan_cancel_sync(void *, dmu_tx_t *);
-static void dsl_scan_sync_state(dsl_scan_t *, dmu_tx_t *);
-static boolean_t dsl_scan_restarting(dsl_scan_t *, dmu_tx_t *);
-
-int zfs_top_maxinflight = 32;          /* maximum I/Os per top-level */
-int zfs_resilver_delay = 2;            /* number of ticks to delay resilver */
-int zfs_scrub_delay = 4;               /* number of ticks to delay scrub */
-int zfs_scan_idle = 50;                        /* idle window in clock ticks */
-
-int zfs_scan_min_time_ms = 1000; /* min millisecs to scrub per txg */
-int zfs_free_min_time_ms = 1000; /* min millisecs to free per txg */
-int zfs_resilver_min_time_ms = 3000; /* min millisecs to resilver per txg */
-int zfs_no_scrub_io = B_FALSE; /* set to disable scrub i/o */
-int zfs_no_scrub_prefetch = B_FALSE; /* set to disable scrub prefetch */
-enum ddt_class zfs_scrub_ddt_class_max = DDT_CLASS_DUPLICATE;
-int dsl_scan_delay_completion = B_FALSE; /* set to delay scan completion */
+
+static int scan_ds_queue_compare(const void *a, const void *b);
+static int scan_prefetch_queue_compare(const void *a, const void *b);
+static void scan_ds_queue_clear(dsl_scan_t *scn);
+static void scan_ds_prefetch_queue_clear(dsl_scan_t *scn);
+static boolean_t scan_ds_queue_contains(dsl_scan_t *scn, uint64_t dsobj,
+    uint64_t *txg);
+static void scan_ds_queue_insert(dsl_scan_t *scn, uint64_t dsobj, uint64_t txg);
+static void scan_ds_queue_remove(dsl_scan_t *scn, uint64_t dsobj);
+static void scan_ds_queue_sync(dsl_scan_t *scn, dmu_tx_t *tx);
+static uint64_t dsl_scan_count_data_disks(spa_t *spa);
+
+extern uint_t zfs_vdev_async_write_active_min_dirty_percent;
+static int zfs_scan_blkstats = 0;
+
+/*
+ * 'zpool status' uses bytes processed per pass to report throughput and
+ * estimate time remaining.  We define a pass to start when the scanning
+ * phase completes for a sequential resilver.  Optionally, this value
+ * may be used to reset the pass statistics every N txgs to provide an
+ * estimated completion time based on currently observed performance.
+ */
+static uint_t zfs_scan_report_txgs = 0;
+
+/*
+ * By default zfs will check to ensure it is not over the hard memory
+ * limit before each txg. If finer-grained control of this is needed
+ * this value can be set to 1 to enable checking before scanning each
+ * block.
+ */
+static int zfs_scan_strict_mem_lim = B_FALSE;
+
+/*
+ * Maximum number of parallelly executed bytes per leaf vdev. We attempt
+ * to strike a balance here between keeping the vdev queues full of I/Os
+ * at all times and not overflowing the queues to cause long latency,
+ * which would cause long txg sync times. No matter what, we will not
+ * overload the drives with I/O, since that is protected by
+ * zfs_vdev_scrub_max_active.
+ */
+static uint64_t zfs_scan_vdev_limit = 16 << 20;
+
+static uint_t zfs_scan_issue_strategy = 0;
+
+/* don't queue & sort zios, go direct */
+static int zfs_scan_legacy = B_FALSE;
+static uint64_t zfs_scan_max_ext_gap = 2 << 20; /* in bytes */
+
+/*
+ * fill_weight is non-tunable at runtime, so we copy it at module init from
+ * zfs_scan_fill_weight. Runtime adjustments to zfs_scan_fill_weight would
+ * break queue sorting.
+ */
+static uint_t zfs_scan_fill_weight = 3;
+static uint64_t fill_weight;
+
+/* See dsl_scan_should_clear() for details on the memory limit tunables */
+static const uint64_t zfs_scan_mem_lim_min = 16 << 20; /* bytes */
+static const uint64_t zfs_scan_mem_lim_soft_max = 128 << 20;   /* bytes */
+
+
+/* fraction of physmem */
+static uint_t zfs_scan_mem_lim_fact = 20;
+
+/* fraction of mem lim above */
+static uint_t zfs_scan_mem_lim_soft_fact = 20;
+
+/* minimum milliseconds to scrub per txg */
+static uint_t zfs_scrub_min_time_ms = 1000;
+
+/* minimum milliseconds to obsolete per txg */
+static uint_t zfs_obsolete_min_time_ms = 500;
+
+/* minimum milliseconds to free per txg */
+static uint_t zfs_free_min_time_ms = 1000;
+
+/* minimum milliseconds to resilver per txg */
+static uint_t zfs_resilver_min_time_ms = 3000;
+
+static uint_t zfs_scan_checkpoint_intval = 7200; /* in seconds */
+int zfs_scan_suspend_progress = 0; /* set to prevent scans from progressing */
+static int zfs_no_scrub_io = B_FALSE; /* set to disable scrub i/o */
+static int zfs_no_scrub_prefetch = B_FALSE; /* set to disable scrub prefetch */
+static const enum ddt_class zfs_scrub_ddt_class_max = DDT_CLASS_DUPLICATE;
 /* max number of blocks to free in a single TXG */
-unsigned long zfs_free_max_blocks = 100000;
+static uint64_t zfs_async_block_max_blocks = UINT64_MAX;
+/* max number of dedup blocks to free in a single TXG */
+static uint64_t zfs_max_async_dedup_frees = 100000;
+
+/* set to disable resilver deferring */
+static int zfs_resilver_disable_defer = B_FALSE;
+
+/*
+ * We wait a few txgs after importing a pool to begin scanning so that
+ * the import / mounting code isn't held up by scrub / resilver IO.
+ * Unfortunately, it is a bit difficult to determine exactly how long
+ * this will take since userspace will trigger fs mounts asynchronously
+ * and the kernel will create zvol minors asynchronously. As a result,
+ * the value provided here is a bit arbitrary, but represents a
+ * reasonable estimate of how many txgs it will take to finish fully
+ * importing a pool
+ */
+#define        SCAN_IMPORT_WAIT_TXGS           5
 
 #define        DSL_SCAN_IS_SCRUB_RESILVER(scn) \
        ((scn)->scn_phys.scn_func == POOL_SCAN_SCRUB || \
@@ -82,7 +229,7 @@ unsigned long zfs_free_max_blocks = 100000;
 /*
  * Enable/disable the processing of the free_bpobj object.
  */
-int zfs_free_bpobj_enabled = 1;
+static int zfs_free_bpobj_enabled = 1;
 
 /* the order has to match pool_scan_type */
 static scan_cb_t *scan_funcs[POOL_SCAN_FUNCS] = {
@@ -91,6 +238,223 @@ static scan_cb_t *scan_funcs[POOL_SCAN_FUNCS] = {
        dsl_scan_scrub_cb,      /* POOL_SCAN_RESILVER */
 };
 
+/* In core node for the scn->scn_queue. Represents a dataset to be scanned */
+typedef struct {
+       uint64_t        sds_dsobj;
+       uint64_t        sds_txg;
+       avl_node_t      sds_node;
+} scan_ds_t;
+
+/*
+ * This controls what conditions are placed on dsl_scan_sync_state():
+ * SYNC_OPTIONAL) write out scn_phys iff scn_queues_pending == 0
+ * SYNC_MANDATORY) write out scn_phys always. scn_queues_pending must be 0.
+ * SYNC_CACHED) if scn_queues_pending == 0, write out scn_phys. Otherwise
+ *     write out the scn_phys_cached version.
+ * See dsl_scan_sync_state for details.
+ */
+typedef enum {
+       SYNC_OPTIONAL,
+       SYNC_MANDATORY,
+       SYNC_CACHED
+} state_sync_type_t;
+
+/*
+ * This struct represents the minimum information needed to reconstruct a
+ * zio for sequential scanning. This is useful because many of these will
+ * accumulate in the sequential IO queues before being issued, so saving
+ * memory matters here.
+ */
+typedef struct scan_io {
+       /* fields from blkptr_t */
+       uint64_t                sio_blk_prop;
+       uint64_t                sio_phys_birth;
+       uint64_t                sio_birth;
+       zio_cksum_t             sio_cksum;
+       uint32_t                sio_nr_dvas;
+
+       /* fields from zio_t */
+       uint32_t                sio_flags;
+       zbookmark_phys_t        sio_zb;
+
+       /* members for queue sorting */
+       union {
+               avl_node_t      sio_addr_node; /* link into issuing queue */
+               list_node_t     sio_list_node; /* link for issuing to disk */
+       } sio_nodes;
+
+       /*
+        * There may be up to SPA_DVAS_PER_BP DVAs here from the bp,
+        * depending on how many were in the original bp. Only the
+        * first DVA is really used for sorting and issuing purposes.
+        * The other DVAs (if provided) simply exist so that the zio
+        * layer can find additional copies to repair from in the
+        * event of an error. This array must go at the end of the
+        * struct to allow this for the variable number of elements.
+        */
+       dva_t                   sio_dva[];
+} scan_io_t;
+
+#define        SIO_SET_OFFSET(sio, x)          DVA_SET_OFFSET(&(sio)->sio_dva[0], x)
+#define        SIO_SET_ASIZE(sio, x)           DVA_SET_ASIZE(&(sio)->sio_dva[0], x)
+#define        SIO_GET_OFFSET(sio)             DVA_GET_OFFSET(&(sio)->sio_dva[0])
+#define        SIO_GET_ASIZE(sio)              DVA_GET_ASIZE(&(sio)->sio_dva[0])
+#define        SIO_GET_END_OFFSET(sio)         \
+       (SIO_GET_OFFSET(sio) + SIO_GET_ASIZE(sio))
+#define        SIO_GET_MUSED(sio)              \
+       (sizeof (scan_io_t) + ((sio)->sio_nr_dvas * sizeof (dva_t)))
+
+struct dsl_scan_io_queue {
+       dsl_scan_t      *q_scn; /* associated dsl_scan_t */
+       vdev_t          *q_vd; /* top-level vdev that this queue represents */
+       zio_t           *q_zio; /* scn_zio_root child for waiting on IO */
+
+       /* trees used for sorting I/Os and extents of I/Os */
+       range_tree_t    *q_exts_by_addr;
+       zfs_btree_t     q_exts_by_size;
+       avl_tree_t      q_sios_by_addr;
+       uint64_t        q_sio_memused;
+       uint64_t        q_last_ext_addr;
+
+       /* members for zio rate limiting */
+       uint64_t        q_maxinflight_bytes;
+       uint64_t        q_inflight_bytes;
+       kcondvar_t      q_zio_cv; /* used under vd->vdev_scan_io_queue_lock */
+
+       /* per txg statistics */
+       uint64_t        q_total_seg_size_this_txg;
+       uint64_t        q_segs_this_txg;
+       uint64_t        q_total_zio_size_this_txg;
+       uint64_t        q_zios_this_txg;
+};
+
+/* private data for dsl_scan_prefetch_cb() */
+typedef struct scan_prefetch_ctx {
+       zfs_refcount_t spc_refcnt;      /* refcount for memory management */
+       dsl_scan_t *spc_scn;            /* dsl_scan_t for the pool */
+       boolean_t spc_root;             /* is this prefetch for an objset? */
+       uint8_t spc_indblkshift;        /* dn_indblkshift of current dnode */
+       uint16_t spc_datablkszsec;      /* dn_idatablkszsec of current dnode */
+} scan_prefetch_ctx_t;
+
+/* private data for dsl_scan_prefetch() */
+typedef struct scan_prefetch_issue_ctx {
+       avl_node_t spic_avl_node;       /* link into scn->scn_prefetch_queue */
+       scan_prefetch_ctx_t *spic_spc;  /* spc for the callback */
+       blkptr_t spic_bp;               /* bp to prefetch */
+       zbookmark_phys_t spic_zb;       /* bookmark to prefetch */
+} scan_prefetch_issue_ctx_t;
+
+static void scan_exec_io(dsl_pool_t *dp, const blkptr_t *bp, int zio_flags,
+    const zbookmark_phys_t *zb, dsl_scan_io_queue_t *queue);
+static void scan_io_queue_insert_impl(dsl_scan_io_queue_t *queue,
+    scan_io_t *sio);
+
+static dsl_scan_io_queue_t *scan_io_queue_create(vdev_t *vd);
+static void scan_io_queues_destroy(dsl_scan_t *scn);
+
+static kmem_cache_t *sio_cache[SPA_DVAS_PER_BP];
+
+/* sio->sio_nr_dvas must be set so we know which cache to free from */
+static void
+sio_free(scan_io_t *sio)
+{
+       ASSERT3U(sio->sio_nr_dvas, >, 0);
+       ASSERT3U(sio->sio_nr_dvas, <=, SPA_DVAS_PER_BP);
+
+       kmem_cache_free(sio_cache[sio->sio_nr_dvas - 1], sio);
+}
+
+/* It is up to the caller to set sio->sio_nr_dvas for freeing */
+static scan_io_t *
+sio_alloc(unsigned short nr_dvas)
+{
+       ASSERT3U(nr_dvas, >, 0);
+       ASSERT3U(nr_dvas, <=, SPA_DVAS_PER_BP);
+
+       return (kmem_cache_alloc(sio_cache[nr_dvas - 1], KM_SLEEP));
+}
+
+void
+scan_init(void)
+{
+       /*
+        * This is used in ext_size_compare() to weight segments
+        * based on how sparse they are. This cannot be changed
+        * mid-scan and the tree comparison functions don't currently
+        * have a mechanism for passing additional context to the
+        * compare functions. Thus we store this value globally and
+        * we only allow it to be set at module initialization time
+        */
+       fill_weight = zfs_scan_fill_weight;
+
+       for (int i = 0; i < SPA_DVAS_PER_BP; i++) {
+               char name[36];
+
+               (void) snprintf(name, sizeof (name), "sio_cache_%d", i);
+               sio_cache[i] = kmem_cache_create(name,
+                   (sizeof (scan_io_t) + ((i + 1) * sizeof (dva_t))),
+                   0, NULL, NULL, NULL, NULL, NULL, 0);
+       }
+}
+
+void
+scan_fini(void)
+{
+       for (int i = 0; i < SPA_DVAS_PER_BP; i++) {
+               kmem_cache_destroy(sio_cache[i]);
+       }
+}
+
+static inline boolean_t
+dsl_scan_is_running(const dsl_scan_t *scn)
+{
+       return (scn->scn_phys.scn_state == DSS_SCANNING);
+}
+
+boolean_t
+dsl_scan_resilvering(dsl_pool_t *dp)
+{
+       return (dsl_scan_is_running(dp->dp_scan) &&
+           dp->dp_scan->scn_phys.scn_func == POOL_SCAN_RESILVER);
+}
+
+static inline void
+sio2bp(const scan_io_t *sio, blkptr_t *bp)
+{
+       memset(bp, 0, sizeof (*bp));
+       bp->blk_prop = sio->sio_blk_prop;
+       bp->blk_phys_birth = sio->sio_phys_birth;
+       bp->blk_birth = sio->sio_birth;
+       bp->blk_fill = 1;       /* we always only work with data pointers */
+       bp->blk_cksum = sio->sio_cksum;
+
+       ASSERT3U(sio->sio_nr_dvas, >, 0);
+       ASSERT3U(sio->sio_nr_dvas, <=, SPA_DVAS_PER_BP);
+
+       memcpy(bp->blk_dva, sio->sio_dva, sio->sio_nr_dvas * sizeof (dva_t));
+}
+
+static inline void
+bp2sio(const blkptr_t *bp, scan_io_t *sio, int dva_i)
+{
+       sio->sio_blk_prop = bp->blk_prop;
+       sio->sio_phys_birth = bp->blk_phys_birth;
+       sio->sio_birth = bp->blk_birth;
+       sio->sio_cksum = bp->blk_cksum;
+       sio->sio_nr_dvas = BP_GET_NDVAS(bp);
+
+       /*
+        * Copy the DVAs to the sio. We need all copies of the block so
+        * that the self healing code can use the alternate copies if the
+        * first is corrupted. We want the DVA at index dva_i to be first
+        * in the sio since this is the primary one that we want to issue.
+        */
+       for (int i = 0, j = dva_i; i < sio->sio_nr_dvas; i++, j++) {
+               sio->sio_dva[i] = bp->blk_dva[j % sio->sio_nr_dvas];
+       }
+}
+
 int
 dsl_scan_init(dsl_pool_t *dp, uint64_t txg)
 {
@@ -111,6 +475,21 @@ dsl_scan_init(dsl_pool_t *dp, uint64_t txg)
        scn->scn_async_destroying = spa_feature_is_active(dp->dp_spa,
            SPA_FEATURE_ASYNC_DESTROY);
 
+       /*
+        * Calculate the max number of in-flight bytes for pool-wide
+        * scanning operations (minimum 1MB, maximum 1/4 of arc_c_max).
+        * Limits for the issuing phase are done per top-level vdev and
+        * are handled separately.
+        */
+       scn->scn_maxinflight_bytes = MIN(arc_c_max / 4, MAX(1ULL << 20,
+           zfs_scan_vdev_limit * dsl_scan_count_data_disks(spa)));
+
+       avl_create(&scn->scn_queue, scan_ds_queue_compare, sizeof (scan_ds_t),
+           offsetof(scan_ds_t, sds_node));
+       avl_create(&scn->scn_prefetch_queue, scan_prefetch_queue_compare,
+           sizeof (scan_prefetch_issue_ctx_t),
+           offsetof(scan_prefetch_issue_ctx_t, spic_avl_node));
+
        err = zap_lookup(dp->dp_meta_objset, DMU_POOL_DIRECTORY_OBJECT,
            "scrub_func", sizeof (uint64_t), 1, &f);
        if (err == 0) {
@@ -119,9 +498,10 @@ dsl_scan_init(dsl_pool_t *dp, uint64_t txg)
                 * new-style scrub from the beginning.
                 */
                scn->scn_restart_txg = txg;
-               zfs_dbgmsg("old-style scrub was in progress; "
+               zfs_dbgmsg("old-style scrub was in progress for %s; "
                    "restarting new-style scrub in txg %llu",
-                   scn->scn_restart_txg);
+                   spa->spa_name,
+                   (longlong_t)scn->scn_restart_txg);
 
                /*
                 * Load the queue obj from the old location so that it
@@ -158,7 +538,7 @@ dsl_scan_init(dsl_pool_t *dp, uint64_t txg)
                                        return (EOVERFLOW);
                                }
 
-                               bcopy(zaptmp, &scn->scn_phys,
+                               memcpy(&scn->scn_phys, zaptmp,
                                    SCAN_PHYS_NUMINTS * sizeof (uint64_t));
                                scn->scn_phys.scn_flags = overflow;
 
@@ -175,7 +555,14 @@ dsl_scan_init(dsl_pool_t *dp, uint64_t txg)
                else if (err)
                        return (err);
 
-               if (scn->scn_phys.scn_state == DSS_SCANNING &&
+               /*
+                * We might be restarting after a reboot, so jump the issued
+                * counter to how far we've scanned. We know we're consistent
+                * up to here.
+                */
+               scn->scn_issued_before_pass = scn->scn_phys.scn_examined;
+
+               if (dsl_scan_is_running(scn) &&
                    spa_prev_software_version(dp->dp_spa) < SPA_VERSION_SCAN) {
                        /*
                         * A new-type scrub was in progress on an old
@@ -185,38 +572,186 @@ dsl_scan_init(dsl_pool_t *dp, uint64_t txg)
                         * the meantime.
                         */
                        scn->scn_restart_txg = txg;
-                       zfs_dbgmsg("new-style scrub was modified "
+                       zfs_dbgmsg("new-style scrub for %s was modified "
                            "by old software; restarting in txg %llu",
-                           scn->scn_restart_txg);
+                           spa->spa_name,
+                           (longlong_t)scn->scn_restart_txg);
+               } else if (dsl_scan_resilvering(dp)) {
+                       /*
+                        * If a resilver is in progress and there are already
+                        * errors, restart it instead of finishing this scan and
+                        * then restarting it. If there haven't been any errors
+                        * then remember that the incore DTL is valid.
+                        */
+                       if (scn->scn_phys.scn_errors > 0) {
+                               scn->scn_restart_txg = txg;
+                               zfs_dbgmsg("resilver can't excise DTL_MISSING "
+                                   "when finished; restarting on %s in txg "
+                                   "%llu",
+                                   spa->spa_name,
+                                   (u_longlong_t)scn->scn_restart_txg);
+                       } else {
+                               /* it's safe to excise DTL when finished */
+                               spa->spa_scrub_started = B_TRUE;
+                       }
+               }
+       }
+
+       memcpy(&scn->scn_phys_cached, &scn->scn_phys, sizeof (scn->scn_phys));
+
+       /* reload the queue into the in-core state */
+       if (scn->scn_phys.scn_queue_obj != 0) {
+               zap_cursor_t zc;
+               zap_attribute_t za;
+
+               for (zap_cursor_init(&zc, dp->dp_meta_objset,
+                   scn->scn_phys.scn_queue_obj);
+                   zap_cursor_retrieve(&zc, &za) == 0;
+                   (void) zap_cursor_advance(&zc)) {
+                       scan_ds_queue_insert(scn,
+                           zfs_strtonum(za.za_name, NULL),
+                           za.za_first_integer);
                }
+               zap_cursor_fini(&zc);
        }
 
        spa_scan_stat_init(spa);
+       vdev_scan_stat_init(spa->spa_root_vdev);
+
        return (0);
 }
 
 void
 dsl_scan_fini(dsl_pool_t *dp)
 {
-       if (dp->dp_scan) {
+       if (dp->dp_scan != NULL) {
+               dsl_scan_t *scn = dp->dp_scan;
+
+               if (scn->scn_taskq != NULL)
+                       taskq_destroy(scn->scn_taskq);
+
+               scan_ds_queue_clear(scn);
+               avl_destroy(&scn->scn_queue);
+               scan_ds_prefetch_queue_clear(scn);
+               avl_destroy(&scn->scn_prefetch_queue);
+
                kmem_free(dp->dp_scan, sizeof (dsl_scan_t));
                dp->dp_scan = NULL;
        }
 }
 
-/* ARGSUSED */
-static int
+static boolean_t
+dsl_scan_restarting(dsl_scan_t *scn, dmu_tx_t *tx)
+{
+       return (scn->scn_restart_txg != 0 &&
+           scn->scn_restart_txg <= tx->tx_txg);
+}
+
+boolean_t
+dsl_scan_resilver_scheduled(dsl_pool_t *dp)
+{
+       return ((dp->dp_scan && dp->dp_scan->scn_restart_txg != 0) ||
+           (spa_async_tasks(dp->dp_spa) & SPA_ASYNC_RESILVER));
+}
+
+boolean_t
+dsl_scan_scrubbing(const dsl_pool_t *dp)
+{
+       dsl_scan_phys_t *scn_phys = &dp->dp_scan->scn_phys;
+
+       return (scn_phys->scn_state == DSS_SCANNING &&
+           scn_phys->scn_func == POOL_SCAN_SCRUB);
+}
+
+boolean_t
+dsl_scan_is_paused_scrub(const dsl_scan_t *scn)
+{
+       return (dsl_scan_scrubbing(scn->scn_dp) &&
+           scn->scn_phys.scn_flags & DSF_SCRUB_PAUSED);
+}
+
+/*
+ * Writes out a persistent dsl_scan_phys_t record to the pool directory.
+ * Because we can be running in the block sorting algorithm, we do not always
+ * want to write out the record, only when it is "safe" to do so. This safety
+ * condition is achieved by making sure that the sorting queues are empty
+ * (scn_queues_pending == 0). When this condition is not true, the sync'd state
+ * is inconsistent with how much actual scanning progress has been made. The
+ * kind of sync to be performed is specified by the sync_type argument. If the
+ * sync is optional, we only sync if the queues are empty. If the sync is
+ * mandatory, we do a hard ASSERT to make sure that the queues are empty. The
+ * third possible state is a "cached" sync. This is done in response to:
+ * 1) The dataset that was in the last sync'd dsl_scan_phys_t having been
+ *     destroyed, so we wouldn't be able to restart scanning from it.
+ * 2) The snapshot that was in the last sync'd dsl_scan_phys_t having been
+ *     superseded by a newer snapshot.
+ * 3) The dataset that was in the last sync'd dsl_scan_phys_t having been
+ *     swapped with its clone.
+ * In all cases, a cached sync simply rewrites the last record we've written,
+ * just slightly modified. For the modifications that are performed to the
+ * last written dsl_scan_phys_t, see dsl_scan_ds_destroyed,
+ * dsl_scan_ds_snapshotted and dsl_scan_ds_clone_swapped.
+ */
+static void
+dsl_scan_sync_state(dsl_scan_t *scn, dmu_tx_t *tx, state_sync_type_t sync_type)
+{
+       int i;
+       spa_t *spa = scn->scn_dp->dp_spa;
+
+       ASSERT(sync_type != SYNC_MANDATORY || scn->scn_queues_pending == 0);
+       if (scn->scn_queues_pending == 0) {
+               for (i = 0; i < spa->spa_root_vdev->vdev_children; i++) {
+                       vdev_t *vd = spa->spa_root_vdev->vdev_child[i];
+                       dsl_scan_io_queue_t *q = vd->vdev_scan_io_queue;
+
+                       if (q == NULL)
+                               continue;
+
+                       mutex_enter(&vd->vdev_scan_io_queue_lock);
+                       ASSERT3P(avl_first(&q->q_sios_by_addr), ==, NULL);
+                       ASSERT3P(zfs_btree_first(&q->q_exts_by_size, NULL), ==,
+                           NULL);
+                       ASSERT3P(range_tree_first(q->q_exts_by_addr), ==, NULL);
+                       mutex_exit(&vd->vdev_scan_io_queue_lock);
+               }
+
+               if (scn->scn_phys.scn_queue_obj != 0)
+                       scan_ds_queue_sync(scn, tx);
+               VERIFY0(zap_update(scn->scn_dp->dp_meta_objset,
+                   DMU_POOL_DIRECTORY_OBJECT,
+                   DMU_POOL_SCAN, sizeof (uint64_t), SCAN_PHYS_NUMINTS,
+                   &scn->scn_phys, tx));
+               memcpy(&scn->scn_phys_cached, &scn->scn_phys,
+                   sizeof (scn->scn_phys));
+
+               if (scn->scn_checkpointing)
+                       zfs_dbgmsg("finish scan checkpoint for %s",
+                           spa->spa_name);
+
+               scn->scn_checkpointing = B_FALSE;
+               scn->scn_last_checkpoint = ddi_get_lbolt();
+       } else if (sync_type == SYNC_CACHED) {
+               VERIFY0(zap_update(scn->scn_dp->dp_meta_objset,
+                   DMU_POOL_DIRECTORY_OBJECT,
+                   DMU_POOL_SCAN, sizeof (uint64_t), SCAN_PHYS_NUMINTS,
+                   &scn->scn_phys_cached, tx));
+       }
+}
+
+int
 dsl_scan_setup_check(void *arg, dmu_tx_t *tx)
 {
+       (void) arg;
        dsl_scan_t *scn = dmu_tx_pool(tx)->dp_scan;
+       vdev_t *rvd = scn->scn_dp->dp_spa->spa_root_vdev;
 
-       if (scn->scn_phys.scn_state == DSS_SCANNING)
+       if (dsl_scan_is_running(scn) || vdev_rebuild_active(rvd))
                return (SET_ERROR(EBUSY));
 
        return (0);
 }
 
-static void
+void
 dsl_scan_setup_sync(void *arg, dmu_tx_t *tx)
 {
        dsl_scan_t *scn = dmu_tx_pool(tx)->dp_scan;
@@ -225,9 +760,9 @@ dsl_scan_setup_sync(void *arg, dmu_tx_t *tx)
        dsl_pool_t *dp = scn->scn_dp;
        spa_t *spa = dp->dp_spa;
 
-       ASSERT(scn->scn_phys.scn_state != DSS_SCANNING);
+       ASSERT(!dsl_scan_is_running(scn));
        ASSERT(*funcp > POOL_SCAN_NONE && *funcp < POOL_SCAN_FUNCS);
-       bzero(&scn->scn_phys, sizeof (scn->scn_phys));
+       memset(&scn->scn_phys, 0, sizeof (scn->scn_phys));
        scn->scn_phys.scn_func = *funcp;
        scn->scn_phys.scn_state = DSS_SCANNING;
        scn->scn_phys.scn_min_txg = 0;
@@ -236,9 +771,13 @@ dsl_scan_setup_sync(void *arg, dmu_tx_t *tx)
        scn->scn_phys.scn_start_time = gethrestime_sec();
        scn->scn_phys.scn_errors = 0;
        scn->scn_phys.scn_to_examine = spa->spa_root_vdev->vdev_stat.vs_alloc;
+       scn->scn_issued_before_pass = 0;
        scn->scn_restart_txg = 0;
        scn->scn_done_txg = 0;
+       scn->scn_last_checkpoint = 0;
+       scn->scn_checkpointing = B_FALSE;
        spa_scan_stat_init(spa);
+       vdev_scan_stat_init(spa->spa_root_vdev);
 
        if (DSL_SCAN_IS_SCRUB_RESILVER(scn)) {
                scn->scn_phys.scn_ddt_class_max = zfs_scrub_ddt_class_max;
@@ -248,9 +787,14 @@ dsl_scan_setup_sync(void *arg, dmu_tx_t *tx)
 
                if (vdev_resilver_needed(spa->spa_root_vdev,
                    &scn->scn_phys.scn_min_txg, &scn->scn_phys.scn_max_txg)) {
-                       spa_event_notify(spa, NULL, ESC_ZFS_RESILVER_START);
+                       nvlist_t *aux = fnvlist_alloc();
+                       fnvlist_add_string(aux, ZFS_EV_RESILVER_TYPE,
+                           "healing");
+                       spa_event_notify(spa, NULL, aux,
+                           ESC_ZFS_RESILVER_START);
+                       nvlist_free(aux);
                } else {
-                       spa_event_notify(spa, NULL, ESC_ZFS_SCRUB_START);
+                       spa_event_notify(spa, NULL, NULL, ESC_ZFS_SCRUB_START);
                }
 
                spa->spa_scrub_started = B_TRUE;
@@ -262,15 +806,38 @@ dsl_scan_setup_sync(void *arg, dmu_tx_t *tx)
                if (scn->scn_phys.scn_min_txg > TXG_INITIAL)
                        scn->scn_phys.scn_ddt_class_max = DDT_CLASS_DITTO;
 
+               /*
+                * When starting a resilver clear any existing rebuild state.
+                * This is required to prevent stale rebuild status from
+                * being reported when a rebuild is run, then a resilver and
+                * finally a scrub.  In which case only the scrub status
+                * should be reported by 'zpool status'.
+                */
+               if (scn->scn_phys.scn_func == POOL_SCAN_RESILVER) {
+                       vdev_t *rvd = spa->spa_root_vdev;
+                       for (uint64_t i = 0; i < rvd->vdev_children; i++) {
+                               vdev_t *vd = rvd->vdev_child[i];
+                               vdev_rebuild_clear_sync(
+                                   (void *)(uintptr_t)vd->vdev_id, tx);
+                       }
+               }
        }
 
        /* back to the generic stuff */
 
-       if (dp->dp_blkstats == NULL) {
-               dp->dp_blkstats =
-                   vmem_alloc(sizeof (zfs_all_blkstats_t), KM_SLEEP);
+       if (zfs_scan_blkstats) {
+               if (dp->dp_blkstats == NULL) {
+                       dp->dp_blkstats =
+                           vmem_alloc(sizeof (zfs_all_blkstats_t), KM_SLEEP);
+               }
+               memset(&dp->dp_blkstats->zab_type, 0,
+                   sizeof (dp->dp_blkstats->zab_type));
+       } else {
+               if (dp->dp_blkstats) {
+                       vmem_free(dp->dp_blkstats, sizeof (zfs_all_blkstats_t));
+                       dp->dp_blkstats = NULL;
+               }
        }
-       bzero(dp->dp_blkstats, sizeof (zfs_all_blkstats_t));
 
        if (spa_version(spa) < SPA_VERSION_DSL_SCRUB)
                ot = DMU_OT_ZAP_OTHER;
@@ -278,14 +845,60 @@ dsl_scan_setup_sync(void *arg, dmu_tx_t *tx)
        scn->scn_phys.scn_queue_obj = zap_create(dp->dp_meta_objset,
            ot ? ot : DMU_OT_SCAN_QUEUE, DMU_OT_NONE, 0, tx);
 
-       dsl_scan_sync_state(scn, tx);
+       memcpy(&scn->scn_phys_cached, &scn->scn_phys, sizeof (scn->scn_phys));
+
+       dsl_scan_sync_state(scn, tx, SYNC_MANDATORY);
 
        spa_history_log_internal(spa, "scan setup", tx,
            "func=%u mintxg=%llu maxtxg=%llu",
-           *funcp, scn->scn_phys.scn_min_txg, scn->scn_phys.scn_max_txg);
+           *funcp, (u_longlong_t)scn->scn_phys.scn_min_txg,
+           (u_longlong_t)scn->scn_phys.scn_max_txg);
+}
+
+/*
+ * Called by the ZFS_IOC_POOL_SCAN ioctl to start a scrub or resilver.
+ * Can also be called to resume a paused scrub.
+ */
+int
+dsl_scan(dsl_pool_t *dp, pool_scan_func_t func)
+{
+       spa_t *spa = dp->dp_spa;
+       dsl_scan_t *scn = dp->dp_scan;
+
+       /*
+        * Purge all vdev caches and probe all devices.  We do this here
+        * rather than in sync context because this requires a writer lock
+        * on the spa_config lock, which we can't do from sync context.  The
+        * spa_scrub_reopen flag indicates that vdev_open() should not
+        * attempt to start another scrub.
+        */
+       spa_vdev_state_enter(spa, SCL_NONE);
+       spa->spa_scrub_reopen = B_TRUE;
+       vdev_reopen(spa->spa_root_vdev);
+       spa->spa_scrub_reopen = B_FALSE;
+       (void) spa_vdev_state_exit(spa, NULL, 0);
+
+       if (func == POOL_SCAN_RESILVER) {
+               dsl_scan_restart_resilver(spa->spa_dsl_pool, 0);
+               return (0);
+       }
+
+       if (func == POOL_SCAN_SCRUB && dsl_scan_is_paused_scrub(scn)) {
+               /* got scrub start cmd, resume paused scrub */
+               int err = dsl_scrub_set_pause_resume(scn->scn_dp,
+                   POOL_SCRUB_NORMAL);
+               if (err == 0) {
+                       spa_event_notify(spa, NULL, NULL, ESC_ZFS_SCRUB_RESUME);
+                       return (SET_ERROR(ECANCELED));
+               }
+
+               return (SET_ERROR(err));
+       }
+
+       return (dsl_sync_task(spa_name(spa), dsl_scan_setup_check,
+           dsl_scan_setup_sync, &func, 0, ZFS_SPACE_CHECK_EXTRA_RESERVED));
 }
 
-/* ARGSUSED */
 static void
 dsl_scan_done(dsl_scan_t *scn, boolean_t complete, dmu_tx_t *tx)
 {
@@ -312,88 +925,146 @@ dsl_scan_done(dsl_scan_t *scn, boolean_t complete, dmu_tx_t *tx)
        }
 
        if (scn->scn_phys.scn_queue_obj != 0) {
-               VERIFY(0 == dmu_object_free(dp->dp_meta_objset,
+               VERIFY0(dmu_object_free(dp->dp_meta_objset,
                    scn->scn_phys.scn_queue_obj, tx));
                scn->scn_phys.scn_queue_obj = 0;
        }
+       scan_ds_queue_clear(scn);
+       scan_ds_prefetch_queue_clear(scn);
+
+       scn->scn_phys.scn_flags &= ~DSF_SCRUB_PAUSED;
 
        /*
         * If we were "restarted" from a stopped state, don't bother
         * with anything else.
         */
-       if (scn->scn_phys.scn_state != DSS_SCANNING)
+       if (!dsl_scan_is_running(scn)) {
+               ASSERT(!scn->scn_is_sorted);
                return;
+       }
 
-       if (complete)
-               scn->scn_phys.scn_state = DSS_FINISHED;
-       else
-               scn->scn_phys.scn_state = DSS_CANCELED;
+       if (scn->scn_is_sorted) {
+               scan_io_queues_destroy(scn);
+               scn->scn_is_sorted = B_FALSE;
+
+               if (scn->scn_taskq != NULL) {
+                       taskq_destroy(scn->scn_taskq);
+                       scn->scn_taskq = NULL;
+               }
+       }
+
+       scn->scn_phys.scn_state = complete ? DSS_FINISHED : DSS_CANCELED;
+
+       spa_notify_waiters(spa);
 
        if (dsl_scan_restarting(scn, tx))
                spa_history_log_internal(spa, "scan aborted, restarting", tx,
-                   "errors=%llu", spa_get_errlog_size(spa));
+                   "errors=%llu", (u_longlong_t)spa_approx_errlog_size(spa));
        else if (!complete)
                spa_history_log_internal(spa, "scan cancelled", tx,
-                   "errors=%llu", spa_get_errlog_size(spa));
+                   "errors=%llu", (u_longlong_t)spa_approx_errlog_size(spa));
        else
                spa_history_log_internal(spa, "scan done", tx,
-                   "errors=%llu", spa_get_errlog_size(spa));
+                   "errors=%llu", (u_longlong_t)spa_approx_errlog_size(spa));
 
        if (DSL_SCAN_IS_SCRUB_RESILVER(scn)) {
-               mutex_enter(&spa->spa_scrub_lock);
-               while (spa->spa_scrub_inflight > 0) {
-                       cv_wait(&spa->spa_scrub_io_cv,
-                           &spa->spa_scrub_lock);
-               }
-               mutex_exit(&spa->spa_scrub_lock);
-               spa->spa_scrub_started = B_FALSE;
                spa->spa_scrub_active = B_FALSE;
 
                /*
                 * If the scrub/resilver completed, update all DTLs to
                 * reflect this.  Whether it succeeded or not, vacate
                 * all temporary scrub DTLs.
+                *
+                * As the scrub does not currently support traversing
+                * data that have been freed but are part of a checkpoint,
+                * we don't mark the scrub as done in the DTLs as faults
+                * may still exist in those vdevs.
                 */
-               vdev_dtl_reassess(spa->spa_root_vdev, tx->tx_txg,
-                   complete ? scn->scn_phys.scn_max_txg : 0, B_TRUE);
-               if (complete) {
-                       spa_event_notify(spa, NULL, scn->scn_phys.scn_min_txg ?
-                           ESC_ZFS_RESILVER_FINISH : ESC_ZFS_SCRUB_FINISH);
+               if (complete &&
+                   !spa_feature_is_active(spa, SPA_FEATURE_POOL_CHECKPOINT)) {
+                       vdev_dtl_reassess(spa->spa_root_vdev, tx->tx_txg,
+                           scn->scn_phys.scn_max_txg, B_TRUE, B_FALSE);
+
+                       if (scn->scn_phys.scn_min_txg) {
+                               nvlist_t *aux = fnvlist_alloc();
+                               fnvlist_add_string(aux, ZFS_EV_RESILVER_TYPE,
+                                   "healing");
+                               spa_event_notify(spa, NULL, aux,
+                                   ESC_ZFS_RESILVER_FINISH);
+                               nvlist_free(aux);
+                       } else {
+                               spa_event_notify(spa, NULL, NULL,
+                                   ESC_ZFS_SCRUB_FINISH);
+                       }
+               } else {
+                       vdev_dtl_reassess(spa->spa_root_vdev, tx->tx_txg,
+                           0, B_TRUE, B_FALSE);
                }
                spa_errlog_rotate(spa);
 
+               /*
+                * Don't clear flag until after vdev_dtl_reassess to ensure that
+                * DTL_MISSING will get updated when possible.
+                */
+               spa->spa_scrub_started = B_FALSE;
+
                /*
                 * We may have finished replacing a device.
                 * Let the async thread assess this and handle the detach.
                 */
                spa_async_request(spa, SPA_ASYNC_RESILVER_DONE);
+
+               /*
+                * Clear any resilver_deferred flags in the config.
+                * If there are drives that need resilvering, kick
+                * off an asynchronous request to start resilver.
+                * vdev_clear_resilver_deferred() may update the config
+                * before the resilver can restart. In the event of
+                * a crash during this period, the spa loading code
+                * will find the drives that need to be resilvered
+                * and start the resilver then.
+                */
+               if (spa_feature_is_enabled(spa, SPA_FEATURE_RESILVER_DEFER) &&
+                   vdev_clear_resilver_deferred(spa->spa_root_vdev, tx)) {
+                       spa_history_log_internal(spa,
+                           "starting deferred resilver", tx, "errors=%llu",
+                           (u_longlong_t)spa_approx_errlog_size(spa));
+                       spa_async_request(spa, SPA_ASYNC_RESILVER);
+               }
+
+               /* Clear recent error events (i.e. duplicate events tracking) */
+               if (complete)
+                       zfs_ereport_clear(spa, NULL);
        }
 
        scn->scn_phys.scn_end_time = gethrestime_sec();
 
        if (spa->spa_errata == ZPOOL_ERRATA_ZOL_2094_SCRUB)
                spa->spa_errata = 0;
+
+       ASSERT(!dsl_scan_is_running(scn));
 }
 
-/* ARGSUSED */
 static int
 dsl_scan_cancel_check(void *arg, dmu_tx_t *tx)
 {
+       (void) arg;
        dsl_scan_t *scn = dmu_tx_pool(tx)->dp_scan;
 
-       if (scn->scn_phys.scn_state != DSS_SCANNING)
+       if (!dsl_scan_is_running(scn))
                return (SET_ERROR(ENOENT));
        return (0);
 }
 
-/* ARGSUSED */
 static void
 dsl_scan_cancel_sync(void *arg, dmu_tx_t *tx)
 {
+       (void) arg;
        dsl_scan_t *scn = dmu_tx_pool(tx)->dp_scan;
 
        dsl_scan_done(scn, B_FALSE, tx);
-       dsl_scan_sync_state(scn, tx);
+       dsl_scan_sync_state(scn, tx, SYNC_MANDATORY);
+       spa_event_notify(scn->scn_dp->dp_spa, NULL, NULL, ESC_ZFS_SCRUB_ABORT);
 }
 
 int
@@ -403,104 +1074,349 @@ dsl_scan_cancel(dsl_pool_t *dp)
            dsl_scan_cancel_sync, NULL, 3, ZFS_SPACE_CHECK_RESERVED));
 }
 
-static void dsl_scan_visitbp(blkptr_t *bp, const zbookmark_phys_t *zb,
-    dnode_phys_t *dnp, dsl_dataset_t *ds, dsl_scan_t *scn,
-    dmu_objset_type_t ostype, dmu_tx_t *tx);
-inline __attribute__((always_inline)) static void dsl_scan_visitdnode(
-    dsl_scan_t *, dsl_dataset_t *ds, dmu_objset_type_t ostype,
-    dnode_phys_t *dnp, uint64_t object, dmu_tx_t *tx);
-
-void
-dsl_free(dsl_pool_t *dp, uint64_t txg, const blkptr_t *bp)
+static int
+dsl_scrub_pause_resume_check(void *arg, dmu_tx_t *tx)
 {
-       zio_free(dp->dp_spa, txg, bp);
-}
+       pool_scrub_cmd_t *cmd = arg;
+       dsl_pool_t *dp = dmu_tx_pool(tx);
+       dsl_scan_t *scn = dp->dp_scan;
 
-void
-dsl_free_sync(zio_t *pio, dsl_pool_t *dp, uint64_t txg, const blkptr_t *bpp)
-{
-       ASSERT(dsl_pool_sync_context(dp));
-       zio_nowait(zio_free_sync(pio, dp->dp_spa, txg, bpp, pio->io_flags));
-}
+       if (*cmd == POOL_SCRUB_PAUSE) {
+               /* can't pause a scrub when there is no in-progress scrub */
+               if (!dsl_scan_scrubbing(dp))
+                       return (SET_ERROR(ENOENT));
 
-static uint64_t
-dsl_scan_ds_maxtxg(dsl_dataset_t *ds)
-{
-       uint64_t smt = ds->ds_dir->dd_pool->dp_scan->scn_phys.scn_max_txg;
-       if (ds->ds_is_snapshot)
-               return (MIN(smt, dsl_dataset_phys(ds)->ds_creation_txg));
-       return (smt);
+               /* can't pause a paused scrub */
+               if (dsl_scan_is_paused_scrub(scn))
+                       return (SET_ERROR(EBUSY));
+       } else if (*cmd != POOL_SCRUB_NORMAL) {
+               return (SET_ERROR(ENOTSUP));
+       }
+
+       return (0);
 }
 
 static void
-dsl_scan_sync_state(dsl_scan_t *scn, dmu_tx_t *tx)
+dsl_scrub_pause_resume_sync(void *arg, dmu_tx_t *tx)
 {
-       VERIFY0(zap_update(scn->scn_dp->dp_meta_objset,
-           DMU_POOL_DIRECTORY_OBJECT,
-           DMU_POOL_SCAN, sizeof (uint64_t), SCAN_PHYS_NUMINTS,
-           &scn->scn_phys, tx));
-}
+       pool_scrub_cmd_t *cmd = arg;
+       dsl_pool_t *dp = dmu_tx_pool(tx);
+       spa_t *spa = dp->dp_spa;
+       dsl_scan_t *scn = dp->dp_scan;
 
-extern int zfs_vdev_async_write_active_min_dirty_percent;
+       if (*cmd == POOL_SCRUB_PAUSE) {
+               /* can't pause a scrub when there is no in-progress scrub */
+               spa->spa_scan_pass_scrub_pause = gethrestime_sec();
+               scn->scn_phys.scn_flags |= DSF_SCRUB_PAUSED;
+               scn->scn_phys_cached.scn_flags |= DSF_SCRUB_PAUSED;
+               dsl_scan_sync_state(scn, tx, SYNC_CACHED);
+               spa_event_notify(spa, NULL, NULL, ESC_ZFS_SCRUB_PAUSED);
+               spa_notify_waiters(spa);
+       } else {
+               ASSERT3U(*cmd, ==, POOL_SCRUB_NORMAL);
+               if (dsl_scan_is_paused_scrub(scn)) {
+                       /*
+                        * We need to keep track of how much time we spend
+                        * paused per pass so that we can adjust the scrub rate
+                        * shown in the output of 'zpool status'
+                        */
+                       spa->spa_scan_pass_scrub_spent_paused +=
+                           gethrestime_sec() - spa->spa_scan_pass_scrub_pause;
+                       spa->spa_scan_pass_scrub_pause = 0;
+                       scn->scn_phys.scn_flags &= ~DSF_SCRUB_PAUSED;
+                       scn->scn_phys_cached.scn_flags &= ~DSF_SCRUB_PAUSED;
+                       dsl_scan_sync_state(scn, tx, SYNC_CACHED);
+               }
+       }
+}
 
-static boolean_t
-dsl_scan_check_pause(dsl_scan_t *scn, const zbookmark_phys_t *zb)
+/*
+ * Set scrub pause/resume state if it makes sense to do so
+ */
+int
+dsl_scrub_set_pause_resume(const dsl_pool_t *dp, pool_scrub_cmd_t cmd)
 {
-       uint64_t elapsed_nanosecs;
-       int mintime;
-       int dirty_pct;
+       return (dsl_sync_task(spa_name(dp->dp_spa),
+           dsl_scrub_pause_resume_check, dsl_scrub_pause_resume_sync, &cmd, 3,
+           ZFS_SPACE_CHECK_RESERVED));
+}
+
+
+/* start a new scan, or restart an existing one. */
+void
+dsl_scan_restart_resilver(dsl_pool_t *dp, uint64_t txg)
+{
+       if (txg == 0) {
+               dmu_tx_t *tx;
+               tx = dmu_tx_create_dd(dp->dp_mos_dir);
+               VERIFY(0 == dmu_tx_assign(tx, TXG_WAIT));
+
+               txg = dmu_tx_get_txg(tx);
+               dp->dp_scan->scn_restart_txg = txg;
+               dmu_tx_commit(tx);
+       } else {
+               dp->dp_scan->scn_restart_txg = txg;
+       }
+       zfs_dbgmsg("restarting resilver for %s at txg=%llu",
+           dp->dp_spa->spa_name, (longlong_t)txg);
+}
+
+void
+dsl_free(dsl_pool_t *dp, uint64_t txg, const blkptr_t *bp)
+{
+       zio_free(dp->dp_spa, txg, bp);
+}
+
+void
+dsl_free_sync(zio_t *pio, dsl_pool_t *dp, uint64_t txg, const blkptr_t *bpp)
+{
+       ASSERT(dsl_pool_sync_context(dp));
+       zio_nowait(zio_free_sync(pio, dp->dp_spa, txg, bpp, pio->io_flags));
+}
+
+static int
+scan_ds_queue_compare(const void *a, const void *b)
+{
+       const scan_ds_t *sds_a = a, *sds_b = b;
+
+       if (sds_a->sds_dsobj < sds_b->sds_dsobj)
+               return (-1);
+       if (sds_a->sds_dsobj == sds_b->sds_dsobj)
+               return (0);
+       return (1);
+}
+
+static void
+scan_ds_queue_clear(dsl_scan_t *scn)
+{
+       void *cookie = NULL;
+       scan_ds_t *sds;
+       while ((sds = avl_destroy_nodes(&scn->scn_queue, &cookie)) != NULL) {
+               kmem_free(sds, sizeof (*sds));
+       }
+}
+
+static boolean_t
+scan_ds_queue_contains(dsl_scan_t *scn, uint64_t dsobj, uint64_t *txg)
+{
+       scan_ds_t srch, *sds;
+
+       srch.sds_dsobj = dsobj;
+       sds = avl_find(&scn->scn_queue, &srch, NULL);
+       if (sds != NULL && txg != NULL)
+               *txg = sds->sds_txg;
+       return (sds != NULL);
+}
+
+static void
+scan_ds_queue_insert(dsl_scan_t *scn, uint64_t dsobj, uint64_t txg)
+{
+       scan_ds_t *sds;
+       avl_index_t where;
+
+       sds = kmem_zalloc(sizeof (*sds), KM_SLEEP);
+       sds->sds_dsobj = dsobj;
+       sds->sds_txg = txg;
+
+       VERIFY3P(avl_find(&scn->scn_queue, sds, &where), ==, NULL);
+       avl_insert(&scn->scn_queue, sds, where);
+}
+
+static void
+scan_ds_queue_remove(dsl_scan_t *scn, uint64_t dsobj)
+{
+       scan_ds_t srch, *sds;
+
+       srch.sds_dsobj = dsobj;
+
+       sds = avl_find(&scn->scn_queue, &srch, NULL);
+       VERIFY(sds != NULL);
+       avl_remove(&scn->scn_queue, sds);
+       kmem_free(sds, sizeof (*sds));
+}
+
+static void
+scan_ds_queue_sync(dsl_scan_t *scn, dmu_tx_t *tx)
+{
+       dsl_pool_t *dp = scn->scn_dp;
+       spa_t *spa = dp->dp_spa;
+       dmu_object_type_t ot = (spa_version(spa) >= SPA_VERSION_DSL_SCRUB) ?
+           DMU_OT_SCAN_QUEUE : DMU_OT_ZAP_OTHER;
+
+       ASSERT0(scn->scn_queues_pending);
+       ASSERT(scn->scn_phys.scn_queue_obj != 0);
+
+       VERIFY0(dmu_object_free(dp->dp_meta_objset,
+           scn->scn_phys.scn_queue_obj, tx));
+       scn->scn_phys.scn_queue_obj = zap_create(dp->dp_meta_objset, ot,
+           DMU_OT_NONE, 0, tx);
+       for (scan_ds_t *sds = avl_first(&scn->scn_queue);
+           sds != NULL; sds = AVL_NEXT(&scn->scn_queue, sds)) {
+               VERIFY0(zap_add_int_key(dp->dp_meta_objset,
+                   scn->scn_phys.scn_queue_obj, sds->sds_dsobj,
+                   sds->sds_txg, tx));
+       }
+}
+
+/*
+ * Computes the memory limit state that we're currently in. A sorted scan
+ * needs quite a bit of memory to hold the sorting queue, so we need to
+ * reasonably constrain the size so it doesn't impact overall system
+ * performance. We compute two limits:
+ * 1) Hard memory limit: if the amount of memory used by the sorting
+ *     queues on a pool gets above this value, we stop the metadata
+ *     scanning portion and start issuing the queued up and sorted
+ *     I/Os to reduce memory usage.
+ *     This limit is calculated as a fraction of physmem (by default 5%).
+ *     We constrain the lower bound of the hard limit to an absolute
+ *     minimum of zfs_scan_mem_lim_min (default: 16 MiB). We also constrain
+ *     the upper bound to 5% of the total pool size - no chance we'll
+ *     ever need that much memory, but just to keep the value in check.
+ * 2) Soft memory limit: once we hit the hard memory limit, we start
+ *     issuing I/O to reduce queue memory usage, but we don't want to
+ *     completely empty out the queues, since we might be able to find I/Os
+ *     that will fill in the gaps of our non-sequential IOs at some point
+ *     in the future. So we stop the issuing of I/Os once the amount of
+ *     memory used drops below the soft limit (at which point we stop issuing
+ *     I/O and start scanning metadata again).
+ *
+ *     This limit is calculated by subtracting a fraction of the hard
+ *     limit from the hard limit. By default this fraction is 5%, so
+ *     the soft limit is 95% of the hard limit. We cap the size of the
+ *     difference between the hard and soft limits at an absolute
+ *     maximum of zfs_scan_mem_lim_soft_max (default: 128 MiB) - this is
+ *     sufficient to not cause too frequent switching between the
+ *     metadata scan and I/O issue (even at 2k recordsize, 128 MiB's
+ *     worth of queues is about 1.2 GiB of on-pool data, so scanning
+ *     that should take at least a decent fraction of a second).
+ */
+static boolean_t
+dsl_scan_should_clear(dsl_scan_t *scn)
+{
+       spa_t *spa = scn->scn_dp->dp_spa;
+       vdev_t *rvd = scn->scn_dp->dp_spa->spa_root_vdev;
+       uint64_t alloc, mlim_hard, mlim_soft, mused;
+
+       alloc = metaslab_class_get_alloc(spa_normal_class(spa));
+       alloc += metaslab_class_get_alloc(spa_special_class(spa));
+       alloc += metaslab_class_get_alloc(spa_dedup_class(spa));
+
+       mlim_hard = MAX((physmem / zfs_scan_mem_lim_fact) * PAGESIZE,
+           zfs_scan_mem_lim_min);
+       mlim_hard = MIN(mlim_hard, alloc / 20);
+       mlim_soft = mlim_hard - MIN(mlim_hard / zfs_scan_mem_lim_soft_fact,
+           zfs_scan_mem_lim_soft_max);
+       mused = 0;
+       for (uint64_t i = 0; i < rvd->vdev_children; i++) {
+               vdev_t *tvd = rvd->vdev_child[i];
+               dsl_scan_io_queue_t *queue;
+
+               mutex_enter(&tvd->vdev_scan_io_queue_lock);
+               queue = tvd->vdev_scan_io_queue;
+               if (queue != NULL) {
+                       /*
+                        * # of extents in exts_by_addr = # in exts_by_size.
+                        * B-tree efficiency is ~75%, but can be as low as 50%.
+                        */
+                       mused += zfs_btree_numnodes(&queue->q_exts_by_size) *
+                           ((sizeof (range_seg_gap_t) + sizeof (uint64_t)) *
+                           3 / 2) + queue->q_sio_memused;
+               }
+               mutex_exit(&tvd->vdev_scan_io_queue_lock);
+       }
 
+       dprintf("current scan memory usage: %llu bytes\n", (longlong_t)mused);
+
+       if (mused == 0)
+               ASSERT0(scn->scn_queues_pending);
+
+       /*
+        * If we are above our hard limit, we need to clear out memory.
+        * If we are below our soft limit, we need to accumulate sequential IOs.
+        * Otherwise, we should keep doing whatever we are currently doing.
+        */
+       if (mused >= mlim_hard)
+               return (B_TRUE);
+       else if (mused < mlim_soft)
+               return (B_FALSE);
+       else
+               return (scn->scn_clearing);
+}
+
+static boolean_t
+dsl_scan_check_suspend(dsl_scan_t *scn, const zbookmark_phys_t *zb)
+{
        /* we never skip user/group accounting objects */
        if (zb && (int64_t)zb->zb_object < 0)
                return (B_FALSE);
 
-       if (scn->scn_pausing)
-               return (B_TRUE); /* we're already pausing */
+       if (scn->scn_suspending)
+               return (B_TRUE); /* we're already suspending */
 
        if (!ZB_IS_ZERO(&scn->scn_phys.scn_bookmark))
                return (B_FALSE); /* we're resuming */
 
-       /* We only know how to resume from level-0 blocks. */
-       if (zb && zb->zb_level != 0)
+       /* We only know how to resume from level-0 and objset blocks. */
+       if (zb && (zb->zb_level != 0 && zb->zb_level != ZB_ROOT_LEVEL))
                return (B_FALSE);
 
        /*
-        * We pause if:
-        *  - we have scanned for the maximum time: an entire txg
-        *    timeout (default 5 sec)
-        *  or
+        * We suspend if:
         *  - we have scanned for at least the minimum time (default 1 sec
         *    for scrub, 3 sec for resilver), and either we have sufficient
         *    dirty data that we are starting to write more quickly
-        *    (default 30%), or someone is explicitly waiting for this txg
-        *    to complete.
+        *    (default 30%), someone is explicitly waiting for this txg
+        *    to complete, or we have used up all of the time in the txg
+        *    timeout (default 5 sec).
         *  or
         *  - the spa is shutting down because this pool is being exported
         *    or the machine is rebooting.
+        *  or
+        *  - the scan queue has reached its memory use limit
         */
-       mintime = (scn->scn_phys.scn_func == POOL_SCAN_RESILVER) ?
-           zfs_resilver_min_time_ms : zfs_scan_min_time_ms;
-       elapsed_nanosecs = gethrtime() - scn->scn_sync_start_time;
-       dirty_pct = scn->scn_dp->dp_dirty_total * 100 / zfs_dirty_data_max;
-       if (elapsed_nanosecs / NANOSEC >= zfs_txg_timeout ||
-           (NSEC2MSEC(elapsed_nanosecs) > mintime &&
-           (txg_sync_waiting(scn->scn_dp) ||
-           dirty_pct >= zfs_vdev_async_write_active_min_dirty_percent)) ||
-           spa_shutting_down(scn->scn_dp->dp_spa)) {
-               if (zb) {
-                       dprintf("pausing at bookmark %llx/%llx/%llx/%llx\n",
+       uint64_t curr_time_ns = gethrtime();
+       uint64_t scan_time_ns = curr_time_ns - scn->scn_sync_start_time;
+       uint64_t sync_time_ns = curr_time_ns -
+           scn->scn_dp->dp_spa->spa_sync_starttime;
+       uint64_t dirty_min_bytes = zfs_dirty_data_max *
+           zfs_vdev_async_write_active_min_dirty_percent / 100;
+       uint_t mintime = (scn->scn_phys.scn_func == POOL_SCAN_RESILVER) ?
+           zfs_resilver_min_time_ms : zfs_scrub_min_time_ms;
+
+       if ((NSEC2MSEC(scan_time_ns) > mintime &&
+           (scn->scn_dp->dp_dirty_total >= dirty_min_bytes ||
+           txg_sync_waiting(scn->scn_dp) ||
+           NSEC2SEC(sync_time_ns) >= zfs_txg_timeout)) ||
+           spa_shutting_down(scn->scn_dp->dp_spa) ||
+           (zfs_scan_strict_mem_lim && dsl_scan_should_clear(scn))) {
+               if (zb && zb->zb_level == ZB_ROOT_LEVEL) {
+                       dprintf("suspending at first available bookmark "
+                           "%llx/%llx/%llx/%llx\n",
+                           (longlong_t)zb->zb_objset,
+                           (longlong_t)zb->zb_object,
+                           (longlong_t)zb->zb_level,
+                           (longlong_t)zb->zb_blkid);
+                       SET_BOOKMARK(&scn->scn_phys.scn_bookmark,
+                           zb->zb_objset, 0, 0, 0);
+               } else if (zb != NULL) {
+                       dprintf("suspending at bookmark %llx/%llx/%llx/%llx\n",
                            (longlong_t)zb->zb_objset,
                            (longlong_t)zb->zb_object,
                            (longlong_t)zb->zb_level,
                            (longlong_t)zb->zb_blkid);
                        scn->scn_phys.scn_bookmark = *zb;
+               } else {
+#ifdef ZFS_DEBUG
+                       dsl_scan_phys_t *scnp = &scn->scn_phys;
+                       dprintf("suspending at at DDT bookmark "
+                           "%llx/%llx/%llx/%llx\n",
+                           (longlong_t)scnp->scn_ddt_bookmark.ddb_class,
+                           (longlong_t)scnp->scn_ddt_bookmark.ddb_type,
+                           (longlong_t)scnp->scn_ddt_bookmark.ddb_checksum,
+                           (longlong_t)scnp->scn_ddt_bookmark.ddb_cursor);
+#endif
                }
-               dprintf("pausing at DDT bookmark %llx/%llx/%llx/%llx\n",
-                   (longlong_t)scn->scn_phys.scn_ddt_bookmark.ddb_class,
-                   (longlong_t)scn->scn_phys.scn_ddt_bookmark.ddb_type,
-                   (longlong_t)scn->scn_phys.scn_ddt_bookmark.ddb_checksum,
-                   (longlong_t)scn->scn_phys.scn_ddt_bookmark.ddb_cursor);
-               scn->scn_pausing = B_TRUE;
+               scn->scn_suspending = B_TRUE;
                return (B_TRUE);
        }
        return (B_FALSE);
@@ -511,16 +1427,18 @@ typedef struct zil_scan_arg {
        zil_header_t    *zsa_zh;
 } zil_scan_arg_t;
 
-/* ARGSUSED */
 static int
-dsl_scan_zil_block(zilog_t *zilog, blkptr_t *bp, void *arg, uint64_t claim_txg)
+dsl_scan_zil_block(zilog_t *zilog, const blkptr_t *bp, void *arg,
+    uint64_t claim_txg)
 {
+       (void) zilog;
        zil_scan_arg_t *zsa = arg;
        dsl_pool_t *dp = zsa->zsa_dp;
        dsl_scan_t *scn = dp->dp_scan;
        zil_header_t *zh = zsa->zsa_zh;
        zbookmark_phys_t zb;
 
+       ASSERT(!BP_IS_REDACTED(bp));
        if (BP_IS_HOLE(bp) || bp->blk_birth <= scn->scn_phys.scn_cur_min_txg)
                return (0);
 
@@ -530,7 +1448,7 @@ dsl_scan_zil_block(zilog_t *zilog, blkptr_t *bp, void *arg, uint64_t claim_txg)
         * (on-disk) even if it hasn't been claimed (even though for
         * scrub there's nothing to do to it).
         */
-       if (claim_txg == 0 && bp->blk_birth >= spa_first_txg(dp->dp_spa))
+       if (claim_txg == 0 && bp->blk_birth >= spa_min_claim_txg(dp->dp_spa))
                return (0);
 
        SET_BOOKMARK(&zb, zh->zh_log.blk_cksum.zc_word[ZIL_ZC_OBJSET],
@@ -540,19 +1458,21 @@ dsl_scan_zil_block(zilog_t *zilog, blkptr_t *bp, void *arg, uint64_t claim_txg)
        return (0);
 }
 
-/* ARGSUSED */
 static int
-dsl_scan_zil_record(zilog_t *zilog, lr_t *lrc, void *arg, uint64_t claim_txg)
+dsl_scan_zil_record(zilog_t *zilog, const lr_t *lrc, void *arg,
+    uint64_t claim_txg)
 {
+       (void) zilog;
        if (lrc->lrc_txtype == TX_WRITE) {
                zil_scan_arg_t *zsa = arg;
                dsl_pool_t *dp = zsa->zsa_dp;
                dsl_scan_t *scn = dp->dp_scan;
                zil_header_t *zh = zsa->zsa_zh;
-               lr_write_t *lr = (lr_write_t *)lrc;
-               blkptr_t *bp = &lr->lr_blkptr;
+               const lr_write_t *lr = (const lr_write_t *)lrc;
+               const blkptr_t *bp = &lr->lr_blkptr;
                zbookmark_phys_t zb;
 
+               ASSERT(!BP_IS_REDACTED(bp));
                if (BP_IS_HOLE(bp) ||
                    bp->blk_birth <= scn->scn_phys.scn_cur_min_txg)
                        return (0);
@@ -565,6 +1485,7 @@ dsl_scan_zil_record(zilog_t *zilog, lr_t *lrc, void *arg, uint64_t claim_txg)
                if (claim_txg == 0 || bp->blk_birth < claim_txg)
                        return (0);
 
+               ASSERT3U(BP_GET_LSIZE(bp), !=, 0);
                SET_BOOKMARK(&zb, zh->zh_log.blk_cksum.zc_word[ZIL_ZC_OBJSET],
                    lr->lr_foid, ZB_ZIL_LEVEL,
                    lr->lr_offset / BP_GET_LSIZE(bp));
@@ -581,41 +1502,316 @@ dsl_scan_zil(dsl_pool_t *dp, zil_header_t *zh)
        zil_scan_arg_t zsa = { dp, zh };
        zilog_t *zilog;
 
+       ASSERT(spa_writeable(dp->dp_spa));
+
        /*
         * We only want to visit blocks that have been claimed but not yet
         * replayed (or, in read-only mode, blocks that *would* be claimed).
         */
-       if (claim_txg == 0 && spa_writeable(dp->dp_spa))
+       if (claim_txg == 0)
                return;
 
        zilog = zil_alloc(dp->dp_meta_objset, zh);
 
        (void) zil_parse(zilog, dsl_scan_zil_block, dsl_scan_zil_record, &zsa,
-           claim_txg);
+           claim_txg, B_FALSE);
 
        zil_free(zilog);
 }
 
-/* ARGSUSED */
+/*
+ * We compare scan_prefetch_issue_ctx_t's based on their bookmarks. The idea
+ * here is to sort the AVL tree by the order each block will be needed.
+ */
+static int
+scan_prefetch_queue_compare(const void *a, const void *b)
+{
+       const scan_prefetch_issue_ctx_t *spic_a = a, *spic_b = b;
+       const scan_prefetch_ctx_t *spc_a = spic_a->spic_spc;
+       const scan_prefetch_ctx_t *spc_b = spic_b->spic_spc;
+
+       return (zbookmark_compare(spc_a->spc_datablkszsec,
+           spc_a->spc_indblkshift, spc_b->spc_datablkszsec,
+           spc_b->spc_indblkshift, &spic_a->spic_zb, &spic_b->spic_zb));
+}
+
+static void
+scan_prefetch_ctx_rele(scan_prefetch_ctx_t *spc, const void *tag)
+{
+       if (zfs_refcount_remove(&spc->spc_refcnt, tag) == 0) {
+               zfs_refcount_destroy(&spc->spc_refcnt);
+               kmem_free(spc, sizeof (scan_prefetch_ctx_t));
+       }
+}
+
+static scan_prefetch_ctx_t *
+scan_prefetch_ctx_create(dsl_scan_t *scn, dnode_phys_t *dnp, const void *tag)
+{
+       scan_prefetch_ctx_t *spc;
+
+       spc = kmem_alloc(sizeof (scan_prefetch_ctx_t), KM_SLEEP);
+       zfs_refcount_create(&spc->spc_refcnt);
+       zfs_refcount_add(&spc->spc_refcnt, tag);
+       spc->spc_scn = scn;
+       if (dnp != NULL) {
+               spc->spc_datablkszsec = dnp->dn_datablkszsec;
+               spc->spc_indblkshift = dnp->dn_indblkshift;
+               spc->spc_root = B_FALSE;
+       } else {
+               spc->spc_datablkszsec = 0;
+               spc->spc_indblkshift = 0;
+               spc->spc_root = B_TRUE;
+       }
+
+       return (spc);
+}
+
+static void
+scan_prefetch_ctx_add_ref(scan_prefetch_ctx_t *spc, const void *tag)
+{
+       zfs_refcount_add(&spc->spc_refcnt, tag);
+}
+
+static void
+scan_ds_prefetch_queue_clear(dsl_scan_t *scn)
+{
+       spa_t *spa = scn->scn_dp->dp_spa;
+       void *cookie = NULL;
+       scan_prefetch_issue_ctx_t *spic = NULL;
+
+       mutex_enter(&spa->spa_scrub_lock);
+       while ((spic = avl_destroy_nodes(&scn->scn_prefetch_queue,
+           &cookie)) != NULL) {
+               scan_prefetch_ctx_rele(spic->spic_spc, scn);
+               kmem_free(spic, sizeof (scan_prefetch_issue_ctx_t));
+       }
+       mutex_exit(&spa->spa_scrub_lock);
+}
+
+static boolean_t
+dsl_scan_check_prefetch_resume(scan_prefetch_ctx_t *spc,
+    const zbookmark_phys_t *zb)
+{
+       zbookmark_phys_t *last_zb = &spc->spc_scn->scn_prefetch_bookmark;
+       dnode_phys_t tmp_dnp;
+       dnode_phys_t *dnp = (spc->spc_root) ? NULL : &tmp_dnp;
+
+       if (zb->zb_objset != last_zb->zb_objset)
+               return (B_TRUE);
+       if ((int64_t)zb->zb_object < 0)
+               return (B_FALSE);
+
+       tmp_dnp.dn_datablkszsec = spc->spc_datablkszsec;
+       tmp_dnp.dn_indblkshift = spc->spc_indblkshift;
+
+       if (zbookmark_subtree_completed(dnp, zb, last_zb))
+               return (B_TRUE);
+
+       return (B_FALSE);
+}
+
 static void
-dsl_scan_prefetch(dsl_scan_t *scn, arc_buf_t *buf, blkptr_t *bp,
-    uint64_t objset, uint64_t object, uint64_t blkid)
+dsl_scan_prefetch(scan_prefetch_ctx_t *spc, blkptr_t *bp, zbookmark_phys_t *zb)
 {
-       zbookmark_phys_t czb;
-       arc_flags_t flags = ARC_FLAG_NOWAIT | ARC_FLAG_PREFETCH;
+       avl_index_t idx;
+       dsl_scan_t *scn = spc->spc_scn;
+       spa_t *spa = scn->scn_dp->dp_spa;
+       scan_prefetch_issue_ctx_t *spic;
+
+       if (zfs_no_scrub_prefetch || BP_IS_REDACTED(bp))
+               return;
+
+       if (BP_IS_HOLE(bp) || bp->blk_birth <= scn->scn_phys.scn_cur_min_txg ||
+           (BP_GET_LEVEL(bp) == 0 && BP_GET_TYPE(bp) != DMU_OT_DNODE &&
+           BP_GET_TYPE(bp) != DMU_OT_OBJSET))
+               return;
+
+       if (dsl_scan_check_prefetch_resume(spc, zb))
+               return;
+
+       scan_prefetch_ctx_add_ref(spc, scn);
+       spic = kmem_alloc(sizeof (scan_prefetch_issue_ctx_t), KM_SLEEP);
+       spic->spic_spc = spc;
+       spic->spic_bp = *bp;
+       spic->spic_zb = *zb;
 
-       if (zfs_no_scrub_prefetch)
+       /*
+        * Add the IO to the queue of blocks to prefetch. This allows us to
+        * prioritize blocks that we will need first for the main traversal
+        * thread.
+        */
+       mutex_enter(&spa->spa_scrub_lock);
+       if (avl_find(&scn->scn_prefetch_queue, spic, &idx) != NULL) {
+               /* this block is already queued for prefetch */
+               kmem_free(spic, sizeof (scan_prefetch_issue_ctx_t));
+               scan_prefetch_ctx_rele(spc, scn);
+               mutex_exit(&spa->spa_scrub_lock);
                return;
+       }
+
+       avl_insert(&scn->scn_prefetch_queue, spic, idx);
+       cv_broadcast(&spa->spa_scrub_io_cv);
+       mutex_exit(&spa->spa_scrub_lock);
+}
+
+static void
+dsl_scan_prefetch_dnode(dsl_scan_t *scn, dnode_phys_t *dnp,
+    uint64_t objset, uint64_t object)
+{
+       int i;
+       zbookmark_phys_t zb;
+       scan_prefetch_ctx_t *spc;
 
-       if (BP_IS_HOLE(bp) || bp->blk_birth <= scn->scn_phys.scn_min_txg ||
-           (BP_GET_LEVEL(bp) == 0 && BP_GET_TYPE(bp) != DMU_OT_DNODE))
+       if (dnp->dn_nblkptr == 0 && !(dnp->dn_flags & DNODE_FLAG_SPILL_BLKPTR))
                return;
 
-       SET_BOOKMARK(&czb, objset, object, BP_GET_LEVEL(bp), blkid);
+       SET_BOOKMARK(&zb, objset, object, 0, 0);
+
+       spc = scan_prefetch_ctx_create(scn, dnp, FTAG);
+
+       for (i = 0; i < dnp->dn_nblkptr; i++) {
+               zb.zb_level = BP_GET_LEVEL(&dnp->dn_blkptr[i]);
+               zb.zb_blkid = i;
+               dsl_scan_prefetch(spc, &dnp->dn_blkptr[i], &zb);
+       }
+
+       if (dnp->dn_flags & DNODE_FLAG_SPILL_BLKPTR) {
+               zb.zb_level = 0;
+               zb.zb_blkid = DMU_SPILL_BLKID;
+               dsl_scan_prefetch(spc, DN_SPILL_BLKPTR(dnp), &zb);
+       }
+
+       scan_prefetch_ctx_rele(spc, FTAG);
+}
+
+static void
+dsl_scan_prefetch_cb(zio_t *zio, const zbookmark_phys_t *zb, const blkptr_t *bp,
+    arc_buf_t *buf, void *private)
+{
+       (void) zio;
+       scan_prefetch_ctx_t *spc = private;
+       dsl_scan_t *scn = spc->spc_scn;
+       spa_t *spa = scn->scn_dp->dp_spa;
+
+       /* broadcast that the IO has completed for rate limiting purposes */
+       mutex_enter(&spa->spa_scrub_lock);
+       ASSERT3U(spa->spa_scrub_inflight, >=, BP_GET_PSIZE(bp));
+       spa->spa_scrub_inflight -= BP_GET_PSIZE(bp);
+       cv_broadcast(&spa->spa_scrub_io_cv);
+       mutex_exit(&spa->spa_scrub_lock);
+
+       /* if there was an error or we are done prefetching, just cleanup */
+       if (buf == NULL || scn->scn_prefetch_stop)
+               goto out;
+
+       if (BP_GET_LEVEL(bp) > 0) {
+               int i;
+               blkptr_t *cbp;
+               int epb = BP_GET_LSIZE(bp) >> SPA_BLKPTRSHIFT;
+               zbookmark_phys_t czb;
+
+               for (i = 0, cbp = buf->b_data; i < epb; i++, cbp++) {
+                       SET_BOOKMARK(&czb, zb->zb_objset, zb->zb_object,
+                           zb->zb_level - 1, zb->zb_blkid * epb + i);
+                       dsl_scan_prefetch(spc, cbp, &czb);
+               }
+       } else if (BP_GET_TYPE(bp) == DMU_OT_DNODE) {
+               dnode_phys_t *cdnp;
+               int i;
+               int epb = BP_GET_LSIZE(bp) >> DNODE_SHIFT;
+
+               for (i = 0, cdnp = buf->b_data; i < epb;
+                   i += cdnp->dn_extra_slots + 1,
+                   cdnp += cdnp->dn_extra_slots + 1) {
+                       dsl_scan_prefetch_dnode(scn, cdnp,
+                           zb->zb_objset, zb->zb_blkid * epb + i);
+               }
+       } else if (BP_GET_TYPE(bp) == DMU_OT_OBJSET) {
+               objset_phys_t *osp = buf->b_data;
+
+               dsl_scan_prefetch_dnode(scn, &osp->os_meta_dnode,
+                   zb->zb_objset, DMU_META_DNODE_OBJECT);
+
+               if (OBJSET_BUF_HAS_USERUSED(buf)) {
+                       dsl_scan_prefetch_dnode(scn,
+                           &osp->os_groupused_dnode, zb->zb_objset,
+                           DMU_GROUPUSED_OBJECT);
+                       dsl_scan_prefetch_dnode(scn,
+                           &osp->os_userused_dnode, zb->zb_objset,
+                           DMU_USERUSED_OBJECT);
+               }
+       }
+
+out:
+       if (buf != NULL)
+               arc_buf_destroy(buf, private);
+       scan_prefetch_ctx_rele(spc, scn);
+}
+
+static void
+dsl_scan_prefetch_thread(void *arg)
+{
+       dsl_scan_t *scn = arg;
+       spa_t *spa = scn->scn_dp->dp_spa;
+       scan_prefetch_issue_ctx_t *spic;
+
+       /* loop until we are told to stop */
+       while (!scn->scn_prefetch_stop) {
+               arc_flags_t flags = ARC_FLAG_NOWAIT |
+                   ARC_FLAG_PRESCIENT_PREFETCH | ARC_FLAG_PREFETCH;
+               int zio_flags = ZIO_FLAG_CANFAIL | ZIO_FLAG_SCAN_THREAD;
+
+               mutex_enter(&spa->spa_scrub_lock);
+
+               /*
+                * Wait until we have an IO to issue and are not above our
+                * maximum in flight limit.
+                */
+               while (!scn->scn_prefetch_stop &&
+                   (avl_numnodes(&scn->scn_prefetch_queue) == 0 ||
+                   spa->spa_scrub_inflight >= scn->scn_maxinflight_bytes)) {
+                       cv_wait(&spa->spa_scrub_io_cv, &spa->spa_scrub_lock);
+               }
+
+               /* recheck if we should stop since we waited for the cv */
+               if (scn->scn_prefetch_stop) {
+                       mutex_exit(&spa->spa_scrub_lock);
+                       break;
+               }
+
+               /* remove the prefetch IO from the tree */
+               spic = avl_first(&scn->scn_prefetch_queue);
+               spa->spa_scrub_inflight += BP_GET_PSIZE(&spic->spic_bp);
+               avl_remove(&scn->scn_prefetch_queue, spic);
+
+               mutex_exit(&spa->spa_scrub_lock);
+
+               if (BP_IS_PROTECTED(&spic->spic_bp)) {
+                       ASSERT(BP_GET_TYPE(&spic->spic_bp) == DMU_OT_DNODE ||
+                           BP_GET_TYPE(&spic->spic_bp) == DMU_OT_OBJSET);
+                       ASSERT3U(BP_GET_LEVEL(&spic->spic_bp), ==, 0);
+                       zio_flags |= ZIO_FLAG_RAW;
+               }
+
+               /* issue the prefetch asynchronously */
+               (void) arc_read(scn->scn_zio_root, scn->scn_dp->dp_spa,
+                   &spic->spic_bp, dsl_scan_prefetch_cb, spic->spic_spc,
+                   ZIO_PRIORITY_SCRUB, zio_flags, &flags, &spic->spic_zb);
 
-       (void) arc_read(scn->scn_zio_root, scn->scn_dp->dp_spa, bp,
-           NULL, NULL, ZIO_PRIORITY_ASYNC_READ,
-           ZIO_FLAG_CANFAIL | ZIO_FLAG_SCAN_THREAD, &flags, &czb);
+               kmem_free(spic, sizeof (scan_prefetch_issue_ctx_t));
+       }
+
+       ASSERT(scn->scn_prefetch_stop);
+
+       /* free any prefetches we didn't get to complete */
+       mutex_enter(&spa->spa_scrub_lock);
+       while ((spic = avl_first(&scn->scn_prefetch_queue)) != NULL) {
+               avl_remove(&scn->scn_prefetch_queue, spic);
+               scan_prefetch_ctx_rele(spic->spic_spc, scn);
+               kmem_free(spic, sizeof (scan_prefetch_issue_ctx_t));
+       }
+       ASSERT0(avl_numnodes(&scn->scn_prefetch_queue));
+       mutex_exit(&spa->spa_scrub_lock);
 }
 
 static boolean_t
@@ -637,23 +1833,29 @@ dsl_scan_check_resume(dsl_scan_t *scn, const dnode_phys_t *dnp,
 
                /*
                 * If we found the block we're trying to resume from, or
-                * we went past it to a different object, zero it out to
-                * indicate that it's OK to start checking for pausing
-                * again.
+                * we went past it, zero it out to indicate that it's OK
+                * to start checking for suspending again.
                 */
-               if (bcmp(zb, &scn->scn_phys.scn_bookmark, sizeof (*zb)) == 0 ||
-                   zb->zb_object > scn->scn_phys.scn_bookmark.zb_object) {
+               if (zbookmark_subtree_tbd(dnp, zb,
+                   &scn->scn_phys.scn_bookmark)) {
                        dprintf("resuming at %llx/%llx/%llx/%llx\n",
                            (longlong_t)zb->zb_objset,
                            (longlong_t)zb->zb_object,
                            (longlong_t)zb->zb_level,
                            (longlong_t)zb->zb_blkid);
-                       bzero(&scn->scn_phys.scn_bookmark, sizeof (*zb));
+                       memset(&scn->scn_phys.scn_bookmark, 0, sizeof (*zb));
                }
        }
        return (B_FALSE);
 }
 
+static void dsl_scan_visitbp(blkptr_t *bp, const zbookmark_phys_t *zb,
+    dnode_phys_t *dnp, dsl_dataset_t *ds, dsl_scan_t *scn,
+    dmu_objset_type_t ostype, dmu_tx_t *tx);
+inline __attribute__((always_inline)) static void dsl_scan_visitdnode(
+    dsl_scan_t *, dsl_dataset_t *ds, dmu_objset_type_t ostype,
+    dnode_phys_t *dnp, uint64_t object, dmu_tx_t *tx);
+
 /*
  * Return nonzero on i/o error.
  * Return new buf to write out in *bufp.
@@ -664,9 +1866,25 @@ dsl_scan_recurse(dsl_scan_t *scn, dsl_dataset_t *ds, dmu_objset_type_t ostype,
     const zbookmark_phys_t *zb, dmu_tx_t *tx)
 {
        dsl_pool_t *dp = scn->scn_dp;
+       spa_t *spa = dp->dp_spa;
        int zio_flags = ZIO_FLAG_CANFAIL | ZIO_FLAG_SCAN_THREAD;
        int err;
 
+       ASSERT(!BP_IS_REDACTED(bp));
+
+       /*
+        * There is an unlikely case of encountering dnodes with contradicting
+        * dn_bonuslen and DNODE_FLAG_SPILL_BLKPTR flag before in files created
+        * or modified before commit 4254acb was merged. As it is not possible
+        * to know which of the two is correct, report an error.
+        */
+       if (dnp != NULL &&
+           dnp->dn_bonuslen > DN_MAX_BONUS_LEN(dnp)) {
+               scn->scn_phys.scn_errors++;
+               spa_log_error(spa, zb, &bp->blk_birth);
+               return (SET_ERROR(EINVAL));
+       }
+
        if (BP_GET_LEVEL(bp) > 0) {
                arc_flags_t flags = ARC_FLAG_WAIT;
                int i;
@@ -674,16 +1892,12 @@ dsl_scan_recurse(dsl_scan_t *scn, dsl_dataset_t *ds, dmu_objset_type_t ostype,
                int epb = BP_GET_LSIZE(bp) >> SPA_BLKPTRSHIFT;
                arc_buf_t *buf;
 
-               err = arc_read(NULL, dp->dp_spa, bp, arc_getbuf_func, &buf,
-                   ZIO_PRIORITY_ASYNC_READ, zio_flags, &flags, zb);
+               err = arc_read(NULL, spa, bp, arc_getbuf_func, &buf,
+                   ZIO_PRIORITY_SCRUB, zio_flags, &flags, zb);
                if (err) {
                        scn->scn_phys.scn_errors++;
                        return (err);
                }
-               for (i = 0, cbp = buf->b_data; i < epb; i++, cbp++) {
-                       dsl_scan_prefetch(scn, buf, cbp, zb->zb_objset,
-                           zb->zb_object, zb->zb_blkid * epb + i);
-               }
                for (i = 0, cbp = buf->b_data; i < epb; i++, cbp++) {
                        zbookmark_phys_t czb;
 
@@ -697,25 +1911,21 @@ dsl_scan_recurse(dsl_scan_t *scn, dsl_dataset_t *ds, dmu_objset_type_t ostype,
        } else if (BP_GET_TYPE(bp) == DMU_OT_DNODE) {
                arc_flags_t flags = ARC_FLAG_WAIT;
                dnode_phys_t *cdnp;
-               int i, j;
+               int i;
                int epb = BP_GET_LSIZE(bp) >> DNODE_SHIFT;
                arc_buf_t *buf;
 
-               err = arc_read(NULL, dp->dp_spa, bp, arc_getbuf_func, &buf,
-                   ZIO_PRIORITY_ASYNC_READ, zio_flags, &flags, zb);
+               if (BP_IS_PROTECTED(bp)) {
+                       ASSERT3U(BP_GET_COMPRESS(bp), ==, ZIO_COMPRESS_OFF);
+                       zio_flags |= ZIO_FLAG_RAW;
+               }
+
+               err = arc_read(NULL, spa, bp, arc_getbuf_func, &buf,
+                   ZIO_PRIORITY_SCRUB, zio_flags, &flags, zb);
                if (err) {
                        scn->scn_phys.scn_errors++;
                        return (err);
                }
-               for (i = 0, cdnp = buf->b_data; i < epb;
-                   i += cdnp->dn_extra_slots + 1,
-                   cdnp += cdnp->dn_extra_slots + 1) {
-                       for (j = 0; j < cdnp->dn_nblkptr; j++) {
-                               blkptr_t *cbp = &cdnp->dn_blkptr[j];
-                               dsl_scan_prefetch(scn, buf, cbp,
-                                   zb->zb_objset, zb->zb_blkid * epb + i, j);
-                       }
-               }
                for (i = 0, cdnp = buf->b_data; i < epb;
                    i += cdnp->dn_extra_slots + 1,
                    cdnp += cdnp->dn_extra_slots + 1) {
@@ -729,8 +1939,8 @@ dsl_scan_recurse(dsl_scan_t *scn, dsl_dataset_t *ds, dmu_objset_type_t ostype,
                objset_phys_t *osp;
                arc_buf_t *buf;
 
-               err = arc_read(NULL, dp->dp_spa, bp, arc_getbuf_func, &buf,
-                   ZIO_PRIORITY_ASYNC_READ, zio_flags, &flags, zb);
+               err = arc_read(NULL, spa, bp, arc_getbuf_func, &buf,
+                   ZIO_PRIORITY_SCRUB, zio_flags, &flags, zb);
                if (err) {
                        scn->scn_phys.scn_errors++;
                        return (err);
@@ -743,11 +1953,15 @@ dsl_scan_recurse(dsl_scan_t *scn, dsl_dataset_t *ds, dmu_objset_type_t ostype,
 
                if (OBJSET_BUF_HAS_USERUSED(buf)) {
                        /*
-                        * We also always visit user/group accounting
+                        * We also always visit user/group/project accounting
                         * objects, and never skip them, even if we are
-                        * pausing.  This is necessary so that the space
-                        * deltas from this txg get integrated.
+                        * suspending. This is necessary so that the
+                        * space deltas from this txg get integrated.
                         */
+                       if (OBJSET_BUF_HAS_PROJECTUSED(buf))
+                               dsl_scan_visitdnode(scn, ds, osp->os_type,
+                                   &osp->os_projectused_dnode,
+                                   DMU_PROJECTUSED_OBJECT, tx);
                        dsl_scan_visitdnode(scn, ds, osp->os_type,
                            &osp->os_groupused_dnode,
                            DMU_GROUPUSED_OBJECT, tx);
@@ -756,6 +1970,15 @@ dsl_scan_recurse(dsl_scan_t *scn, dsl_dataset_t *ds, dmu_objset_type_t ostype,
                            DMU_USERUSED_OBJECT, tx);
                }
                arc_buf_destroy(buf, &buf);
+       } else if (!zfs_blkptr_verify(spa, bp,
+           BLK_CONFIG_NEEDED, BLK_VERIFY_LOG)) {
+               /*
+                * Sanity check the block pointer contents, this is handled
+                * by arc_read() for the cases above.
+                */
+               scn->scn_phys.scn_errors++;
+               spa_log_error(spa, zb, &bp->blk_birth);
+               return (SET_ERROR(EINVAL));
        }
 
        return (0);
@@ -796,39 +2019,49 @@ dsl_scan_visitbp(blkptr_t *bp, const zbookmark_phys_t *zb,
     dmu_objset_type_t ostype, dmu_tx_t *tx)
 {
        dsl_pool_t *dp = scn->scn_dp;
-       blkptr_t *bp_toread;
-
-       bp_toread = kmem_alloc(sizeof (blkptr_t), KM_SLEEP);
-       *bp_toread = *bp;
+       blkptr_t *bp_toread = NULL;
 
-       /* ASSERT(pbuf == NULL || arc_released(pbuf)); */
-
-       if (dsl_scan_check_pause(scn, zb))
-               goto out;
+       if (dsl_scan_check_suspend(scn, zb))
+               return;
 
        if (dsl_scan_check_resume(scn, dnp, zb))
-               goto out;
-
-       if (BP_IS_HOLE(bp))
-               goto out;
+               return;
 
        scn->scn_visited_this_txg++;
 
+       if (BP_IS_HOLE(bp)) {
+               scn->scn_holes_this_txg++;
+               return;
+       }
+
+       if (BP_IS_REDACTED(bp)) {
+               ASSERT(dsl_dataset_feature_is_active(ds,
+                   SPA_FEATURE_REDACTED_DATASETS));
+               return;
+       }
+
        /*
-        * This debugging is commented out to conserve stack space.  This
-        * function is called recursively and the debugging addes several
-        * bytes to the stack for each call.  It can be commented back in
-        * if required to debug an issue in dsl_scan_visitbp().
-        *
-        * dprintf_bp(bp,
-        *    "visiting ds=%p/%llu zb=%llx/%llx/%llx/%llx bp=%p",
-        *    ds, ds ? ds->ds_object : 0,
-        *    zb->zb_objset, zb->zb_object, zb->zb_level, zb->zb_blkid,
-        *    bp);
+        * Check if this block contradicts any filesystem flags.
         */
+       spa_feature_t f = SPA_FEATURE_LARGE_BLOCKS;
+       if (BP_GET_LSIZE(bp) > SPA_OLD_MAXBLOCKSIZE)
+               ASSERT(dsl_dataset_feature_is_active(ds, f));
 
-       if (bp->blk_birth <= scn->scn_phys.scn_cur_min_txg)
-               goto out;
+       f = zio_checksum_to_feature(BP_GET_CHECKSUM(bp));
+       if (f != SPA_FEATURE_NONE)
+               ASSERT(dsl_dataset_feature_is_active(ds, f));
+
+       f = zio_compress_to_feature(BP_GET_COMPRESS(bp));
+       if (f != SPA_FEATURE_NONE)
+               ASSERT(dsl_dataset_feature_is_active(ds, f));
+
+       if (bp->blk_birth <= scn->scn_phys.scn_cur_min_txg) {
+               scn->scn_lt_min_this_txg++;
+               return;
+       }
+
+       bp_toread = kmem_alloc(sizeof (blkptr_t), KM_SLEEP);
+       *bp_toread = *bp;
 
        if (dsl_scan_recurse(scn, ds, ostype, dnp, bp_toread, zb, tx) != 0)
                goto out;
@@ -840,6 +2073,7 @@ dsl_scan_visitbp(blkptr_t *bp, const zbookmark_phys_t *zb,
         */
        if (ddt_class_contains(dp->dp_spa,
            scn->scn_phys.scn_ddt_class_max, bp)) {
+               scn->scn_ddt_contained_this_txg++;
                goto out;
        }
 
@@ -850,9 +2084,13 @@ dsl_scan_visitbp(blkptr_t *bp, const zbookmark_phys_t *zb,
         * Don't scan it now unless we need to because something
         * under it was modified.
         */
-       if (BP_PHYSICAL_BIRTH(bp) <= scn->scn_phys.scn_cur_max_txg) {
-               scan_funcs[scn->scn_phys.scn_func](dp, bp, zb);
+       if (BP_PHYSICAL_BIRTH(bp) > scn->scn_phys.scn_cur_max_txg) {
+               scn->scn_gt_max_this_txg++;
+               goto out;
        }
+
+       scan_funcs[scn->scn_phys.scn_func](dp, bp, zb);
+
 out:
        kmem_free(bp_toread, sizeof (blkptr_t));
 }
@@ -862,26 +2100,33 @@ dsl_scan_visit_rootbp(dsl_scan_t *scn, dsl_dataset_t *ds, blkptr_t *bp,
     dmu_tx_t *tx)
 {
        zbookmark_phys_t zb;
+       scan_prefetch_ctx_t *spc;
 
        SET_BOOKMARK(&zb, ds ? ds->ds_object : DMU_META_OBJSET,
            ZB_ROOT_OBJECT, ZB_ROOT_LEVEL, ZB_ROOT_BLKID);
-       dsl_scan_visitbp(bp, &zb, NULL,
-           ds, scn, DMU_OST_NONE, tx);
+
+       if (ZB_IS_ZERO(&scn->scn_phys.scn_bookmark)) {
+               SET_BOOKMARK(&scn->scn_prefetch_bookmark,
+                   zb.zb_objset, 0, 0, 0);
+       } else {
+               scn->scn_prefetch_bookmark = scn->scn_phys.scn_bookmark;
+       }
+
+       scn->scn_objsets_visited_this_txg++;
+
+       spc = scan_prefetch_ctx_create(scn, NULL, FTAG);
+       dsl_scan_prefetch(spc, bp, &zb);
+       scan_prefetch_ctx_rele(spc, FTAG);
+
+       dsl_scan_visitbp(bp, &zb, NULL, ds, scn, DMU_OST_NONE, tx);
 
        dprintf_ds(ds, "finished scan%s", "");
 }
 
-void
-dsl_scan_ds_destroyed(dsl_dataset_t *ds, dmu_tx_t *tx)
+static void
+ds_destroyed_scn_phys(dsl_dataset_t *ds, dsl_scan_phys_t *scn_phys)
 {
-       dsl_pool_t *dp = ds->ds_dir->dd_pool;
-       dsl_scan_t *scn = dp->dp_scan;
-       uint64_t mintxg;
-
-       if (scn->scn_phys.scn_state != DSS_SCANNING)
-               return;
-
-       if (scn->scn_phys.scn_bookmark.zb_objset == ds->ds_object) {
+       if (scn_phys->scn_bookmark.zb_objset == ds->ds_object) {
                if (ds->ds_is_snapshot) {
                        /*
                         * Note:
@@ -893,23 +2138,59 @@ dsl_scan_ds_destroyed(dsl_dataset_t *ds, dmu_tx_t *tx)
                         *    ignore it when we retraverse it in
                         *    dsl_scan_visitds().
                         */
-                       scn->scn_phys.scn_bookmark.zb_objset =
+                       scn_phys->scn_bookmark.zb_objset =
                            dsl_dataset_phys(ds)->ds_next_snap_obj;
-                       zfs_dbgmsg("destroying ds %llu; currently traversing; "
-                           "reset zb_objset to %llu",
+                       zfs_dbgmsg("destroying ds %llu on %s; currently "
+                           "traversing; reset zb_objset to %llu",
                            (u_longlong_t)ds->ds_object,
+                           ds->ds_dir->dd_pool->dp_spa->spa_name,
                            (u_longlong_t)dsl_dataset_phys(ds)->
                            ds_next_snap_obj);
-                       scn->scn_phys.scn_flags |= DSF_VISIT_DS_AGAIN;
+                       scn_phys->scn_flags |= DSF_VISIT_DS_AGAIN;
                } else {
-                       SET_BOOKMARK(&scn->scn_phys.scn_bookmark,
+                       SET_BOOKMARK(&scn_phys->scn_bookmark,
                            ZB_DESTROYED_OBJSET, 0, 0, 0);
-                       zfs_dbgmsg("destroying ds %llu; currently traversing; "
-                           "reset bookmark to -1,0,0,0",
-                           (u_longlong_t)ds->ds_object);
+                       zfs_dbgmsg("destroying ds %llu on %s; currently "
+                           "traversing; reset bookmark to -1,0,0,0",
+                           (u_longlong_t)ds->ds_object,
+                           ds->ds_dir->dd_pool->dp_spa->spa_name);
                }
-       } else if (zap_lookup_int_key(dp->dp_meta_objset,
-           scn->scn_phys.scn_queue_obj, ds->ds_object, &mintxg) == 0) {
+       }
+}
+
+/*
+ * Invoked when a dataset is destroyed. We need to make sure that:
+ *
+ * 1) If it is the dataset that was currently being scanned, we write
+ *     a new dsl_scan_phys_t and marking the objset reference in it
+ *     as destroyed.
+ * 2) Remove it from the work queue, if it was present.
+ *
+ * If the dataset was actually a snapshot, instead of marking the dataset
+ * as destroyed, we instead substitute the next snapshot in line.
+ */
+void
+dsl_scan_ds_destroyed(dsl_dataset_t *ds, dmu_tx_t *tx)
+{
+       dsl_pool_t *dp = ds->ds_dir->dd_pool;
+       dsl_scan_t *scn = dp->dp_scan;
+       uint64_t mintxg;
+
+       if (!dsl_scan_is_running(scn))
+               return;
+
+       ds_destroyed_scn_phys(ds, &scn->scn_phys);
+       ds_destroyed_scn_phys(ds, &scn->scn_phys_cached);
+
+       if (scan_ds_queue_contains(scn, ds->ds_object, &mintxg)) {
+               scan_ds_queue_remove(scn, ds->ds_object);
+               if (ds->ds_is_snapshot)
+                       scan_ds_queue_insert(scn,
+                           dsl_dataset_phys(ds)->ds_next_snap_obj, mintxg);
+       }
+
+       if (zap_lookup_int_key(dp->dp_meta_objset, scn->scn_phys.scn_queue_obj,
+           ds->ds_object, &mintxg) == 0) {
                ASSERT3U(dsl_dataset_phys(ds)->ds_num_children, <=, 1);
                VERIFY3U(0, ==, zap_remove_int(dp->dp_meta_objset,
                    scn->scn_phys.scn_queue_obj, ds->ds_object, tx));
@@ -923,14 +2204,17 @@ dsl_scan_ds_destroyed(dsl_dataset_t *ds, dmu_tx_t *tx)
                            scn->scn_phys.scn_queue_obj,
                            dsl_dataset_phys(ds)->ds_next_snap_obj,
                            mintxg, tx) == 0);
-                       zfs_dbgmsg("destroying ds %llu; in queue; "
+                       zfs_dbgmsg("destroying ds %llu on %s; in queue; "
                            "replacing with %llu",
                            (u_longlong_t)ds->ds_object,
+                           dp->dp_spa->spa_name,
                            (u_longlong_t)dsl_dataset_phys(ds)->
                            ds_next_snap_obj);
                } else {
-                       zfs_dbgmsg("destroying ds %llu; in queue; removing",
-                           (u_longlong_t)ds->ds_object);
+                       zfs_dbgmsg("destroying ds %llu on %s; in queue; "
+                           "removing",
+                           (u_longlong_t)ds->ds_object,
+                           dp->dp_spa->spa_name);
                }
        }
 
@@ -938,9 +2222,29 @@ dsl_scan_ds_destroyed(dsl_dataset_t *ds, dmu_tx_t *tx)
         * dsl_scan_sync() should be called after this, and should sync
         * out our changed state, but just to be safe, do it here.
         */
-       dsl_scan_sync_state(scn, tx);
+       dsl_scan_sync_state(scn, tx, SYNC_CACHED);
+}
+
+static void
+ds_snapshotted_bookmark(dsl_dataset_t *ds, zbookmark_phys_t *scn_bookmark)
+{
+       if (scn_bookmark->zb_objset == ds->ds_object) {
+               scn_bookmark->zb_objset =
+                   dsl_dataset_phys(ds)->ds_prev_snap_obj;
+               zfs_dbgmsg("snapshotting ds %llu on %s; currently traversing; "
+                   "reset zb_objset to %llu",
+                   (u_longlong_t)ds->ds_object,
+                   ds->ds_dir->dd_pool->dp_spa->spa_name,
+                   (u_longlong_t)dsl_dataset_phys(ds)->ds_prev_snap_obj);
+       }
 }
 
+/*
+ * Called when a dataset is snapshotted. If we were currently traversing
+ * this snapshot, we reset our bookmark to point at the newly created
+ * snapshot. We also modify our work queue to remove the old snapshot and
+ * replace with the new one.
+ */
 void
 dsl_scan_ds_snapshotted(dsl_dataset_t *ds, dmu_tx_t *tx)
 {
@@ -948,117 +2252,179 @@ dsl_scan_ds_snapshotted(dsl_dataset_t *ds, dmu_tx_t *tx)
        dsl_scan_t *scn = dp->dp_scan;
        uint64_t mintxg;
 
-       if (scn->scn_phys.scn_state != DSS_SCANNING)
+       if (!dsl_scan_is_running(scn))
                return;
 
        ASSERT(dsl_dataset_phys(ds)->ds_prev_snap_obj != 0);
 
-       if (scn->scn_phys.scn_bookmark.zb_objset == ds->ds_object) {
-               scn->scn_phys.scn_bookmark.zb_objset =
-                   dsl_dataset_phys(ds)->ds_prev_snap_obj;
-               zfs_dbgmsg("snapshotting ds %llu; currently traversing; "
-                   "reset zb_objset to %llu",
-                   (u_longlong_t)ds->ds_object,
-                   (u_longlong_t)dsl_dataset_phys(ds)->ds_prev_snap_obj);
-       } else if (zap_lookup_int_key(dp->dp_meta_objset,
-           scn->scn_phys.scn_queue_obj, ds->ds_object, &mintxg) == 0) {
+       ds_snapshotted_bookmark(ds, &scn->scn_phys.scn_bookmark);
+       ds_snapshotted_bookmark(ds, &scn->scn_phys_cached.scn_bookmark);
+
+       if (scan_ds_queue_contains(scn, ds->ds_object, &mintxg)) {
+               scan_ds_queue_remove(scn, ds->ds_object);
+               scan_ds_queue_insert(scn,
+                   dsl_dataset_phys(ds)->ds_prev_snap_obj, mintxg);
+       }
+
+       if (zap_lookup_int_key(dp->dp_meta_objset, scn->scn_phys.scn_queue_obj,
+           ds->ds_object, &mintxg) == 0) {
                VERIFY3U(0, ==, zap_remove_int(dp->dp_meta_objset,
                    scn->scn_phys.scn_queue_obj, ds->ds_object, tx));
                VERIFY(zap_add_int_key(dp->dp_meta_objset,
                    scn->scn_phys.scn_queue_obj,
                    dsl_dataset_phys(ds)->ds_prev_snap_obj, mintxg, tx) == 0);
-               zfs_dbgmsg("snapshotting ds %llu; in queue; "
+               zfs_dbgmsg("snapshotting ds %llu on %s; in queue; "
                    "replacing with %llu",
                    (u_longlong_t)ds->ds_object,
+                   dp->dp_spa->spa_name,
                    (u_longlong_t)dsl_dataset_phys(ds)->ds_prev_snap_obj);
        }
-       dsl_scan_sync_state(scn, tx);
+
+       dsl_scan_sync_state(scn, tx, SYNC_CACHED);
+}
+
+static void
+ds_clone_swapped_bookmark(dsl_dataset_t *ds1, dsl_dataset_t *ds2,
+    zbookmark_phys_t *scn_bookmark)
+{
+       if (scn_bookmark->zb_objset == ds1->ds_object) {
+               scn_bookmark->zb_objset = ds2->ds_object;
+               zfs_dbgmsg("clone_swap ds %llu on %s; currently traversing; "
+                   "reset zb_objset to %llu",
+                   (u_longlong_t)ds1->ds_object,
+                   ds1->ds_dir->dd_pool->dp_spa->spa_name,
+                   (u_longlong_t)ds2->ds_object);
+       } else if (scn_bookmark->zb_objset == ds2->ds_object) {
+               scn_bookmark->zb_objset = ds1->ds_object;
+               zfs_dbgmsg("clone_swap ds %llu on %s; currently traversing; "
+                   "reset zb_objset to %llu",
+                   (u_longlong_t)ds2->ds_object,
+                   ds2->ds_dir->dd_pool->dp_spa->spa_name,
+                   (u_longlong_t)ds1->ds_object);
+       }
 }
 
+/*
+ * Called when an origin dataset and its clone are swapped.  If we were
+ * currently traversing the dataset, we need to switch to traversing the
+ * newly promoted clone.
+ */
 void
 dsl_scan_ds_clone_swapped(dsl_dataset_t *ds1, dsl_dataset_t *ds2, dmu_tx_t *tx)
 {
        dsl_pool_t *dp = ds1->ds_dir->dd_pool;
        dsl_scan_t *scn = dp->dp_scan;
-       uint64_t mintxg;
+       uint64_t mintxg1, mintxg2;
+       boolean_t ds1_queued, ds2_queued;
 
-       if (scn->scn_phys.scn_state != DSS_SCANNING)
+       if (!dsl_scan_is_running(scn))
                return;
 
-       if (scn->scn_phys.scn_bookmark.zb_objset == ds1->ds_object) {
-               scn->scn_phys.scn_bookmark.zb_objset = ds2->ds_object;
-               zfs_dbgmsg("clone_swap ds %llu; currently traversing; "
-                   "reset zb_objset to %llu",
-                   (u_longlong_t)ds1->ds_object,
-                   (u_longlong_t)ds2->ds_object);
-       } else if (scn->scn_phys.scn_bookmark.zb_objset == ds2->ds_object) {
-               scn->scn_phys.scn_bookmark.zb_objset = ds1->ds_object;
-               zfs_dbgmsg("clone_swap ds %llu; currently traversing; "
-                   "reset zb_objset to %llu",
-                   (u_longlong_t)ds2->ds_object,
-                   (u_longlong_t)ds1->ds_object);
+       ds_clone_swapped_bookmark(ds1, ds2, &scn->scn_phys.scn_bookmark);
+       ds_clone_swapped_bookmark(ds1, ds2, &scn->scn_phys_cached.scn_bookmark);
+
+       /*
+        * Handle the in-memory scan queue.
+        */
+       ds1_queued = scan_ds_queue_contains(scn, ds1->ds_object, &mintxg1);
+       ds2_queued = scan_ds_queue_contains(scn, ds2->ds_object, &mintxg2);
+
+       /* Sanity checking. */
+       if (ds1_queued) {
+               ASSERT3U(mintxg1, ==, dsl_dataset_phys(ds1)->ds_prev_snap_txg);
+               ASSERT3U(mintxg1, ==, dsl_dataset_phys(ds2)->ds_prev_snap_txg);
+       }
+       if (ds2_queued) {
+               ASSERT3U(mintxg2, ==, dsl_dataset_phys(ds1)->ds_prev_snap_txg);
+               ASSERT3U(mintxg2, ==, dsl_dataset_phys(ds2)->ds_prev_snap_txg);
        }
 
-       if (zap_lookup_int_key(dp->dp_meta_objset, scn->scn_phys.scn_queue_obj,
-           ds1->ds_object, &mintxg) == 0) {
-               int err;
+       if (ds1_queued && ds2_queued) {
+               /*
+                * If both are queued, we don't need to do anything.
+                * The swapping code below would not handle this case correctly,
+                * since we can't insert ds2 if it is already there. That's
+                * because scan_ds_queue_insert() prohibits a duplicate insert
+                * and panics.
+                */
+       } else if (ds1_queued) {
+               scan_ds_queue_remove(scn, ds1->ds_object);
+               scan_ds_queue_insert(scn, ds2->ds_object, mintxg1);
+       } else if (ds2_queued) {
+               scan_ds_queue_remove(scn, ds2->ds_object);
+               scan_ds_queue_insert(scn, ds1->ds_object, mintxg2);
+       }
 
-               ASSERT3U(mintxg, ==, dsl_dataset_phys(ds1)->ds_prev_snap_txg);
-               ASSERT3U(mintxg, ==, dsl_dataset_phys(ds2)->ds_prev_snap_txg);
-               VERIFY3U(0, ==, zap_remove_int(dp->dp_meta_objset,
+       /*
+        * Handle the on-disk scan queue.
+        * The on-disk state is an out-of-date version of the in-memory state,
+        * so the in-memory and on-disk values for ds1_queued and ds2_queued may
+        * be different. Therefore we need to apply the swap logic to the
+        * on-disk state independently of the in-memory state.
+        */
+       ds1_queued = zap_lookup_int_key(dp->dp_meta_objset,
+           scn->scn_phys.scn_queue_obj, ds1->ds_object, &mintxg1) == 0;
+       ds2_queued = zap_lookup_int_key(dp->dp_meta_objset,
+           scn->scn_phys.scn_queue_obj, ds2->ds_object, &mintxg2) == 0;
+
+       /* Sanity checking. */
+       if (ds1_queued) {
+               ASSERT3U(mintxg1, ==, dsl_dataset_phys(ds1)->ds_prev_snap_txg);
+               ASSERT3U(mintxg1, ==, dsl_dataset_phys(ds2)->ds_prev_snap_txg);
+       }
+       if (ds2_queued) {
+               ASSERT3U(mintxg2, ==, dsl_dataset_phys(ds1)->ds_prev_snap_txg);
+               ASSERT3U(mintxg2, ==, dsl_dataset_phys(ds2)->ds_prev_snap_txg);
+       }
+
+       if (ds1_queued && ds2_queued) {
+               /*
+                * If both are queued, we don't need to do anything.
+                * Alternatively, we could check for EEXIST from
+                * zap_add_int_key() and back out to the original state, but
+                * that would be more work than checking for this case upfront.
+                */
+       } else if (ds1_queued) {
+               VERIFY3S(0, ==, zap_remove_int(dp->dp_meta_objset,
                    scn->scn_phys.scn_queue_obj, ds1->ds_object, tx));
-               err = zap_add_int_key(dp->dp_meta_objset,
-                   scn->scn_phys.scn_queue_obj, ds2->ds_object, mintxg, tx);
-               VERIFY(err == 0 || err == EEXIST);
-               if (err == EEXIST) {
-                       /* Both were there to begin with */
-                       VERIFY(0 == zap_add_int_key(dp->dp_meta_objset,
-                           scn->scn_phys.scn_queue_obj,
-                           ds1->ds_object, mintxg, tx));
-               }
-               zfs_dbgmsg("clone_swap ds %llu; in queue; "
+               VERIFY3S(0, ==, zap_add_int_key(dp->dp_meta_objset,
+                   scn->scn_phys.scn_queue_obj, ds2->ds_object, mintxg1, tx));
+               zfs_dbgmsg("clone_swap ds %llu on %s; in queue; "
                    "replacing with %llu",
                    (u_longlong_t)ds1->ds_object,
+                   dp->dp_spa->spa_name,
                    (u_longlong_t)ds2->ds_object);
-       } else if (zap_lookup_int_key(dp->dp_meta_objset,
-           scn->scn_phys.scn_queue_obj, ds2->ds_object, &mintxg) == 0) {
-               ASSERT3U(mintxg, ==, dsl_dataset_phys(ds1)->ds_prev_snap_txg);
-               ASSERT3U(mintxg, ==, dsl_dataset_phys(ds2)->ds_prev_snap_txg);
-               VERIFY3U(0, ==, zap_remove_int(dp->dp_meta_objset,
+       } else if (ds2_queued) {
+               VERIFY3S(0, ==, zap_remove_int(dp->dp_meta_objset,
                    scn->scn_phys.scn_queue_obj, ds2->ds_object, tx));
-               VERIFY(0 == zap_add_int_key(dp->dp_meta_objset,
-                   scn->scn_phys.scn_queue_obj, ds1->ds_object, mintxg, tx));
-               zfs_dbgmsg("clone_swap ds %llu; in queue; "
+               VERIFY3S(0, ==, zap_add_int_key(dp->dp_meta_objset,
+                   scn->scn_phys.scn_queue_obj, ds1->ds_object, mintxg2, tx));
+               zfs_dbgmsg("clone_swap ds %llu on %s; in queue; "
                    "replacing with %llu",
                    (u_longlong_t)ds2->ds_object,
+                   dp->dp_spa->spa_name,
                    (u_longlong_t)ds1->ds_object);
        }
 
-       dsl_scan_sync_state(scn, tx);
+       dsl_scan_sync_state(scn, tx, SYNC_CACHED);
 }
 
-struct enqueue_clones_arg {
-       dmu_tx_t *tx;
-       uint64_t originobj;
-};
-
-/* ARGSUSED */
 static int
 enqueue_clones_cb(dsl_pool_t *dp, dsl_dataset_t *hds, void *arg)
 {
-       struct enqueue_clones_arg *eca = arg;
+       uint64_t originobj = *(uint64_t *)arg;
        dsl_dataset_t *ds;
        int err;
        dsl_scan_t *scn = dp->dp_scan;
 
-       if (dsl_dir_phys(hds->ds_dir)->dd_origin_obj != eca->originobj)
+       if (dsl_dir_phys(hds->ds_dir)->dd_origin_obj != originobj)
                return (0);
 
        err = dsl_dataset_hold_obj(dp, hds->ds_object, FTAG, &ds);
        if (err)
                return (err);
 
-       while (dsl_dataset_phys(ds)->ds_prev_snap_obj != eca->originobj) {
+       while (dsl_dataset_phys(ds)->ds_prev_snap_obj != originobj) {
                dsl_dataset_t *prev;
                err = dsl_dataset_hold_obj(dp,
                    dsl_dataset_phys(ds)->ds_prev_snap_obj, FTAG, &prev);
@@ -1068,9 +2434,8 @@ enqueue_clones_cb(dsl_pool_t *dp, dsl_dataset_t *hds, void *arg)
                        return (err);
                ds = prev;
        }
-       VERIFY(zap_add_int_key(dp->dp_meta_objset,
-           scn->scn_phys.scn_queue_obj, ds->ds_object,
-           dsl_dataset_phys(ds)->ds_prev_snap_txg, eca->tx) == 0);
+       scan_ds_queue_insert(scn, ds->ds_object,
+           dsl_dataset_phys(ds)->ds_prev_snap_txg);
        dsl_dataset_rele(ds, FTAG);
        return (0);
 }
@@ -1080,8 +2445,6 @@ dsl_scan_visitds(dsl_scan_t *scn, uint64_t dsobj, dmu_tx_t *tx)
 {
        dsl_pool_t *dp = scn->scn_dp;
        dsl_dataset_t *ds;
-       objset_t *os;
-       char *dsname;
 
        VERIFY3U(0, ==, dsl_dataset_hold_obj(dp, dsobj, FTAG, &ds));
 
@@ -1117,26 +2480,32 @@ dsl_scan_visitds(dsl_scan_t *scn, uint64_t dsobj, dmu_tx_t *tx)
                dsl_dataset_name(ds, dsname);
                zfs_dbgmsg("scanning dataset %llu (%s) is unnecessary because "
                    "cur_min_txg (%llu) >= max_txg (%llu)",
-                   dsobj, dsname,
-                   scn->scn_phys.scn_cur_min_txg,
-                   scn->scn_phys.scn_max_txg);
+                   (longlong_t)dsobj, dsname,
+                   (longlong_t)scn->scn_phys.scn_cur_min_txg,
+                   (longlong_t)scn->scn_phys.scn_max_txg);
                kmem_free(dsname, MAXNAMELEN);
 
                goto out;
        }
 
-       if (dmu_objset_from_ds(ds, &os))
-               goto out;
-
        /*
-        * Only the ZIL in the head (non-snapshot) is valid.  Even though
+        * Only the ZIL in the head (non-snapshot) is valid. Even though
         * snapshots can have ZIL block pointers (which may be the same
-        * BP as in the head), they must be ignored.  So we traverse the
-        * ZIL here, rather than in scan_recurse(), because the regular
-        * snapshot block-sharing rules don't apply to it.
+        * BP as in the head), they must be ignored. In addition, $ORIGIN
+        * doesn't have a objset (i.e. its ds_bp is a hole) so we don't
+        * need to look for a ZIL in it either. So we traverse the ZIL here,
+        * rather than in scan_recurse(), because the regular snapshot
+        * block-sharing rules don't apply to it.
         */
-       if (DSL_SCAN_IS_SCRUB_RESILVER(scn) && !ds->ds_is_snapshot)
+       if (!dsl_dataset_is_snapshot(ds) &&
+           (dp->dp_origin_snap == NULL ||
+           ds->ds_dir != dp->dp_origin_snap->ds_dir)) {
+               objset_t *os;
+               if (dmu_objset_from_ds(ds, &os) != 0) {
+                       goto out;
+               }
                dsl_scan_zil(dp, &os->os_zil_header);
+       }
 
        /*
         * Iterate over the bps in this ds.
@@ -1146,17 +2515,17 @@ dsl_scan_visitds(dsl_scan_t *scn, uint64_t dsobj, dmu_tx_t *tx)
        dsl_scan_visit_rootbp(scn, ds, &dsl_dataset_phys(ds)->ds_bp, tx);
        rrw_exit(&ds->ds_bp_rwlock, FTAG);
 
-       dsname = kmem_alloc(ZFS_MAX_DATASET_NAME_LEN, KM_SLEEP);
+       char *dsname = kmem_alloc(ZFS_MAX_DATASET_NAME_LEN, KM_SLEEP);
        dsl_dataset_name(ds, dsname);
        zfs_dbgmsg("scanned dataset %llu (%s) with min=%llu max=%llu; "
-           "pausing=%u",
+           "suspending=%u",
            (longlong_t)dsobj, dsname,
            (longlong_t)scn->scn_phys.scn_cur_min_txg,
            (longlong_t)scn->scn_phys.scn_cur_max_txg,
-           (int)scn->scn_pausing);
+           (int)scn->scn_suspending);
        kmem_free(dsname, ZFS_MAX_DATASET_NAME_LEN);
 
-       if (scn->scn_pausing)
+       if (scn->scn_suspending)
                goto out;
 
        /*
@@ -1167,22 +2536,21 @@ dsl_scan_visitds(dsl_scan_t *scn, uint64_t dsobj, dmu_tx_t *tx)
         * If we did not completely visit this dataset, do another pass.
         */
        if (scn->scn_phys.scn_flags & DSF_VISIT_DS_AGAIN) {
-               zfs_dbgmsg("incomplete pass; visiting again");
+               zfs_dbgmsg("incomplete pass on %s; visiting again",
+                   dp->dp_spa->spa_name);
                scn->scn_phys.scn_flags &= ~DSF_VISIT_DS_AGAIN;
-               VERIFY(zap_add_int_key(dp->dp_meta_objset,
-                   scn->scn_phys.scn_queue_obj, ds->ds_object,
-                   scn->scn_phys.scn_cur_max_txg, tx) == 0);
+               scan_ds_queue_insert(scn, ds->ds_object,
+                   scn->scn_phys.scn_cur_max_txg);
                goto out;
        }
 
        /*
-        * Add descendent datasets to work queue.
+        * Add descendant datasets to work queue.
         */
        if (dsl_dataset_phys(ds)->ds_next_snap_obj != 0) {
-               VERIFY(zap_add_int_key(dp->dp_meta_objset,
-                   scn->scn_phys.scn_queue_obj,
+               scan_ds_queue_insert(scn,
                    dsl_dataset_phys(ds)->ds_next_snap_obj,
-                   dsl_dataset_phys(ds)->ds_creation_txg, tx) == 0);
+                   dsl_dataset_phys(ds)->ds_creation_txg);
        }
        if (dsl_dataset_phys(ds)->ds_num_children > 1) {
                boolean_t usenext = B_FALSE;
@@ -1203,17 +2571,21 @@ dsl_scan_visitds(dsl_scan_t *scn, uint64_t dsobj, dmu_tx_t *tx)
                }
 
                if (usenext) {
-                       VERIFY0(zap_join_key(dp->dp_meta_objset,
-                           dsl_dataset_phys(ds)->ds_next_clones_obj,
-                           scn->scn_phys.scn_queue_obj,
-                           dsl_dataset_phys(ds)->ds_creation_txg, tx));
+                       zap_cursor_t zc;
+                       zap_attribute_t za;
+                       for (zap_cursor_init(&zc, dp->dp_meta_objset,
+                           dsl_dataset_phys(ds)->ds_next_clones_obj);
+                           zap_cursor_retrieve(&zc, &za) == 0;
+                           (void) zap_cursor_advance(&zc)) {
+                               scan_ds_queue_insert(scn,
+                                   zfs_strtonum(za.za_name, NULL),
+                                   dsl_dataset_phys(ds)->ds_creation_txg);
+                       }
+                       zap_cursor_fini(&zc);
                } else {
-                       struct enqueue_clones_arg eca;
-                       eca.tx = tx;
-                       eca.originobj = ds->ds_object;
-
                        VERIFY0(dmu_objset_find_dp(dp, dp->dp_root_dir_obj,
-                           enqueue_clones_cb, &eca, DS_FIND_CHILDREN));
+                           enqueue_clones_cb, &ds->ds_object,
+                           DS_FIND_CHILDREN));
                }
        }
 
@@ -1221,11 +2593,10 @@ out:
        dsl_dataset_rele(ds, FTAG);
 }
 
-/* ARGSUSED */
 static int
 enqueue_cb(dsl_pool_t *dp, dsl_dataset_t *hds, void *arg)
 {
-       dmu_tx_t *tx = arg;
+       (void) arg;
        dsl_dataset_t *ds;
        int err;
        dsl_scan_t *scn = dp->dp_scan;
@@ -1255,12 +2626,50 @@ enqueue_cb(dsl_pool_t *dp, dsl_dataset_t *hds, void *arg)
                ds = prev;
        }
 
-       VERIFY(zap_add_int_key(dp->dp_meta_objset, scn->scn_phys.scn_queue_obj,
-           ds->ds_object, dsl_dataset_phys(ds)->ds_prev_snap_txg, tx) == 0);
+       scan_ds_queue_insert(scn, ds->ds_object,
+           dsl_dataset_phys(ds)->ds_prev_snap_txg);
        dsl_dataset_rele(ds, FTAG);
        return (0);
 }
 
+void
+dsl_scan_ddt_entry(dsl_scan_t *scn, enum zio_checksum checksum,
+    ddt_entry_t *dde, dmu_tx_t *tx)
+{
+       (void) tx;
+       const ddt_key_t *ddk = &dde->dde_key;
+       ddt_phys_t *ddp = dde->dde_phys;
+       blkptr_t bp;
+       zbookmark_phys_t zb = { 0 };
+
+       if (!dsl_scan_is_running(scn))
+               return;
+
+       /*
+        * This function is special because it is the only thing
+        * that can add scan_io_t's to the vdev scan queues from
+        * outside dsl_scan_sync(). For the most part this is ok
+        * as long as it is called from within syncing context.
+        * However, dsl_scan_sync() expects that no new sio's will
+        * be added between when all the work for a scan is done
+        * and the next txg when the scan is actually marked as
+        * completed. This check ensures we do not issue new sio's
+        * during this period.
+        */
+       if (scn->scn_done_txg != 0)
+               return;
+
+       for (int p = 0; p < DDT_PHYS_TYPES; p++, ddp++) {
+               if (ddp->ddp_phys_birth == 0 ||
+                   ddp->ddp_phys_birth > scn->scn_phys.scn_max_txg)
+                       continue;
+               ddt_bp_create(checksum, ddk, ddp, &bp);
+
+               scn->scn_visited_this_txg++;
+               scan_funcs[scn->scn_phys.scn_func](scn->scn_dp, &bp, &zb);
+       }
+}
+
 /*
  * Scrub/dedup interaction.
  *
@@ -1298,12 +2707,10 @@ static void
 dsl_scan_ddt(dsl_scan_t *scn, dmu_tx_t *tx)
 {
        ddt_bookmark_t *ddb = &scn->scn_phys.scn_ddt_bookmark;
-       ddt_entry_t dde;
+       ddt_entry_t dde = {{{{0}}}};
        int error;
        uint64_t n = 0;
 
-       bzero(&dde, sizeof (ddt_entry_t));
-
        while ((error = ddt_walk(scn->scn_dp->dp_spa, ddb, &dde)) == 0) {
                ddt_t *ddt;
 
@@ -1322,57 +2729,40 @@ dsl_scan_ddt(dsl_scan_t *scn, dmu_tx_t *tx)
                dsl_scan_ddt_entry(scn, ddb->ddb_checksum, &dde, tx);
                n++;
 
-               if (dsl_scan_check_pause(scn, NULL))
+               if (dsl_scan_check_suspend(scn, NULL))
                        break;
        }
 
-       zfs_dbgmsg("scanned %llu ddt entries with class_max = %u; pausing=%u",
-           (longlong_t)n, (int)scn->scn_phys.scn_ddt_class_max,
-           (int)scn->scn_pausing);
+       zfs_dbgmsg("scanned %llu ddt entries on %s with class_max = %u; "
+           "suspending=%u", (longlong_t)n, scn->scn_dp->dp_spa->spa_name,
+           (int)scn->scn_phys.scn_ddt_class_max, (int)scn->scn_suspending);
 
        ASSERT(error == 0 || error == ENOENT);
        ASSERT(error != ENOENT ||
            ddb->ddb_class > scn->scn_phys.scn_ddt_class_max);
 }
 
-/* ARGSUSED */
-void
-dsl_scan_ddt_entry(dsl_scan_t *scn, enum zio_checksum checksum,
-    ddt_entry_t *dde, dmu_tx_t *tx)
+static uint64_t
+dsl_scan_ds_maxtxg(dsl_dataset_t *ds)
 {
-       const ddt_key_t *ddk = &dde->dde_key;
-       ddt_phys_t *ddp = dde->dde_phys;
-       blkptr_t bp;
-       zbookmark_phys_t zb = { 0 };
-       int p;
-
-       if (scn->scn_phys.scn_state != DSS_SCANNING)
-               return;
-
-       for (p = 0; p < DDT_PHYS_TYPES; p++, ddp++) {
-               if (ddp->ddp_phys_birth == 0 ||
-                   ddp->ddp_phys_birth > scn->scn_phys.scn_max_txg)
-                       continue;
-               ddt_bp_create(checksum, ddk, ddp, &bp);
-
-               scn->scn_visited_this_txg++;
-               scan_funcs[scn->scn_phys.scn_func](scn->scn_dp, &bp, &zb);
-       }
+       uint64_t smt = ds->ds_dir->dd_pool->dp_scan->scn_phys.scn_max_txg;
+       if (ds->ds_is_snapshot)
+               return (MIN(smt, dsl_dataset_phys(ds)->ds_creation_txg));
+       return (smt);
 }
 
 static void
 dsl_scan_visit(dsl_scan_t *scn, dmu_tx_t *tx)
 {
+       scan_ds_t *sds;
        dsl_pool_t *dp = scn->scn_dp;
-       zap_cursor_t *zc;
-       zap_attribute_t *za;
 
        if (scn->scn_phys.scn_ddt_bookmark.ddb_class <=
            scn->scn_phys.scn_ddt_class_max) {
                scn->scn_phys.scn_cur_min_txg = scn->scn_phys.scn_min_txg;
                scn->scn_phys.scn_cur_max_txg = scn->scn_phys.scn_max_txg;
                dsl_scan_ddt(scn, tx);
-               if (scn->scn_pausing)
+               if (scn->scn_suspending)
                        return;
        }
 
@@ -1384,55 +2774,55 @@ dsl_scan_visit(dsl_scan_t *scn, dmu_tx_t *tx)
                dsl_scan_visit_rootbp(scn, NULL,
                    &dp->dp_meta_rootbp, tx);
                spa_set_rootblkptr(dp->dp_spa, &dp->dp_meta_rootbp);
-               if (scn->scn_pausing)
+               if (scn->scn_suspending)
                        return;
 
                if (spa_version(dp->dp_spa) < SPA_VERSION_DSL_SCRUB) {
                        VERIFY0(dmu_objset_find_dp(dp, dp->dp_root_dir_obj,
-                           enqueue_cb, tx, DS_FIND_CHILDREN));
+                           enqueue_cb, NULL, DS_FIND_CHILDREN));
                } else {
                        dsl_scan_visitds(scn,
                            dp->dp_origin_snap->ds_object, tx);
                }
-               ASSERT(!scn->scn_pausing);
+               ASSERT(!scn->scn_suspending);
        } else if (scn->scn_phys.scn_bookmark.zb_objset !=
            ZB_DESTROYED_OBJSET) {
+               uint64_t dsobj = scn->scn_phys.scn_bookmark.zb_objset;
                /*
-                * If we were paused, continue from here.  Note if the
-                * ds we were paused on was deleted, the zb_objset may
+                * If we were suspended, continue from here. Note if the
+                * ds we were suspended on was deleted, the zb_objset may
                 * be -1, so we will skip this and find a new objset
                 * below.
                 */
-               dsl_scan_visitds(scn, scn->scn_phys.scn_bookmark.zb_objset, tx);
-               if (scn->scn_pausing)
+               dsl_scan_visitds(scn, dsobj, tx);
+               if (scn->scn_suspending)
                        return;
        }
 
        /*
-        * In case we were paused right at the end of the ds, zero the
+        * In case we suspended right at the end of the ds, zero the
         * bookmark so we don't think that we're still trying to resume.
         */
-       bzero(&scn->scn_phys.scn_bookmark, sizeof (zbookmark_phys_t));
-       zc = kmem_alloc(sizeof (zap_cursor_t), KM_SLEEP);
-       za = kmem_alloc(sizeof (zap_attribute_t), KM_SLEEP);
-
-       /* keep pulling things out of the zap-object-as-queue */
-       while (zap_cursor_init(zc, dp->dp_meta_objset,
-           scn->scn_phys.scn_queue_obj),
-           zap_cursor_retrieve(zc, za) == 0) {
+       memset(&scn->scn_phys.scn_bookmark, 0, sizeof (zbookmark_phys_t));
+
+       /*
+        * Keep pulling things out of the dataset avl queue. Updates to the
+        * persistent zap-object-as-queue happen only at checkpoints.
+        */
+       while ((sds = avl_first(&scn->scn_queue)) != NULL) {
                dsl_dataset_t *ds;
-               uint64_t dsobj;
+               uint64_t dsobj = sds->sds_dsobj;
+               uint64_t txg = sds->sds_txg;
 
-               dsobj = strtonum(za->za_name, NULL);
-               VERIFY3U(0, ==, zap_remove_int(dp->dp_meta_objset,
-                   scn->scn_phys.scn_queue_obj, dsobj, tx));
+               /* dequeue and free the ds from the queue */
+               scan_ds_queue_remove(scn, dsobj);
+               sds = NULL;
 
-               /* Set up min/max txg */
+               /* set up min / max txg */
                VERIFY3U(0, ==, dsl_dataset_hold_obj(dp, dsobj, FTAG, &ds));
-               if (za->za_first_integer != 0) {
+               if (txg != 0) {
                        scn->scn_phys.scn_cur_min_txg =
-                           MAX(scn->scn_phys.scn_min_txg,
-                           za->za_first_integer);
+                           MAX(scn->scn_phys.scn_min_txg, txg);
                } else {
                        scn->scn_phys.scn_cur_min_txg =
                            MAX(scn->scn_phys.scn_min_txg,
@@ -1442,137 +2832,616 @@ dsl_scan_visit(dsl_scan_t *scn, dmu_tx_t *tx)
                dsl_dataset_rele(ds, FTAG);
 
                dsl_scan_visitds(scn, dsobj, tx);
-               zap_cursor_fini(zc);
-               if (scn->scn_pausing)
-                       goto out;
+               if (scn->scn_suspending)
+                       return;
        }
-       zap_cursor_fini(zc);
-out:
-       kmem_free(za, sizeof (zap_attribute_t));
-       kmem_free(zc, sizeof (zap_cursor_t));
+
+       /* No more objsets to fetch, we're done */
+       scn->scn_phys.scn_bookmark.zb_objset = ZB_DESTROYED_OBJSET;
+       ASSERT0(scn->scn_suspending);
 }
 
-static boolean_t
-dsl_scan_free_should_pause(dsl_scan_t *scn)
+static uint64_t
+dsl_scan_count_data_disks(spa_t *spa)
 {
-       uint64_t elapsed_nanosecs;
-
-       if (zfs_recover)
-               return (B_FALSE);
-
-       if (scn->scn_visited_this_txg >= zfs_free_max_blocks)
-               return (B_TRUE);
+       vdev_t *rvd = spa->spa_root_vdev;
+       uint64_t i, leaves = 0;
 
-       elapsed_nanosecs = gethrtime() - scn->scn_sync_start_time;
-       return (elapsed_nanosecs / NANOSEC > zfs_txg_timeout ||
-           (NSEC2MSEC(elapsed_nanosecs) > zfs_free_min_time_ms &&
-           txg_sync_waiting(scn->scn_dp)) ||
-           spa_shutting_down(scn->scn_dp->dp_spa));
+       for (i = 0; i < rvd->vdev_children; i++) {
+               vdev_t *vd = rvd->vdev_child[i];
+               if (vd->vdev_islog || vd->vdev_isspare || vd->vdev_isl2cache)
+                       continue;
+               leaves += vdev_get_ndisks(vd) - vdev_get_nparity(vd);
+       }
+       return (leaves);
 }
 
-static int
-dsl_scan_free_block_cb(void *arg, const blkptr_t *bp, dmu_tx_t *tx)
+static void
+scan_io_queues_update_zio_stats(dsl_scan_io_queue_t *q, const blkptr_t *bp)
 {
-       dsl_scan_t *scn = arg;
+       int i;
+       uint64_t cur_size = 0;
 
-       if (!scn->scn_is_bptree ||
-           (BP_GET_LEVEL(bp) == 0 && BP_GET_TYPE(bp) != DMU_OT_OBJSET)) {
-               if (dsl_scan_free_should_pause(scn))
-                       return (SET_ERROR(ERESTART));
+       for (i = 0; i < BP_GET_NDVAS(bp); i++) {
+               cur_size += DVA_GET_ASIZE(&bp->blk_dva[i]);
        }
 
-       zio_nowait(zio_free_sync(scn->scn_zio_root, scn->scn_dp->dp_spa,
-           dmu_tx_get_txg(tx), bp, 0));
-       dsl_dir_diduse_space(tx->tx_pool->dp_free_dir, DD_USED_HEAD,
-           -bp_get_dsize_sync(scn->scn_dp->dp_spa, bp),
-           -BP_GET_PSIZE(bp), -BP_GET_UCSIZE(bp), tx);
-       scn->scn_visited_this_txg++;
-       return (0);
+       q->q_total_zio_size_this_txg += cur_size;
+       q->q_zios_this_txg++;
 }
 
-boolean_t
-dsl_scan_active(dsl_scan_t *scn)
+static void
+scan_io_queues_update_seg_stats(dsl_scan_io_queue_t *q, uint64_t start,
+    uint64_t end)
 {
-       spa_t *spa = scn->scn_dp->dp_spa;
-       uint64_t used = 0, comp, uncomp;
-
-       if (spa->spa_load_state != SPA_LOAD_NONE)
-               return (B_FALSE);
-       if (spa_shutting_down(spa))
-               return (B_FALSE);
-       if (scn->scn_phys.scn_state == DSS_SCANNING ||
-           (scn->scn_async_destroying && !scn->scn_async_stalled))
-               return (B_TRUE);
-
-       if (spa_version(scn->scn_dp->dp_spa) >= SPA_VERSION_DEADLISTS) {
-               (void) bpobj_space(&scn->scn_dp->dp_free_bpobj,
-                   &used, &comp, &uncomp);
-       }
-       return (used != 0);
+       q->q_total_seg_size_this_txg += end - start;
+       q->q_segs_this_txg++;
 }
 
-void
-dsl_scan_sync(dsl_pool_t *dp, dmu_tx_t *tx)
+static boolean_t
+scan_io_queue_check_suspend(dsl_scan_t *scn)
 {
-       dsl_scan_t *scn = dp->dp_scan;
-       spa_t *spa = dp->dp_spa;
-       int err = 0;
+       /* See comment in dsl_scan_check_suspend() */
+       uint64_t curr_time_ns = gethrtime();
+       uint64_t scan_time_ns = curr_time_ns - scn->scn_sync_start_time;
+       uint64_t sync_time_ns = curr_time_ns -
+           scn->scn_dp->dp_spa->spa_sync_starttime;
+       uint64_t dirty_min_bytes = zfs_dirty_data_max *
+           zfs_vdev_async_write_active_min_dirty_percent / 100;
+       uint_t mintime = (scn->scn_phys.scn_func == POOL_SCAN_RESILVER) ?
+           zfs_resilver_min_time_ms : zfs_scrub_min_time_ms;
+
+       return ((NSEC2MSEC(scan_time_ns) > mintime &&
+           (scn->scn_dp->dp_dirty_total >= dirty_min_bytes ||
+           txg_sync_waiting(scn->scn_dp) ||
+           NSEC2SEC(sync_time_ns) >= zfs_txg_timeout)) ||
+           spa_shutting_down(scn->scn_dp->dp_spa));
+}
+
+/*
+ * Given a list of scan_io_t's in io_list, this issues the I/Os out to
+ * disk. This consumes the io_list and frees the scan_io_t's. This is
+ * called when emptying queues, either when we're up against the memory
+ * limit or when we have finished scanning. Returns B_TRUE if we stopped
+ * processing the list before we finished. Any sios that were not issued
+ * will remain in the io_list.
+ */
+static boolean_t
+scan_io_queue_issue(dsl_scan_io_queue_t *queue, list_t *io_list)
+{
+       dsl_scan_t *scn = queue->q_scn;
+       scan_io_t *sio;
+       boolean_t suspended = B_FALSE;
+
+       while ((sio = list_head(io_list)) != NULL) {
+               blkptr_t bp;
+
+               if (scan_io_queue_check_suspend(scn)) {
+                       suspended = B_TRUE;
+                       break;
+               }
+
+               sio2bp(sio, &bp);
+               scan_exec_io(scn->scn_dp, &bp, sio->sio_flags,
+                   &sio->sio_zb, queue);
+               (void) list_remove_head(io_list);
+               scan_io_queues_update_zio_stats(queue, &bp);
+               sio_free(sio);
+       }
+       return (suspended);
+}
+
+/*
+ * This function removes sios from an IO queue which reside within a given
+ * range_seg_t and inserts them (in offset order) into a list. Note that
+ * we only ever return a maximum of 32 sios at once. If there are more sios
+ * to process within this segment that did not make it onto the list we
+ * return B_TRUE and otherwise B_FALSE.
+ */
+static boolean_t
+scan_io_queue_gather(dsl_scan_io_queue_t *queue, range_seg_t *rs, list_t *list)
+{
+       scan_io_t *srch_sio, *sio, *next_sio;
+       avl_index_t idx;
+       uint_t num_sios = 0;
+       int64_t bytes_issued = 0;
+
+       ASSERT(rs != NULL);
+       ASSERT(MUTEX_HELD(&queue->q_vd->vdev_scan_io_queue_lock));
+
+       srch_sio = sio_alloc(1);
+       srch_sio->sio_nr_dvas = 1;
+       SIO_SET_OFFSET(srch_sio, rs_get_start(rs, queue->q_exts_by_addr));
 
        /*
-        * Check for scn_restart_txg before checking spa_load_state, so
-        * that we can restart an old-style scan while the pool is being
-        * imported (see dsl_scan_init).
+        * The exact start of the extent might not contain any matching zios,
+        * so if that's the case, examine the next one in the tree.
         */
-       if (dsl_scan_restarting(scn, tx)) {
-               pool_scan_func_t func = POOL_SCAN_SCRUB;
-               dsl_scan_done(scn, B_FALSE, tx);
-               if (vdev_resilver_needed(spa->spa_root_vdev, NULL, NULL))
-                       func = POOL_SCAN_RESILVER;
-               zfs_dbgmsg("restarting scan func=%u txg=%llu",
-                   func, tx->tx_txg);
-               dsl_scan_setup_sync(&func, tx);
+       sio = avl_find(&queue->q_sios_by_addr, srch_sio, &idx);
+       sio_free(srch_sio);
+
+       if (sio == NULL)
+               sio = avl_nearest(&queue->q_sios_by_addr, idx, AVL_AFTER);
+
+       while (sio != NULL && SIO_GET_OFFSET(sio) < rs_get_end(rs,
+           queue->q_exts_by_addr) && num_sios <= 32) {
+               ASSERT3U(SIO_GET_OFFSET(sio), >=, rs_get_start(rs,
+                   queue->q_exts_by_addr));
+               ASSERT3U(SIO_GET_END_OFFSET(sio), <=, rs_get_end(rs,
+                   queue->q_exts_by_addr));
+
+               next_sio = AVL_NEXT(&queue->q_sios_by_addr, sio);
+               avl_remove(&queue->q_sios_by_addr, sio);
+               if (avl_is_empty(&queue->q_sios_by_addr))
+                       atomic_add_64(&queue->q_scn->scn_queues_pending, -1);
+               queue->q_sio_memused -= SIO_GET_MUSED(sio);
+
+               bytes_issued += SIO_GET_ASIZE(sio);
+               num_sios++;
+               list_insert_tail(list, sio);
+               sio = next_sio;
        }
 
        /*
-        * Only process scans in sync pass 1.
+        * We limit the number of sios we process at once to 32 to avoid
+        * biting off more than we can chew. If we didn't take everything
+        * in the segment we update it to reflect the work we were able to
+        * complete. Otherwise, we remove it from the range tree entirely.
         */
-       if (spa_sync_pass(dp->dp_spa) > 1)
-               return;
+       if (sio != NULL && SIO_GET_OFFSET(sio) < rs_get_end(rs,
+           queue->q_exts_by_addr)) {
+               range_tree_adjust_fill(queue->q_exts_by_addr, rs,
+                   -bytes_issued);
+               range_tree_resize_segment(queue->q_exts_by_addr, rs,
+                   SIO_GET_OFFSET(sio), rs_get_end(rs,
+                   queue->q_exts_by_addr) - SIO_GET_OFFSET(sio));
+               queue->q_last_ext_addr = SIO_GET_OFFSET(sio);
+               return (B_TRUE);
+       } else {
+               uint64_t rstart = rs_get_start(rs, queue->q_exts_by_addr);
+               uint64_t rend = rs_get_end(rs, queue->q_exts_by_addr);
+               range_tree_remove(queue->q_exts_by_addr, rstart, rend - rstart);
+               queue->q_last_ext_addr = -1;
+               return (B_FALSE);
+       }
+}
+
+/*
+ * This is called from the queue emptying thread and selects the next
+ * extent from which we are to issue I/Os. The behavior of this function
+ * depends on the state of the scan, the current memory consumption and
+ * whether or not we are performing a scan shutdown.
+ * 1) We select extents in an elevator algorithm (LBA-order) if the scan
+ *     needs to perform a checkpoint
+ * 2) We select the largest available extent if we are up against the
+ *     memory limit.
+ * 3) Otherwise we don't select any extents.
+ */
+static range_seg_t *
+scan_io_queue_fetch_ext(dsl_scan_io_queue_t *queue)
+{
+       dsl_scan_t *scn = queue->q_scn;
+       range_tree_t *rt = queue->q_exts_by_addr;
+
+       ASSERT(MUTEX_HELD(&queue->q_vd->vdev_scan_io_queue_lock));
+       ASSERT(scn->scn_is_sorted);
+
+       if (!scn->scn_checkpointing && !scn->scn_clearing)
+               return (NULL);
 
        /*
-        * If the spa is shutting down, then stop scanning. This will
-        * ensure that the scan does not dirty any new data during the
-        * shutdown phase.
+        * During normal clearing, we want to issue our largest segments
+        * first, keeping IO as sequential as possible, and leaving the
+        * smaller extents for later with the hope that they might eventually
+        * grow to larger sequential segments. However, when the scan is
+        * checkpointing, no new extents will be added to the sorting queue,
+        * so the way we are sorted now is as good as it will ever get.
+        * In this case, we instead switch to issuing extents in LBA order.
         */
-       if (spa_shutting_down(spa))
+       if ((zfs_scan_issue_strategy < 1 && scn->scn_checkpointing) ||
+           zfs_scan_issue_strategy == 1)
+               return (range_tree_first(rt));
+
+       /*
+        * Try to continue previous extent if it is not completed yet.  After
+        * shrink in scan_io_queue_gather() it may no longer be the best, but
+        * otherwise we leave shorter remnant every txg.
+        */
+       uint64_t start;
+       uint64_t size = 1ULL << rt->rt_shift;
+       range_seg_t *addr_rs;
+       if (queue->q_last_ext_addr != -1) {
+               start = queue->q_last_ext_addr;
+               addr_rs = range_tree_find(rt, start, size);
+               if (addr_rs != NULL)
+                       return (addr_rs);
+       }
+
+       /*
+        * Nothing to continue, so find new best extent.
+        */
+       uint64_t *v = zfs_btree_first(&queue->q_exts_by_size, NULL);
+       if (v == NULL)
+               return (NULL);
+       queue->q_last_ext_addr = start = *v << rt->rt_shift;
+
+       /*
+        * We need to get the original entry in the by_addr tree so we can
+        * modify it.
+        */
+       addr_rs = range_tree_find(rt, start, size);
+       ASSERT3P(addr_rs, !=, NULL);
+       ASSERT3U(rs_get_start(addr_rs, rt), ==, start);
+       ASSERT3U(rs_get_end(addr_rs, rt), >, start);
+       return (addr_rs);
+}
+
+static void
+scan_io_queues_run_one(void *arg)
+{
+       dsl_scan_io_queue_t *queue = arg;
+       kmutex_t *q_lock = &queue->q_vd->vdev_scan_io_queue_lock;
+       boolean_t suspended = B_FALSE;
+       range_seg_t *rs;
+       scan_io_t *sio;
+       zio_t *zio;
+       list_t sio_list;
+
+       ASSERT(queue->q_scn->scn_is_sorted);
+
+       list_create(&sio_list, sizeof (scan_io_t),
+           offsetof(scan_io_t, sio_nodes.sio_list_node));
+       zio = zio_null(queue->q_scn->scn_zio_root, queue->q_scn->scn_dp->dp_spa,
+           NULL, NULL, NULL, ZIO_FLAG_CANFAIL);
+       mutex_enter(q_lock);
+       queue->q_zio = zio;
+
+       /* Calculate maximum in-flight bytes for this vdev. */
+       queue->q_maxinflight_bytes = MAX(1, zfs_scan_vdev_limit *
+           (vdev_get_ndisks(queue->q_vd) - vdev_get_nparity(queue->q_vd)));
+
+       /* reset per-queue scan statistics for this txg */
+       queue->q_total_seg_size_this_txg = 0;
+       queue->q_segs_this_txg = 0;
+       queue->q_total_zio_size_this_txg = 0;
+       queue->q_zios_this_txg = 0;
+
+       /* loop until we run out of time or sios */
+       while ((rs = scan_io_queue_fetch_ext(queue)) != NULL) {
+               uint64_t seg_start = 0, seg_end = 0;
+               boolean_t more_left;
+
+               ASSERT(list_is_empty(&sio_list));
+
+               /* loop while we still have sios left to process in this rs */
+               do {
+                       scan_io_t *first_sio, *last_sio;
+
+                       /*
+                        * We have selected which extent needs to be
+                        * processed next. Gather up the corresponding sios.
+                        */
+                       more_left = scan_io_queue_gather(queue, rs, &sio_list);
+                       ASSERT(!list_is_empty(&sio_list));
+                       first_sio = list_head(&sio_list);
+                       last_sio = list_tail(&sio_list);
+
+                       seg_end = SIO_GET_END_OFFSET(last_sio);
+                       if (seg_start == 0)
+                               seg_start = SIO_GET_OFFSET(first_sio);
+
+                       /*
+                        * Issuing sios can take a long time so drop the
+                        * queue lock. The sio queue won't be updated by
+                        * other threads since we're in syncing context so
+                        * we can be sure that our trees will remain exactly
+                        * as we left them.
+                        */
+                       mutex_exit(q_lock);
+                       suspended = scan_io_queue_issue(queue, &sio_list);
+                       mutex_enter(q_lock);
+
+                       if (suspended)
+                               break;
+               } while (more_left);
+
+               /* update statistics for debugging purposes */
+               scan_io_queues_update_seg_stats(queue, seg_start, seg_end);
+
+               if (suspended)
+                       break;
+       }
+
+       /*
+        * If we were suspended in the middle of processing,
+        * requeue any unfinished sios and exit.
+        */
+       while ((sio = list_head(&sio_list)) != NULL) {
+               list_remove(&sio_list, sio);
+               scan_io_queue_insert_impl(queue, sio);
+       }
+
+       queue->q_zio = NULL;
+       mutex_exit(q_lock);
+       zio_nowait(zio);
+       list_destroy(&sio_list);
+}
+
+/*
+ * Performs an emptying run on all scan queues in the pool. This just
+ * punches out one thread per top-level vdev, each of which processes
+ * only that vdev's scan queue. We can parallelize the I/O here because
+ * we know that each queue's I/Os only affect its own top-level vdev.
+ *
+ * This function waits for the queue runs to complete, and must be
+ * called from dsl_scan_sync (or in general, syncing context).
+ */
+static void
+scan_io_queues_run(dsl_scan_t *scn)
+{
+       spa_t *spa = scn->scn_dp->dp_spa;
+
+       ASSERT(scn->scn_is_sorted);
+       ASSERT(spa_config_held(spa, SCL_CONFIG, RW_READER));
+
+       if (scn->scn_queues_pending == 0)
                return;
 
+       if (scn->scn_taskq == NULL) {
+               int nthreads = spa->spa_root_vdev->vdev_children;
+
+               /*
+                * We need to make this taskq *always* execute as many
+                * threads in parallel as we have top-level vdevs and no
+                * less, otherwise strange serialization of the calls to
+                * scan_io_queues_run_one can occur during spa_sync runs
+                * and that significantly impacts performance.
+                */
+               scn->scn_taskq = taskq_create("dsl_scan_iss", nthreads,
+                   minclsyspri, nthreads, nthreads, TASKQ_PREPOPULATE);
+       }
+
+       for (uint64_t i = 0; i < spa->spa_root_vdev->vdev_children; i++) {
+               vdev_t *vd = spa->spa_root_vdev->vdev_child[i];
+
+               mutex_enter(&vd->vdev_scan_io_queue_lock);
+               if (vd->vdev_scan_io_queue != NULL) {
+                       VERIFY(taskq_dispatch(scn->scn_taskq,
+                           scan_io_queues_run_one, vd->vdev_scan_io_queue,
+                           TQ_SLEEP) != TASKQID_INVALID);
+               }
+               mutex_exit(&vd->vdev_scan_io_queue_lock);
+       }
+
        /*
-        * If the scan is inactive due to a stalled async destroy, try again.
+        * Wait for the queues to finish issuing their IOs for this run
+        * before we return. There may still be IOs in flight at this
+        * point.
         */
-       if (!scn->scn_async_stalled && !dsl_scan_active(scn))
+       taskq_wait(scn->scn_taskq);
+}
+
+static boolean_t
+dsl_scan_async_block_should_pause(dsl_scan_t *scn)
+{
+       uint64_t elapsed_nanosecs;
+
+       if (zfs_recover)
+               return (B_FALSE);
+
+       if (zfs_async_block_max_blocks != 0 &&
+           scn->scn_visited_this_txg >= zfs_async_block_max_blocks) {
+               return (B_TRUE);
+       }
+
+       if (zfs_max_async_dedup_frees != 0 &&
+           scn->scn_dedup_frees_this_txg >= zfs_max_async_dedup_frees) {
+               return (B_TRUE);
+       }
+
+       elapsed_nanosecs = gethrtime() - scn->scn_sync_start_time;
+       return (elapsed_nanosecs / NANOSEC > zfs_txg_timeout ||
+           (NSEC2MSEC(elapsed_nanosecs) > scn->scn_async_block_min_time_ms &&
+           txg_sync_waiting(scn->scn_dp)) ||
+           spa_shutting_down(scn->scn_dp->dp_spa));
+}
+
+static int
+dsl_scan_free_block_cb(void *arg, const blkptr_t *bp, dmu_tx_t *tx)
+{
+       dsl_scan_t *scn = arg;
+
+       if (!scn->scn_is_bptree ||
+           (BP_GET_LEVEL(bp) == 0 && BP_GET_TYPE(bp) != DMU_OT_OBJSET)) {
+               if (dsl_scan_async_block_should_pause(scn))
+                       return (SET_ERROR(ERESTART));
+       }
+
+       zio_nowait(zio_free_sync(scn->scn_zio_root, scn->scn_dp->dp_spa,
+           dmu_tx_get_txg(tx), bp, 0));
+       dsl_dir_diduse_space(tx->tx_pool->dp_free_dir, DD_USED_HEAD,
+           -bp_get_dsize_sync(scn->scn_dp->dp_spa, bp),
+           -BP_GET_PSIZE(bp), -BP_GET_UCSIZE(bp), tx);
+       scn->scn_visited_this_txg++;
+       if (BP_GET_DEDUP(bp))
+               scn->scn_dedup_frees_this_txg++;
+       return (0);
+}
+
+static void
+dsl_scan_update_stats(dsl_scan_t *scn)
+{
+       spa_t *spa = scn->scn_dp->dp_spa;
+       uint64_t i;
+       uint64_t seg_size_total = 0, zio_size_total = 0;
+       uint64_t seg_count_total = 0, zio_count_total = 0;
+
+       for (i = 0; i < spa->spa_root_vdev->vdev_children; i++) {
+               vdev_t *vd = spa->spa_root_vdev->vdev_child[i];
+               dsl_scan_io_queue_t *queue = vd->vdev_scan_io_queue;
+
+               if (queue == NULL)
+                       continue;
+
+               seg_size_total += queue->q_total_seg_size_this_txg;
+               zio_size_total += queue->q_total_zio_size_this_txg;
+               seg_count_total += queue->q_segs_this_txg;
+               zio_count_total += queue->q_zios_this_txg;
+       }
+
+       if (seg_count_total == 0 || zio_count_total == 0) {
+               scn->scn_avg_seg_size_this_txg = 0;
+               scn->scn_avg_zio_size_this_txg = 0;
+               scn->scn_segs_this_txg = 0;
+               scn->scn_zios_this_txg = 0;
                return;
+       }
 
-       scn->scn_visited_this_txg = 0;
-       scn->scn_pausing = B_FALSE;
-       scn->scn_sync_start_time = gethrtime();
-       spa->spa_scrub_active = B_TRUE;
+       scn->scn_avg_seg_size_this_txg = seg_size_total / seg_count_total;
+       scn->scn_avg_zio_size_this_txg = zio_size_total / zio_count_total;
+       scn->scn_segs_this_txg = seg_count_total;
+       scn->scn_zios_this_txg = zio_count_total;
+}
+
+static int
+bpobj_dsl_scan_free_block_cb(void *arg, const blkptr_t *bp, boolean_t bp_freed,
+    dmu_tx_t *tx)
+{
+       ASSERT(!bp_freed);
+       return (dsl_scan_free_block_cb(arg, bp, tx));
+}
+
+static int
+dsl_scan_obsolete_block_cb(void *arg, const blkptr_t *bp, boolean_t bp_freed,
+    dmu_tx_t *tx)
+{
+       ASSERT(!bp_freed);
+       dsl_scan_t *scn = arg;
+       const dva_t *dva = &bp->blk_dva[0];
+
+       if (dsl_scan_async_block_should_pause(scn))
+               return (SET_ERROR(ERESTART));
+
+       spa_vdev_indirect_mark_obsolete(scn->scn_dp->dp_spa,
+           DVA_GET_VDEV(dva), DVA_GET_OFFSET(dva),
+           DVA_GET_ASIZE(dva), tx);
+       scn->scn_visited_this_txg++;
+       return (0);
+}
+
+boolean_t
+dsl_scan_active(dsl_scan_t *scn)
+{
+       spa_t *spa = scn->scn_dp->dp_spa;
+       uint64_t used = 0, comp, uncomp;
+       boolean_t clones_left;
+
+       if (spa->spa_load_state != SPA_LOAD_NONE)
+               return (B_FALSE);
+       if (spa_shutting_down(spa))
+               return (B_FALSE);
+       if ((dsl_scan_is_running(scn) && !dsl_scan_is_paused_scrub(scn)) ||
+           (scn->scn_async_destroying && !scn->scn_async_stalled))
+               return (B_TRUE);
+
+       if (spa_version(scn->scn_dp->dp_spa) >= SPA_VERSION_DEADLISTS) {
+               (void) bpobj_space(&scn->scn_dp->dp_free_bpobj,
+                   &used, &comp, &uncomp);
+       }
+       clones_left = spa_livelist_delete_check(spa);
+       return ((used != 0) || (clones_left));
+}
+
+static boolean_t
+dsl_scan_check_deferred(vdev_t *vd)
+{
+       boolean_t need_resilver = B_FALSE;
+
+       for (int c = 0; c < vd->vdev_children; c++) {
+               need_resilver |=
+                   dsl_scan_check_deferred(vd->vdev_child[c]);
+       }
+
+       if (!vdev_is_concrete(vd) || vd->vdev_aux ||
+           !vd->vdev_ops->vdev_op_leaf)
+               return (need_resilver);
+
+       if (!vd->vdev_resilver_deferred)
+               need_resilver = B_TRUE;
+
+       return (need_resilver);
+}
+
+static boolean_t
+dsl_scan_need_resilver(spa_t *spa, const dva_t *dva, size_t psize,
+    uint64_t phys_birth)
+{
+       vdev_t *vd;
+
+       vd = vdev_lookup_top(spa, DVA_GET_VDEV(dva));
+
+       if (vd->vdev_ops == &vdev_indirect_ops) {
+               /*
+                * The indirect vdev can point to multiple
+                * vdevs.  For simplicity, always create
+                * the resilver zio_t. zio_vdev_io_start()
+                * will bypass the child resilver i/o's if
+                * they are on vdevs that don't have DTL's.
+                */
+               return (B_TRUE);
+       }
+
+       if (DVA_GET_GANG(dva)) {
+               /*
+                * Gang members may be spread across multiple
+                * vdevs, so the best estimate we have is the
+                * scrub range, which has already been checked.
+                * XXX -- it would be better to change our
+                * allocation policy to ensure that all
+                * gang members reside on the same vdev.
+                */
+               return (B_TRUE);
+       }
 
        /*
-        * First process the async destroys.  If we pause, don't do
-        * any scrubbing or resilvering.  This ensures that there are no
-        * async destroys while we are scanning, so the scan code doesn't
-        * have to worry about traversing it.  It is also faster to free the
-        * blocks than to scrub them.
+        * Check if the top-level vdev must resilver this offset.
+        * When the offset does not intersect with a dirty leaf DTL
+        * then it may be possible to skip the resilver IO.  The psize
+        * is provided instead of asize to simplify the check for RAIDZ.
+        */
+       if (!vdev_dtl_need_resilver(vd, dva, psize, phys_birth))
+               return (B_FALSE);
+
+       /*
+        * Check that this top-level vdev has a device under it which
+        * is resilvering and is not deferred.
         */
+       if (!dsl_scan_check_deferred(vd))
+               return (B_FALSE);
+
+       return (B_TRUE);
+}
+
+static int
+dsl_process_async_destroys(dsl_pool_t *dp, dmu_tx_t *tx)
+{
+       dsl_scan_t *scn = dp->dp_scan;
+       spa_t *spa = dp->dp_spa;
+       int err = 0;
+
+       if (spa_suspend_async_destroy(spa))
+               return (0);
+
        if (zfs_free_bpobj_enabled &&
-           spa_version(dp->dp_spa) >= SPA_VERSION_DEADLISTS) {
+           spa_version(spa) >= SPA_VERSION_DEADLISTS) {
                scn->scn_is_bptree = B_FALSE;
-               scn->scn_zio_root = zio_root(dp->dp_spa, NULL,
+               scn->scn_async_block_min_time_ms = zfs_free_min_time_ms;
+               scn->scn_zio_root = zio_root(spa, NULL,
                    NULL, ZIO_FLAG_MUSTSUCCEED);
                err = bpobj_iterate(&dp->dp_free_bpobj,
-                   dsl_scan_free_block_cb, scn, tx);
-               VERIFY3U(0, ==, zio_wait(scn->scn_zio_root));
+                   bpobj_dsl_scan_free_block_cb, scn, tx);
+               VERIFY0(zio_wait(scn->scn_zio_root));
+               scn->scn_zio_root = NULL;
 
                if (err != 0 && err != ERESTART)
                        zfs_panic_recover("error %u from bpobj_iterate()", err);
@@ -1581,11 +3450,12 @@ dsl_scan_sync(dsl_pool_t *dp, dmu_tx_t *tx)
        if (err == 0 && spa_feature_is_active(spa, SPA_FEATURE_ASYNC_DESTROY)) {
                ASSERT(scn->scn_async_destroying);
                scn->scn_is_bptree = B_TRUE;
-               scn->scn_zio_root = zio_root(dp->dp_spa, NULL,
+               scn->scn_zio_root = zio_root(spa, NULL,
                    NULL, ZIO_FLAG_MUSTSUCCEED);
                err = bptree_iterate(dp->dp_meta_objset,
                    dp->dp_bptree_obj, B_TRUE, dsl_scan_free_block_cb, scn, tx);
                VERIFY0(zio_wait(scn->scn_zio_root));
+               scn->scn_zio_root = NULL;
 
                if (err == EIO || err == ECKSUM) {
                        err = 0;
@@ -1622,22 +3492,24 @@ dsl_scan_sync(dsl_pool_t *dp, dmu_tx_t *tx)
        }
        if (scn->scn_visited_this_txg) {
                zfs_dbgmsg("freed %llu blocks in %llums from "
-                   "free_bpobj/bptree txg %llu; err=%u",
+                   "free_bpobj/bptree on %s in txg %llu; err=%u",
                    (longlong_t)scn->scn_visited_this_txg,
                    (longlong_t)
                    NSEC2MSEC(gethrtime() - scn->scn_sync_start_time),
-                   (longlong_t)tx->tx_txg, err);
+                   spa->spa_name, (longlong_t)tx->tx_txg, err);
                scn->scn_visited_this_txg = 0;
+               scn->scn_dedup_frees_this_txg = 0;
 
                /*
-                * Write out changes to the DDT that may be required as a
-                * result of the blocks freed.  This ensures that the DDT
-                * is clean when a scrub/resilver runs.
+                * Write out changes to the DDT and the BRT that may be required
+                * as a result of the blocks freed.  This ensures that the DDT
+                * and the BRT are clean when a scrub/resilver runs.
                 */
                ddt_sync(spa, tx->tx_txg);
+               brt_sync(spa, tx->tx_txg);
        }
        if (err != 0)
-               return;
+               return (err);
        if (dp->dp_free_dir != NULL && !scn->scn_async_destroying &&
            zfs_free_leak_on_eio &&
            (dsl_dir_phys(dp->dp_free_dir)->dd_used_bytes != 0 ||
@@ -1665,114 +3537,363 @@ dsl_scan_sync(dsl_pool_t *dp, dmu_tx_t *tx)
                    -dsl_dir_phys(dp->dp_free_dir)->dd_compressed_bytes,
                    -dsl_dir_phys(dp->dp_free_dir)->dd_uncompressed_bytes, tx);
        }
-       if (dp->dp_free_dir != NULL && !scn->scn_async_destroying) {
+
+       if (dp->dp_free_dir != NULL && !scn->scn_async_destroying &&
+           !spa_livelist_delete_check(spa)) {
                /* finished; verify that space accounting went to zero */
                ASSERT0(dsl_dir_phys(dp->dp_free_dir)->dd_used_bytes);
                ASSERT0(dsl_dir_phys(dp->dp_free_dir)->dd_compressed_bytes);
                ASSERT0(dsl_dir_phys(dp->dp_free_dir)->dd_uncompressed_bytes);
        }
 
-       if (scn->scn_phys.scn_state != DSS_SCANNING)
-               return;
+       spa_notify_waiters(spa);
 
-       if (scn->scn_done_txg == tx->tx_txg) {
-               ASSERT(!scn->scn_pausing);
-               /* finished with scan. */
-               zfs_dbgmsg("txg %llu scan complete", tx->tx_txg);
-               dsl_scan_done(scn, B_TRUE, tx);
-               ASSERT3U(spa->spa_scrub_inflight, ==, 0);
-               dsl_scan_sync_state(scn, tx);
-               return;
+       EQUIV(bpobj_is_open(&dp->dp_obsolete_bpobj),
+           0 == zap_contains(dp->dp_meta_objset, DMU_POOL_DIRECTORY_OBJECT,
+           DMU_POOL_OBSOLETE_BPOBJ));
+       if (err == 0 && bpobj_is_open(&dp->dp_obsolete_bpobj)) {
+               ASSERT(spa_feature_is_active(dp->dp_spa,
+                   SPA_FEATURE_OBSOLETE_COUNTS));
+
+               scn->scn_is_bptree = B_FALSE;
+               scn->scn_async_block_min_time_ms = zfs_obsolete_min_time_ms;
+               err = bpobj_iterate(&dp->dp_obsolete_bpobj,
+                   dsl_scan_obsolete_block_cb, scn, tx);
+               if (err != 0 && err != ERESTART)
+                       zfs_panic_recover("error %u from bpobj_iterate()", err);
+
+               if (bpobj_is_empty(&dp->dp_obsolete_bpobj))
+                       dsl_pool_destroy_obsolete_bpobj(dp, tx);
        }
+       return (0);
+}
 
-       if (scn->scn_phys.scn_ddt_bookmark.ddb_class <=
-           scn->scn_phys.scn_ddt_class_max) {
-               zfs_dbgmsg("doing scan sync txg %llu; "
-                   "ddt bm=%llu/%llu/%llu/%llx",
-                   (longlong_t)tx->tx_txg,
-                   (longlong_t)scn->scn_phys.scn_ddt_bookmark.ddb_class,
-                   (longlong_t)scn->scn_phys.scn_ddt_bookmark.ddb_type,
-                   (longlong_t)scn->scn_phys.scn_ddt_bookmark.ddb_checksum,
-                   (longlong_t)scn->scn_phys.scn_ddt_bookmark.ddb_cursor);
-               ASSERT(scn->scn_phys.scn_bookmark.zb_objset == 0);
-               ASSERT(scn->scn_phys.scn_bookmark.zb_object == 0);
-               ASSERT(scn->scn_phys.scn_bookmark.zb_level == 0);
-               ASSERT(scn->scn_phys.scn_bookmark.zb_blkid == 0);
-       } else {
-               zfs_dbgmsg("doing scan sync txg %llu; bm=%llu/%llu/%llu/%llu",
-                   (longlong_t)tx->tx_txg,
-                   (longlong_t)scn->scn_phys.scn_bookmark.zb_objset,
-                   (longlong_t)scn->scn_phys.scn_bookmark.zb_object,
-                   (longlong_t)scn->scn_phys.scn_bookmark.zb_level,
-                   (longlong_t)scn->scn_phys.scn_bookmark.zb_blkid);
+/*
+ * This is the primary entry point for scans that is called from syncing
+ * context. Scans must happen entirely during syncing context so that we
+ * can guarantee that blocks we are currently scanning will not change out
+ * from under us. While a scan is active, this function controls how quickly
+ * transaction groups proceed, instead of the normal handling provided by
+ * txg_sync_thread().
+ */
+void
+dsl_scan_sync(dsl_pool_t *dp, dmu_tx_t *tx)
+{
+       int err = 0;
+       dsl_scan_t *scn = dp->dp_scan;
+       spa_t *spa = dp->dp_spa;
+       state_sync_type_t sync_type = SYNC_OPTIONAL;
+
+       if (spa->spa_resilver_deferred &&
+           !spa_feature_is_active(dp->dp_spa, SPA_FEATURE_RESILVER_DEFER))
+               spa_feature_incr(spa, SPA_FEATURE_RESILVER_DEFER, tx);
+
+       /*
+        * Check for scn_restart_txg before checking spa_load_state, so
+        * that we can restart an old-style scan while the pool is being
+        * imported (see dsl_scan_init). We also restart scans if there
+        * is a deferred resilver and the user has manually disabled
+        * deferred resilvers via the tunable.
+        */
+       if (dsl_scan_restarting(scn, tx) ||
+           (spa->spa_resilver_deferred && zfs_resilver_disable_defer)) {
+               pool_scan_func_t func = POOL_SCAN_SCRUB;
+               dsl_scan_done(scn, B_FALSE, tx);
+               if (vdev_resilver_needed(spa->spa_root_vdev, NULL, NULL))
+                       func = POOL_SCAN_RESILVER;
+               zfs_dbgmsg("restarting scan func=%u on %s txg=%llu",
+                   func, dp->dp_spa->spa_name, (longlong_t)tx->tx_txg);
+               dsl_scan_setup_sync(&func, tx);
        }
 
-       scn->scn_zio_root = zio_root(dp->dp_spa, NULL,
-           NULL, ZIO_FLAG_CANFAIL);
-       dsl_pool_config_enter(dp, FTAG);
-       dsl_scan_visit(scn, tx);
-       dsl_pool_config_exit(dp, FTAG);
-       (void) zio_wait(scn->scn_zio_root);
-       scn->scn_zio_root = NULL;
+       /*
+        * Only process scans in sync pass 1.
+        */
+       if (spa_sync_pass(spa) > 1)
+               return;
+
+       /*
+        * If the spa is shutting down, then stop scanning. This will
+        * ensure that the scan does not dirty any new data during the
+        * shutdown phase.
+        */
+       if (spa_shutting_down(spa))
+               return;
+
+       /*
+        * If the scan is inactive due to a stalled async destroy, try again.
+        */
+       if (!scn->scn_async_stalled && !dsl_scan_active(scn))
+               return;
+
+       /* reset scan statistics */
+       scn->scn_visited_this_txg = 0;
+       scn->scn_dedup_frees_this_txg = 0;
+       scn->scn_holes_this_txg = 0;
+       scn->scn_lt_min_this_txg = 0;
+       scn->scn_gt_max_this_txg = 0;
+       scn->scn_ddt_contained_this_txg = 0;
+       scn->scn_objsets_visited_this_txg = 0;
+       scn->scn_avg_seg_size_this_txg = 0;
+       scn->scn_segs_this_txg = 0;
+       scn->scn_avg_zio_size_this_txg = 0;
+       scn->scn_zios_this_txg = 0;
+       scn->scn_suspending = B_FALSE;
+       scn->scn_sync_start_time = gethrtime();
+       spa->spa_scrub_active = B_TRUE;
+
+       /*
+        * First process the async destroys.  If we suspend, don't do
+        * any scrubbing or resilvering.  This ensures that there are no
+        * async destroys while we are scanning, so the scan code doesn't
+        * have to worry about traversing it.  It is also faster to free the
+        * blocks than to scrub them.
+        */
+       err = dsl_process_async_destroys(dp, tx);
+       if (err != 0)
+               return;
+
+       if (!dsl_scan_is_running(scn) || dsl_scan_is_paused_scrub(scn))
+               return;
+
+       /*
+        * Wait a few txgs after importing to begin scanning so that
+        * we can get the pool imported quickly.
+        */
+       if (spa->spa_syncing_txg < spa->spa_first_txg + SCAN_IMPORT_WAIT_TXGS)
+               return;
 
-       zfs_dbgmsg("visited %llu blocks in %llums",
-           (longlong_t)scn->scn_visited_this_txg,
-           (longlong_t)NSEC2MSEC(gethrtime() - scn->scn_sync_start_time));
+       /*
+        * zfs_scan_suspend_progress can be set to disable scan progress.
+        * We don't want to spin the txg_sync thread, so we add a delay
+        * here to simulate the time spent doing a scan. This is mostly
+        * useful for testing and debugging.
+        */
+       if (zfs_scan_suspend_progress) {
+               uint64_t scan_time_ns = gethrtime() - scn->scn_sync_start_time;
+               uint_t mintime = (scn->scn_phys.scn_func ==
+                   POOL_SCAN_RESILVER) ? zfs_resilver_min_time_ms :
+                   zfs_scrub_min_time_ms;
+
+               while (zfs_scan_suspend_progress &&
+                   !txg_sync_waiting(scn->scn_dp) &&
+                   !spa_shutting_down(scn->scn_dp->dp_spa) &&
+                   NSEC2MSEC(scan_time_ns) < mintime) {
+                       delay(hz);
+                       scan_time_ns = gethrtime() - scn->scn_sync_start_time;
+               }
+               return;
+       }
 
-       if (!scn->scn_pausing) {
-               scn->scn_done_txg = tx->tx_txg + 1;
-               zfs_dbgmsg("txg %llu traversal complete, waiting till txg %llu",
-                   tx->tx_txg, scn->scn_done_txg);
+       /*
+        * Disabled by default, set zfs_scan_report_txgs to report
+        * average performance over the last zfs_scan_report_txgs TXGs.
+        */
+       if (!dsl_scan_is_paused_scrub(scn) && zfs_scan_report_txgs != 0 &&
+           tx->tx_txg % zfs_scan_report_txgs == 0) {
+               scn->scn_issued_before_pass += spa->spa_scan_pass_issued;
+               spa_scan_stat_init(spa);
        }
 
-       if (DSL_SCAN_IS_SCRUB_RESILVER(scn)) {
-               mutex_enter(&spa->spa_scrub_lock);
-               while (spa->spa_scrub_inflight > 0) {
-                       cv_wait(&spa->spa_scrub_io_cv,
-                           &spa->spa_scrub_lock);
+       /*
+        * It is possible to switch from unsorted to sorted at any time,
+        * but afterwards the scan will remain sorted unless reloaded from
+        * a checkpoint after a reboot.
+        */
+       if (!zfs_scan_legacy) {
+               scn->scn_is_sorted = B_TRUE;
+               if (scn->scn_last_checkpoint == 0)
+                       scn->scn_last_checkpoint = ddi_get_lbolt();
+       }
+
+       /*
+        * For sorted scans, determine what kind of work we will be doing
+        * this txg based on our memory limitations and whether or not we
+        * need to perform a checkpoint.
+        */
+       if (scn->scn_is_sorted) {
+               /*
+                * If we are over our checkpoint interval, set scn_clearing
+                * so that we can begin checkpointing immediately. The
+                * checkpoint allows us to save a consistent bookmark
+                * representing how much data we have scrubbed so far.
+                * Otherwise, use the memory limit to determine if we should
+                * scan for metadata or start issue scrub IOs. We accumulate
+                * metadata until we hit our hard memory limit at which point
+                * we issue scrub IOs until we are at our soft memory limit.
+                */
+               if (scn->scn_checkpointing ||
+                   ddi_get_lbolt() - scn->scn_last_checkpoint >
+                   SEC_TO_TICK(zfs_scan_checkpoint_intval)) {
+                       if (!scn->scn_checkpointing)
+                               zfs_dbgmsg("begin scan checkpoint for %s",
+                                   spa->spa_name);
+
+                       scn->scn_checkpointing = B_TRUE;
+                       scn->scn_clearing = B_TRUE;
+               } else {
+                       boolean_t should_clear = dsl_scan_should_clear(scn);
+                       if (should_clear && !scn->scn_clearing) {
+                               zfs_dbgmsg("begin scan clearing for %s",
+                                   spa->spa_name);
+                               scn->scn_clearing = B_TRUE;
+                       } else if (!should_clear && scn->scn_clearing) {
+                               zfs_dbgmsg("finish scan clearing for %s",
+                                   spa->spa_name);
+                               scn->scn_clearing = B_FALSE;
+                       }
                }
-               mutex_exit(&spa->spa_scrub_lock);
+       } else {
+               ASSERT0(scn->scn_checkpointing);
+               ASSERT0(scn->scn_clearing);
        }
 
-       dsl_scan_sync_state(scn, tx);
-}
+       if (!scn->scn_clearing && scn->scn_done_txg == 0) {
+               /* Need to scan metadata for more blocks to scrub */
+               dsl_scan_phys_t *scnp = &scn->scn_phys;
+               taskqid_t prefetch_tqid;
 
-/*
- * This will start a new scan, or restart an existing one.
- */
-void
-dsl_resilver_restart(dsl_pool_t *dp, uint64_t txg)
-{
-       if (txg == 0) {
-               dmu_tx_t *tx;
-               tx = dmu_tx_create_dd(dp->dp_mos_dir);
-               VERIFY(0 == dmu_tx_assign(tx, TXG_WAIT));
+               /*
+                * Calculate the max number of in-flight bytes for pool-wide
+                * scanning operations (minimum 1MB, maximum 1/4 of arc_c_max).
+                * Limits for the issuing phase are done per top-level vdev and
+                * are handled separately.
+                */
+               scn->scn_maxinflight_bytes = MIN(arc_c_max / 4, MAX(1ULL << 20,
+                   zfs_scan_vdev_limit * dsl_scan_count_data_disks(spa)));
+
+               if (scnp->scn_ddt_bookmark.ddb_class <=
+                   scnp->scn_ddt_class_max) {
+                       ASSERT(ZB_IS_ZERO(&scnp->scn_bookmark));
+                       zfs_dbgmsg("doing scan sync for %s txg %llu; "
+                           "ddt bm=%llu/%llu/%llu/%llx",
+                           spa->spa_name,
+                           (longlong_t)tx->tx_txg,
+                           (longlong_t)scnp->scn_ddt_bookmark.ddb_class,
+                           (longlong_t)scnp->scn_ddt_bookmark.ddb_type,
+                           (longlong_t)scnp->scn_ddt_bookmark.ddb_checksum,
+                           (longlong_t)scnp->scn_ddt_bookmark.ddb_cursor);
+               } else {
+                       zfs_dbgmsg("doing scan sync for %s txg %llu; "
+                           "bm=%llu/%llu/%llu/%llu",
+                           spa->spa_name,
+                           (longlong_t)tx->tx_txg,
+                           (longlong_t)scnp->scn_bookmark.zb_objset,
+                           (longlong_t)scnp->scn_bookmark.zb_object,
+                           (longlong_t)scnp->scn_bookmark.zb_level,
+                           (longlong_t)scnp->scn_bookmark.zb_blkid);
+               }
+
+               scn->scn_zio_root = zio_root(dp->dp_spa, NULL,
+                   NULL, ZIO_FLAG_CANFAIL);
+
+               scn->scn_prefetch_stop = B_FALSE;
+               prefetch_tqid = taskq_dispatch(dp->dp_sync_taskq,
+                   dsl_scan_prefetch_thread, scn, TQ_SLEEP);
+               ASSERT(prefetch_tqid != TASKQID_INVALID);
+
+               dsl_pool_config_enter(dp, FTAG);
+               dsl_scan_visit(scn, tx);
+               dsl_pool_config_exit(dp, FTAG);
+
+               mutex_enter(&dp->dp_spa->spa_scrub_lock);
+               scn->scn_prefetch_stop = B_TRUE;
+               cv_broadcast(&spa->spa_scrub_io_cv);
+               mutex_exit(&dp->dp_spa->spa_scrub_lock);
+
+               taskq_wait_id(dp->dp_sync_taskq, prefetch_tqid);
+               (void) zio_wait(scn->scn_zio_root);
+               scn->scn_zio_root = NULL;
+
+               zfs_dbgmsg("scan visited %llu blocks of %s in %llums "
+                   "(%llu os's, %llu holes, %llu < mintxg, "
+                   "%llu in ddt, %llu > maxtxg)",
+                   (longlong_t)scn->scn_visited_this_txg,
+                   spa->spa_name,
+                   (longlong_t)NSEC2MSEC(gethrtime() -
+                   scn->scn_sync_start_time),
+                   (longlong_t)scn->scn_objsets_visited_this_txg,
+                   (longlong_t)scn->scn_holes_this_txg,
+                   (longlong_t)scn->scn_lt_min_this_txg,
+                   (longlong_t)scn->scn_ddt_contained_this_txg,
+                   (longlong_t)scn->scn_gt_max_this_txg);
+
+               if (!scn->scn_suspending) {
+                       ASSERT0(avl_numnodes(&scn->scn_queue));
+                       scn->scn_done_txg = tx->tx_txg + 1;
+                       if (scn->scn_is_sorted) {
+                               scn->scn_checkpointing = B_TRUE;
+                               scn->scn_clearing = B_TRUE;
+                               scn->scn_issued_before_pass +=
+                                   spa->spa_scan_pass_issued;
+                               spa_scan_stat_init(spa);
+                       }
+                       zfs_dbgmsg("scan complete for %s txg %llu",
+                           spa->spa_name,
+                           (longlong_t)tx->tx_txg);
+               }
+       } else if (scn->scn_is_sorted && scn->scn_queues_pending != 0) {
+               ASSERT(scn->scn_clearing);
 
-               txg = dmu_tx_get_txg(tx);
-               dp->dp_scan->scn_restart_txg = txg;
-               dmu_tx_commit(tx);
-       } else {
-               dp->dp_scan->scn_restart_txg = txg;
+               /* need to issue scrubbing IOs from per-vdev queues */
+               scn->scn_zio_root = zio_root(dp->dp_spa, NULL,
+                   NULL, ZIO_FLAG_CANFAIL);
+               scan_io_queues_run(scn);
+               (void) zio_wait(scn->scn_zio_root);
+               scn->scn_zio_root = NULL;
+
+               /* calculate and dprintf the current memory usage */
+               (void) dsl_scan_should_clear(scn);
+               dsl_scan_update_stats(scn);
+
+               zfs_dbgmsg("scan issued %llu blocks for %s (%llu segs) "
+                   "in %llums (avg_block_size = %llu, avg_seg_size = %llu)",
+                   (longlong_t)scn->scn_zios_this_txg,
+                   spa->spa_name,
+                   (longlong_t)scn->scn_segs_this_txg,
+                   (longlong_t)NSEC2MSEC(gethrtime() -
+                   scn->scn_sync_start_time),
+                   (longlong_t)scn->scn_avg_zio_size_this_txg,
+                   (longlong_t)scn->scn_avg_seg_size_this_txg);
+       } else if (scn->scn_done_txg != 0 && scn->scn_done_txg <= tx->tx_txg) {
+               /* Finished with everything. Mark the scrub as complete */
+               zfs_dbgmsg("scan issuing complete txg %llu for %s",
+                   (longlong_t)tx->tx_txg,
+                   spa->spa_name);
+               ASSERT3U(scn->scn_done_txg, !=, 0);
+               ASSERT0(spa->spa_scrub_inflight);
+               ASSERT0(scn->scn_queues_pending);
+               dsl_scan_done(scn, B_TRUE, tx);
+               sync_type = SYNC_MANDATORY;
        }
-       zfs_dbgmsg("restarting resilver txg=%llu", txg);
+
+       dsl_scan_sync_state(scn, tx, sync_type);
 }
 
-boolean_t
-dsl_scan_resilvering(dsl_pool_t *dp)
+static void
+count_block_issued(spa_t *spa, const blkptr_t *bp, boolean_t all)
 {
-       return (dp->dp_scan->scn_phys.scn_state == DSS_SCANNING &&
-           dp->dp_scan->scn_phys.scn_func == POOL_SCAN_RESILVER);
-}
+       /*
+        * Don't count embedded bp's, since we already did the work of
+        * scanning these when we scanned the containing block.
+        */
+       if (BP_IS_EMBEDDED(bp))
+               return;
 
-/*
- * scrub consumers
- */
+       /*
+        * Update the spa's stats on how many bytes we have issued.
+        * Sequential scrubs create a zio for each DVA of the bp. Each
+        * of these will include all DVAs for repair purposes, but the
+        * zio code will only try the first one unless there is an issue.
+        * Therefore, we should only count the first DVA for these IOs.
+        */
+       atomic_add_64(&spa->spa_scan_pass_issued,
+           all ? BP_GET_ASIZE(bp) : DVA_GET_ASIZE(&bp->blk_dva[0]));
+}
 
 static void
 count_block(zfs_all_blkstats_t *zab, const blkptr_t *bp)
 {
-       int i;
-
        /*
         * If we resume after a reboot, zab will be NULL; don't record
         * incomplete stats in that case.
@@ -1780,16 +3901,15 @@ count_block(zfs_all_blkstats_t *zab, const blkptr_t *bp)
        if (zab == NULL)
                return;
 
-       for (i = 0; i < 4; i++) {
+       for (int i = 0; i < 4; i++) {
                int l = (i < 2) ? BP_GET_LEVEL(bp) : DN_MAX_LEVELS;
                int t = (i & 1) ? BP_GET_TYPE(bp) : DMU_OT_TOTAL;
-               int equal;
-               zfs_blkstat_t *zb;
 
                if (t & DMU_OT_NEWTYPE)
                        t = DMU_OT_OTHER;
+               zfs_blkstat_t *zb = &zab->zab_type[l][t];
+               int equal;
 
-               zb = &zab->zab_type[l][t];
                zb->zb_count++;
                zb->zb_asize += BP_GET_ASIZE(bp);
                zb->zb_lsize += BP_GET_LSIZE(bp);
@@ -1819,60 +3939,87 @@ count_block(zfs_all_blkstats_t *zab, const blkptr_t *bp)
 }
 
 static void
-dsl_scan_scrub_done(zio_t *zio)
+scan_io_queue_insert_impl(dsl_scan_io_queue_t *queue, scan_io_t *sio)
 {
-       spa_t *spa = zio->io_spa;
-
-       abd_free(zio->io_abd);
+       avl_index_t idx;
+       dsl_scan_t *scn = queue->q_scn;
 
-       mutex_enter(&spa->spa_scrub_lock);
-       spa->spa_scrub_inflight--;
-       cv_broadcast(&spa->spa_scrub_io_cv);
+       ASSERT(MUTEX_HELD(&queue->q_vd->vdev_scan_io_queue_lock));
 
-       if (zio->io_error && (zio->io_error != ECKSUM ||
-           !(zio->io_flags & ZIO_FLAG_SPECULATIVE))) {
-               spa->spa_dsl_pool->dp_scan->scn_phys.scn_errors++;
+       if (unlikely(avl_is_empty(&queue->q_sios_by_addr)))
+               atomic_add_64(&scn->scn_queues_pending, 1);
+       if (avl_find(&queue->q_sios_by_addr, sio, &idx) != NULL) {
+               /* block is already scheduled for reading */
+               sio_free(sio);
+               return;
        }
-       mutex_exit(&spa->spa_scrub_lock);
+       avl_insert(&queue->q_sios_by_addr, sio, idx);
+       queue->q_sio_memused += SIO_GET_MUSED(sio);
+       range_tree_add(queue->q_exts_by_addr, SIO_GET_OFFSET(sio),
+           SIO_GET_ASIZE(sio));
 }
 
-static boolean_t
-dsl_scan_need_resilver(spa_t *spa, const dva_t *dva, size_t psize,
-    uint64_t phys_birth)
+/*
+ * Given all the info we got from our metadata scanning process, we
+ * construct a scan_io_t and insert it into the scan sorting queue. The
+ * I/O must already be suitable for us to process. This is controlled
+ * by dsl_scan_enqueue().
+ */
+static void
+scan_io_queue_insert(dsl_scan_io_queue_t *queue, const blkptr_t *bp, int dva_i,
+    int zio_flags, const zbookmark_phys_t *zb)
 {
-       vdev_t *vd;
+       scan_io_t *sio = sio_alloc(BP_GET_NDVAS(bp));
 
-       if (DVA_GET_GANG(dva)) {
-               /*
-                * Gang members may be spread across multiple
-                * vdevs, so the best estimate we have is the
-                * scrub range, which has already been checked.
-                * XXX -- it would be better to change our
-                * allocation policy to ensure that all
-                * gang members reside on the same vdev.
-                */
-               return (B_TRUE);
-       }
+       ASSERT0(BP_IS_GANG(bp));
+       ASSERT(MUTEX_HELD(&queue->q_vd->vdev_scan_io_queue_lock));
 
-       vd = vdev_lookup_top(spa, DVA_GET_VDEV(dva));
+       bp2sio(bp, sio, dva_i);
+       sio->sio_flags = zio_flags;
+       sio->sio_zb = *zb;
 
-       /*
-        * Check if the txg falls within the range which must be
-        * resilvered.  DVAs outside this range can always be skipped.
-        */
-       if (!vdev_dtl_contains(vd, DTL_PARTIAL, phys_birth, 1))
-               return (B_FALSE);
+       queue->q_last_ext_addr = -1;
+       scan_io_queue_insert_impl(queue, sio);
+}
+
+/*
+ * Given a set of I/O parameters as discovered by the metadata traversal
+ * process, attempts to place the I/O into the sorted queues (if allowed),
+ * or immediately executes the I/O.
+ */
+static void
+dsl_scan_enqueue(dsl_pool_t *dp, const blkptr_t *bp, int zio_flags,
+    const zbookmark_phys_t *zb)
+{
+       spa_t *spa = dp->dp_spa;
+
+       ASSERT(!BP_IS_EMBEDDED(bp));
 
        /*
-        * Check if the top-level vdev must resilver this offset.
-        * When the offset does not intersect with a dirty leaf DTL
-        * then it may be possible to skip the resilver IO.  The psize
-        * is provided instead of asize to simplify the check for RAIDZ.
+        * Gang blocks are hard to issue sequentially, so we just issue them
+        * here immediately instead of queuing them.
         */
-       if (!vdev_dtl_need_resilver(vd, DVA_GET_OFFSET(dva), psize))
-               return (B_FALSE);
+       if (!dp->dp_scan->scn_is_sorted || BP_IS_GANG(bp)) {
+               scan_exec_io(dp, bp, zio_flags, zb, NULL);
+               return;
+       }
 
-       return (B_TRUE);
+       for (int i = 0; i < BP_GET_NDVAS(bp); i++) {
+               dva_t dva;
+               vdev_t *vdev;
+
+               dva = bp->blk_dva[i];
+               vdev = vdev_lookup_top(spa, DVA_GET_VDEV(&dva));
+               ASSERT(vdev != NULL);
+
+               mutex_enter(&vdev->vdev_scan_io_queue_lock);
+               if (vdev->vdev_scan_io_queue == NULL)
+                       vdev->vdev_scan_io_queue = scan_io_queue_create(vdev);
+               ASSERT(dp->dp_scan != NULL);
+               scan_io_queue_insert(vdev->vdev_scan_io_queue, bp,
+                   i, zio_flags, zb);
+               mutex_exit(&vdev->vdev_scan_io_queue_lock);
+       }
 }
 
 static int
@@ -1880,48 +4027,46 @@ dsl_scan_scrub_cb(dsl_pool_t *dp,
     const blkptr_t *bp, const zbookmark_phys_t *zb)
 {
        dsl_scan_t *scn = dp->dp_scan;
-       size_t psize = BP_GET_PSIZE(bp);
        spa_t *spa = dp->dp_spa;
        uint64_t phys_birth = BP_PHYSICAL_BIRTH(bp);
+       size_t psize = BP_GET_PSIZE(bp);
        boolean_t needs_io = B_FALSE;
        int zio_flags = ZIO_FLAG_SCAN_THREAD | ZIO_FLAG_RAW | ZIO_FLAG_CANFAIL;
-       int scan_delay = 0;
-       int d;
 
+       count_block(dp->dp_blkstats, bp);
        if (phys_birth <= scn->scn_phys.scn_min_txg ||
-           phys_birth >= scn->scn_phys.scn_max_txg)
+           phys_birth >= scn->scn_phys.scn_max_txg) {
+               count_block_issued(spa, bp, B_TRUE);
                return (0);
+       }
 
-       count_block(dp->dp_blkstats, bp);
-
-       if (BP_IS_EMBEDDED(bp))
-               return (0);
+       /* Embedded BP's have phys_birth==0, so we reject them above. */
+       ASSERT(!BP_IS_EMBEDDED(bp));
 
        ASSERT(DSL_SCAN_IS_SCRUB_RESILVER(scn));
        if (scn->scn_phys.scn_func == POOL_SCAN_SCRUB) {
                zio_flags |= ZIO_FLAG_SCRUB;
                needs_io = B_TRUE;
-               scan_delay = zfs_scrub_delay;
        } else {
                ASSERT3U(scn->scn_phys.scn_func, ==, POOL_SCAN_RESILVER);
                zio_flags |= ZIO_FLAG_RESILVER;
                needs_io = B_FALSE;
-               scan_delay = zfs_resilver_delay;
        }
 
        /* If it's an intent log block, failure is expected. */
        if (zb->zb_level == ZB_ZIL_LEVEL)
                zio_flags |= ZIO_FLAG_SPECULATIVE;
 
-       for (d = 0; d < BP_GET_NDVAS(bp); d++) {
+       for (int d = 0; d < BP_GET_NDVAS(bp); d++) {
                const dva_t *dva = &bp->blk_dva[d];
 
                /*
                 * Keep track of how much data we've examined so that
-                * zpool(1M) status can make useful progress reports.
+                * zpool(8) status can make useful progress reports.
                 */
-               scn->scn_phys.scn_examined += DVA_GET_ASIZE(dva);
-               spa->spa_scan_pass_exam += DVA_GET_ASIZE(dva);
+               uint64_t asize = DVA_GET_ASIZE(dva);
+               scn->scn_phys.scn_examined += asize;
+               spa->spa_scan_pass_exam += asize;
 
                /* if it's a resilver, this may not be in the target range */
                if (!needs_io)
@@ -1930,93 +4075,487 @@ dsl_scan_scrub_cb(dsl_pool_t *dp,
        }
 
        if (needs_io && !zfs_no_scrub_io) {
-               vdev_t *rvd = spa->spa_root_vdev;
-               uint64_t maxinflight = rvd->vdev_children * zfs_top_maxinflight;
+               dsl_scan_enqueue(dp, bp, zio_flags, zb);
+       } else {
+               count_block_issued(spa, bp, B_TRUE);
+       }
+
+       /* do not relocate this block */
+       return (0);
+}
+
+static void
+dsl_scan_scrub_done(zio_t *zio)
+{
+       spa_t *spa = zio->io_spa;
+       blkptr_t *bp = zio->io_bp;
+       dsl_scan_io_queue_t *queue = zio->io_private;
+
+       abd_free(zio->io_abd);
+
+       if (queue == NULL) {
+               mutex_enter(&spa->spa_scrub_lock);
+               ASSERT3U(spa->spa_scrub_inflight, >=, BP_GET_PSIZE(bp));
+               spa->spa_scrub_inflight -= BP_GET_PSIZE(bp);
+               cv_broadcast(&spa->spa_scrub_io_cv);
+               mutex_exit(&spa->spa_scrub_lock);
+       } else {
+               mutex_enter(&queue->q_vd->vdev_scan_io_queue_lock);
+               ASSERT3U(queue->q_inflight_bytes, >=, BP_GET_PSIZE(bp));
+               queue->q_inflight_bytes -= BP_GET_PSIZE(bp);
+               cv_broadcast(&queue->q_zio_cv);
+               mutex_exit(&queue->q_vd->vdev_scan_io_queue_lock);
+       }
+
+       if (zio->io_error && (zio->io_error != ECKSUM ||
+           !(zio->io_flags & ZIO_FLAG_SPECULATIVE))) {
+               atomic_inc_64(&spa->spa_dsl_pool->dp_scan->scn_phys.scn_errors);
+       }
+}
+
+/*
+ * Given a scanning zio's information, executes the zio. The zio need
+ * not necessarily be only sortable, this function simply executes the
+ * zio, no matter what it is. The optional queue argument allows the
+ * caller to specify that they want per top level vdev IO rate limiting
+ * instead of the legacy global limiting.
+ */
+static void
+scan_exec_io(dsl_pool_t *dp, const blkptr_t *bp, int zio_flags,
+    const zbookmark_phys_t *zb, dsl_scan_io_queue_t *queue)
+{
+       spa_t *spa = dp->dp_spa;
+       dsl_scan_t *scn = dp->dp_scan;
+       size_t size = BP_GET_PSIZE(bp);
+       abd_t *data = abd_alloc_for_io(size, B_FALSE);
+       zio_t *pio;
 
+       if (queue == NULL) {
+               ASSERT3U(scn->scn_maxinflight_bytes, >, 0);
                mutex_enter(&spa->spa_scrub_lock);
-               while (spa->spa_scrub_inflight >= maxinflight)
+               while (spa->spa_scrub_inflight >= scn->scn_maxinflight_bytes)
                        cv_wait(&spa->spa_scrub_io_cv, &spa->spa_scrub_lock);
-               spa->spa_scrub_inflight++;
+               spa->spa_scrub_inflight += BP_GET_PSIZE(bp);
                mutex_exit(&spa->spa_scrub_lock);
+               pio = scn->scn_zio_root;
+       } else {
+               kmutex_t *q_lock = &queue->q_vd->vdev_scan_io_queue_lock;
+
+               ASSERT3U(queue->q_maxinflight_bytes, >, 0);
+               mutex_enter(q_lock);
+               while (queue->q_inflight_bytes >= queue->q_maxinflight_bytes)
+                       cv_wait(&queue->q_zio_cv, q_lock);
+               queue->q_inflight_bytes += BP_GET_PSIZE(bp);
+               pio = queue->q_zio;
+               mutex_exit(q_lock);
+       }
 
-               /*
-                * If we're seeing recent (zfs_scan_idle) "important" I/Os
-                * then throttle our workload to limit the impact of a scan.
-                */
-               if (ddi_get_lbolt64() - spa->spa_last_io <= zfs_scan_idle)
-                       delay(scan_delay);
+       ASSERT(pio != NULL);
+       count_block_issued(spa, bp, queue == NULL);
+       zio_nowait(zio_read(pio, spa, bp, data, size, dsl_scan_scrub_done,
+           queue, ZIO_PRIORITY_SCRUB, zio_flags, zb));
+}
+
+/*
+ * This is the primary extent sorting algorithm. We balance two parameters:
+ * 1) how many bytes of I/O are in an extent
+ * 2) how well the extent is filled with I/O (as a fraction of its total size)
+ * Since we allow extents to have gaps between their constituent I/Os, it's
+ * possible to have a fairly large extent that contains the same amount of
+ * I/O bytes than a much smaller extent, which just packs the I/O more tightly.
+ * The algorithm sorts based on a score calculated from the extent's size,
+ * the relative fill volume (in %) and a "fill weight" parameter that controls
+ * the split between whether we prefer larger extents or more well populated
+ * extents:
+ *
+ * SCORE = FILL_IN_BYTES + (FILL_IN_PERCENT * FILL_IN_BYTES * FILL_WEIGHT)
+ *
+ * Example:
+ * 1) assume extsz = 64 MiB
+ * 2) assume fill = 32 MiB (extent is half full)
+ * 3) assume fill_weight = 3
+ * 4)  SCORE = 32M + (((32M * 100) / 64M) * 3 * 32M) / 100
+ *     SCORE = 32M + (50 * 3 * 32M) / 100
+ *     SCORE = 32M + (4800M / 100)
+ *     SCORE = 32M + 48M
+ *              ^     ^
+ *              |     +--- final total relative fill-based score
+ *              +--------- final total fill-based score
+ *     SCORE = 80M
+ *
+ * As can be seen, at fill_ratio=3, the algorithm is slightly biased towards
+ * extents that are more completely filled (in a 3:2 ratio) vs just larger.
+ * Note that as an optimization, we replace multiplication and division by
+ * 100 with bitshifting by 7 (which effectively multiplies and divides by 128).
+ *
+ * Since we do not care if one extent is only few percent better than another,
+ * compress the score into 6 bits via binary logarithm AKA highbit64() and
+ * put into otherwise unused due to ashift high bits of offset.  This allows
+ * to reduce q_exts_by_size B-tree elements to only 64 bits and compare them
+ * with single operation.  Plus it makes scrubs more sequential and reduces
+ * chances that minor extent change move it within the B-tree.
+ */
+static int
+ext_size_compare(const void *x, const void *y)
+{
+       const uint64_t *a = x, *b = y;
+
+       return (TREE_CMP(*a, *b));
+}
+
+static void
+ext_size_create(range_tree_t *rt, void *arg)
+{
+       (void) rt;
+       zfs_btree_t *size_tree = arg;
+
+       zfs_btree_create(size_tree, ext_size_compare, sizeof (uint64_t));
+}
+
+static void
+ext_size_destroy(range_tree_t *rt, void *arg)
+{
+       (void) rt;
+       zfs_btree_t *size_tree = arg;
+       ASSERT0(zfs_btree_numnodes(size_tree));
+
+       zfs_btree_destroy(size_tree);
+}
+
+static uint64_t
+ext_size_value(range_tree_t *rt, range_seg_gap_t *rsg)
+{
+       (void) rt;
+       uint64_t size = rsg->rs_end - rsg->rs_start;
+       uint64_t score = rsg->rs_fill + ((((rsg->rs_fill << 7) / size) *
+           fill_weight * rsg->rs_fill) >> 7);
+       ASSERT3U(rt->rt_shift, >=, 8);
+       return (((uint64_t)(64 - highbit64(score)) << 56) | rsg->rs_start);
+}
+
+static void
+ext_size_add(range_tree_t *rt, range_seg_t *rs, void *arg)
+{
+       zfs_btree_t *size_tree = arg;
+       ASSERT3U(rt->rt_type, ==, RANGE_SEG_GAP);
+       uint64_t v = ext_size_value(rt, (range_seg_gap_t *)rs);
+       zfs_btree_add(size_tree, &v);
+}
+
+static void
+ext_size_remove(range_tree_t *rt, range_seg_t *rs, void *arg)
+{
+       zfs_btree_t *size_tree = arg;
+       ASSERT3U(rt->rt_type, ==, RANGE_SEG_GAP);
+       uint64_t v = ext_size_value(rt, (range_seg_gap_t *)rs);
+       zfs_btree_remove(size_tree, &v);
+}
+
+static void
+ext_size_vacate(range_tree_t *rt, void *arg)
+{
+       zfs_btree_t *size_tree = arg;
+       zfs_btree_clear(size_tree);
+       zfs_btree_destroy(size_tree);
+
+       ext_size_create(rt, arg);
+}
 
-               zio_nowait(zio_read(NULL, spa, bp,
-                   abd_alloc_for_io(psize, B_FALSE),
-                   psize, dsl_scan_scrub_done, NULL,
-                   ZIO_PRIORITY_SCRUB, zio_flags, zb));
+static const range_tree_ops_t ext_size_ops = {
+       .rtop_create = ext_size_create,
+       .rtop_destroy = ext_size_destroy,
+       .rtop_add = ext_size_add,
+       .rtop_remove = ext_size_remove,
+       .rtop_vacate = ext_size_vacate
+};
+
+/*
+ * Comparator for the q_sios_by_addr tree. Sorting is simply performed
+ * based on LBA-order (from lowest to highest).
+ */
+static int
+sio_addr_compare(const void *x, const void *y)
+{
+       const scan_io_t *a = x, *b = y;
+
+       return (TREE_CMP(SIO_GET_OFFSET(a), SIO_GET_OFFSET(b)));
+}
+
+/* IO queues are created on demand when they are needed. */
+static dsl_scan_io_queue_t *
+scan_io_queue_create(vdev_t *vd)
+{
+       dsl_scan_t *scn = vd->vdev_spa->spa_dsl_pool->dp_scan;
+       dsl_scan_io_queue_t *q = kmem_zalloc(sizeof (*q), KM_SLEEP);
+
+       q->q_scn = scn;
+       q->q_vd = vd;
+       q->q_sio_memused = 0;
+       q->q_last_ext_addr = -1;
+       cv_init(&q->q_zio_cv, NULL, CV_DEFAULT, NULL);
+       q->q_exts_by_addr = range_tree_create_gap(&ext_size_ops, RANGE_SEG_GAP,
+           &q->q_exts_by_size, 0, vd->vdev_ashift, zfs_scan_max_ext_gap);
+       avl_create(&q->q_sios_by_addr, sio_addr_compare,
+           sizeof (scan_io_t), offsetof(scan_io_t, sio_nodes.sio_addr_node));
+
+       return (q);
+}
+
+/*
+ * Destroys a scan queue and all segments and scan_io_t's contained in it.
+ * No further execution of I/O occurs, anything pending in the queue is
+ * simply freed without being executed.
+ */
+void
+dsl_scan_io_queue_destroy(dsl_scan_io_queue_t *queue)
+{
+       dsl_scan_t *scn = queue->q_scn;
+       scan_io_t *sio;
+       void *cookie = NULL;
+
+       ASSERT(MUTEX_HELD(&queue->q_vd->vdev_scan_io_queue_lock));
+
+       if (!avl_is_empty(&queue->q_sios_by_addr))
+               atomic_add_64(&scn->scn_queues_pending, -1);
+       while ((sio = avl_destroy_nodes(&queue->q_sios_by_addr, &cookie)) !=
+           NULL) {
+               ASSERT(range_tree_contains(queue->q_exts_by_addr,
+                   SIO_GET_OFFSET(sio), SIO_GET_ASIZE(sio)));
+               queue->q_sio_memused -= SIO_GET_MUSED(sio);
+               sio_free(sio);
        }
 
-       /* do not relocate this block */
-       return (0);
+       ASSERT0(queue->q_sio_memused);
+       range_tree_vacate(queue->q_exts_by_addr, NULL, queue);
+       range_tree_destroy(queue->q_exts_by_addr);
+       avl_destroy(&queue->q_sios_by_addr);
+       cv_destroy(&queue->q_zio_cv);
+
+       kmem_free(queue, sizeof (*queue));
 }
 
-int
-dsl_scan(dsl_pool_t *dp, pool_scan_func_t func)
+/*
+ * Properly transfers a dsl_scan_queue_t from `svd' to `tvd'. This is
+ * called on behalf of vdev_top_transfer when creating or destroying
+ * a mirror vdev due to zpool attach/detach.
+ */
+void
+dsl_scan_io_queue_vdev_xfer(vdev_t *svd, vdev_t *tvd)
 {
-       spa_t *spa = dp->dp_spa;
+       mutex_enter(&svd->vdev_scan_io_queue_lock);
+       mutex_enter(&tvd->vdev_scan_io_queue_lock);
+
+       VERIFY3P(tvd->vdev_scan_io_queue, ==, NULL);
+       tvd->vdev_scan_io_queue = svd->vdev_scan_io_queue;
+       svd->vdev_scan_io_queue = NULL;
+       if (tvd->vdev_scan_io_queue != NULL)
+               tvd->vdev_scan_io_queue->q_vd = tvd;
+
+       mutex_exit(&tvd->vdev_scan_io_queue_lock);
+       mutex_exit(&svd->vdev_scan_io_queue_lock);
+}
+
+static void
+scan_io_queues_destroy(dsl_scan_t *scn)
+{
+       vdev_t *rvd = scn->scn_dp->dp_spa->spa_root_vdev;
+
+       for (uint64_t i = 0; i < rvd->vdev_children; i++) {
+               vdev_t *tvd = rvd->vdev_child[i];
+
+               mutex_enter(&tvd->vdev_scan_io_queue_lock);
+               if (tvd->vdev_scan_io_queue != NULL)
+                       dsl_scan_io_queue_destroy(tvd->vdev_scan_io_queue);
+               tvd->vdev_scan_io_queue = NULL;
+               mutex_exit(&tvd->vdev_scan_io_queue_lock);
+       }
+}
+
+static void
+dsl_scan_freed_dva(spa_t *spa, const blkptr_t *bp, int dva_i)
+{
+       dsl_pool_t *dp = spa->spa_dsl_pool;
+       dsl_scan_t *scn = dp->dp_scan;
+       vdev_t *vdev;
+       kmutex_t *q_lock;
+       dsl_scan_io_queue_t *queue;
+       scan_io_t *srch_sio, *sio;
+       avl_index_t idx;
+       uint64_t start, size;
+
+       vdev = vdev_lookup_top(spa, DVA_GET_VDEV(&bp->blk_dva[dva_i]));
+       ASSERT(vdev != NULL);
+       q_lock = &vdev->vdev_scan_io_queue_lock;
+       queue = vdev->vdev_scan_io_queue;
+
+       mutex_enter(q_lock);
+       if (queue == NULL) {
+               mutex_exit(q_lock);
+               return;
+       }
+
+       srch_sio = sio_alloc(BP_GET_NDVAS(bp));
+       bp2sio(bp, srch_sio, dva_i);
+       start = SIO_GET_OFFSET(srch_sio);
+       size = SIO_GET_ASIZE(srch_sio);
 
        /*
-        * Purge all vdev caches and probe all devices.  We do this here
-        * rather than in sync context because this requires a writer lock
-        * on the spa_config lock, which we can't do from sync context.  The
-        * spa_scrub_reopen flag indicates that vdev_open() should not
-        * attempt to start another scrub.
+        * We can find the zio in two states:
+        * 1) Cold, just sitting in the queue of zio's to be issued at
+        *      some point in the future. In this case, all we do is
+        *      remove the zio from the q_sios_by_addr tree, decrement
+        *      its data volume from the containing range_seg_t and
+        *      resort the q_exts_by_size tree to reflect that the
+        *      range_seg_t has lost some of its 'fill'. We don't shorten
+        *      the range_seg_t - this is usually rare enough not to be
+        *      worth the extra hassle of trying keep track of precise
+        *      extent boundaries.
+        * 2) Hot, where the zio is currently in-flight in
+        *      dsl_scan_issue_ios. In this case, we can't simply
+        *      reach in and stop the in-flight zio's, so we instead
+        *      block the caller. Eventually, dsl_scan_issue_ios will
+        *      be done with issuing the zio's it gathered and will
+        *      signal us.
         */
-       spa_vdev_state_enter(spa, SCL_NONE);
-       spa->spa_scrub_reopen = B_TRUE;
-       vdev_reopen(spa->spa_root_vdev);
-       spa->spa_scrub_reopen = B_FALSE;
-       (void) spa_vdev_state_exit(spa, NULL, 0);
+       sio = avl_find(&queue->q_sios_by_addr, srch_sio, &idx);
+       sio_free(srch_sio);
 
-       return (dsl_sync_task(spa_name(spa), dsl_scan_setup_check,
-           dsl_scan_setup_sync, &func, 0, ZFS_SPACE_CHECK_NONE));
+       if (sio != NULL) {
+               blkptr_t tmpbp;
+
+               /* Got it while it was cold in the queue */
+               ASSERT3U(start, ==, SIO_GET_OFFSET(sio));
+               ASSERT3U(size, ==, SIO_GET_ASIZE(sio));
+               avl_remove(&queue->q_sios_by_addr, sio);
+               if (avl_is_empty(&queue->q_sios_by_addr))
+                       atomic_add_64(&scn->scn_queues_pending, -1);
+               queue->q_sio_memused -= SIO_GET_MUSED(sio);
+
+               ASSERT(range_tree_contains(queue->q_exts_by_addr, start, size));
+               range_tree_remove_fill(queue->q_exts_by_addr, start, size);
+
+               /* count the block as though we issued it */
+               sio2bp(sio, &tmpbp);
+               count_block_issued(spa, &tmpbp, B_FALSE);
+
+               sio_free(sio);
+       }
+       mutex_exit(q_lock);
 }
 
-static boolean_t
-dsl_scan_restarting(dsl_scan_t *scn, dmu_tx_t *tx)
+/*
+ * Callback invoked when a zio_free() zio is executing. This needs to be
+ * intercepted to prevent the zio from deallocating a particular portion
+ * of disk space and it then getting reallocated and written to, while we
+ * still have it queued up for processing.
+ */
+void
+dsl_scan_freed(spa_t *spa, const blkptr_t *bp)
 {
-       return (scn->scn_restart_txg != 0 &&
-           scn->scn_restart_txg <= tx->tx_txg);
+       dsl_pool_t *dp = spa->spa_dsl_pool;
+       dsl_scan_t *scn = dp->dp_scan;
+
+       ASSERT(!BP_IS_EMBEDDED(bp));
+       ASSERT(scn != NULL);
+       if (!dsl_scan_is_running(scn))
+               return;
+
+       for (int i = 0; i < BP_GET_NDVAS(bp); i++)
+               dsl_scan_freed_dva(spa, bp, i);
 }
 
-#if defined(_KERNEL) && defined(HAVE_SPL)
-module_param(zfs_top_maxinflight, int, 0644);
-MODULE_PARM_DESC(zfs_top_maxinflight, "Max I/Os per top-level");
+/*
+ * Check if a vdev needs resilvering (non-empty DTL), if so, and resilver has
+ * not started, start it. Otherwise, only restart if max txg in DTL range is
+ * greater than the max txg in the current scan. If the DTL max is less than
+ * the scan max, then the vdev has not missed any new data since the resilver
+ * started, so a restart is not needed.
+ */
+void
+dsl_scan_assess_vdev(dsl_pool_t *dp, vdev_t *vd)
+{
+       uint64_t min, max;
 
-module_param(zfs_resilver_delay, int, 0644);
-MODULE_PARM_DESC(zfs_resilver_delay, "Number of ticks to delay resilver");
+       if (!vdev_resilver_needed(vd, &min, &max))
+               return;
 
-module_param(zfs_scrub_delay, int, 0644);
-MODULE_PARM_DESC(zfs_scrub_delay, "Number of ticks to delay scrub");
+       if (!dsl_scan_resilvering(dp)) {
+               spa_async_request(dp->dp_spa, SPA_ASYNC_RESILVER);
+               return;
+       }
 
-module_param(zfs_scan_idle, int, 0644);
-MODULE_PARM_DESC(zfs_scan_idle, "Idle window in clock ticks");
+       if (max <= dp->dp_scan->scn_phys.scn_max_txg)
+               return;
 
-module_param(zfs_scan_min_time_ms, int, 0644);
-MODULE_PARM_DESC(zfs_scan_min_time_ms, "Min millisecs to scrub per txg");
+       /* restart is needed, check if it can be deferred */
+       if (spa_feature_is_enabled(dp->dp_spa, SPA_FEATURE_RESILVER_DEFER))
+               vdev_defer_resilver(vd);
+       else
+               spa_async_request(dp->dp_spa, SPA_ASYNC_RESILVER);
+}
 
-module_param(zfs_free_min_time_ms, int, 0644);
-MODULE_PARM_DESC(zfs_free_min_time_ms, "Min millisecs to free per txg");
+ZFS_MODULE_PARAM(zfs, zfs_, scan_vdev_limit, U64, ZMOD_RW,
+       "Max bytes in flight per leaf vdev for scrubs and resilvers");
 
-module_param(zfs_resilver_min_time_ms, int, 0644);
-MODULE_PARM_DESC(zfs_resilver_min_time_ms, "Min millisecs to resilver per txg");
+ZFS_MODULE_PARAM(zfs, zfs_, scrub_min_time_ms, UINT, ZMOD_RW,
+       "Min millisecs to scrub per txg");
 
-module_param(zfs_no_scrub_io, int, 0644);
-MODULE_PARM_DESC(zfs_no_scrub_io, "Set to disable scrub I/O");
+ZFS_MODULE_PARAM(zfs, zfs_, obsolete_min_time_ms, UINT, ZMOD_RW,
+       "Min millisecs to obsolete per txg");
 
-module_param(zfs_no_scrub_prefetch, int, 0644);
-MODULE_PARM_DESC(zfs_no_scrub_prefetch, "Set to disable scrub prefetching");
+ZFS_MODULE_PARAM(zfs, zfs_, free_min_time_ms, UINT, ZMOD_RW,
+       "Min millisecs to free per txg");
 
-/* CSTYLED */
-module_param(zfs_free_max_blocks, ulong, 0644);
-MODULE_PARM_DESC(zfs_free_max_blocks, "Max number of blocks freed in one txg");
+ZFS_MODULE_PARAM(zfs, zfs_, resilver_min_time_ms, UINT, ZMOD_RW,
+       "Min millisecs to resilver per txg");
 
-module_param(zfs_free_bpobj_enabled, int, 0644);
-MODULE_PARM_DESC(zfs_free_bpobj_enabled, "Enable processing of the free_bpobj");
-#endif
+ZFS_MODULE_PARAM(zfs, zfs_, scan_suspend_progress, INT, ZMOD_RW,
+       "Set to prevent scans from progressing");
+
+ZFS_MODULE_PARAM(zfs, zfs_, no_scrub_io, INT, ZMOD_RW,
+       "Set to disable scrub I/O");
+
+ZFS_MODULE_PARAM(zfs, zfs_, no_scrub_prefetch, INT, ZMOD_RW,
+       "Set to disable scrub prefetching");
+
+ZFS_MODULE_PARAM(zfs, zfs_, async_block_max_blocks, U64, ZMOD_RW,
+       "Max number of blocks freed in one txg");
+
+ZFS_MODULE_PARAM(zfs, zfs_, max_async_dedup_frees, U64, ZMOD_RW,
+       "Max number of dedup blocks freed in one txg");
+
+ZFS_MODULE_PARAM(zfs, zfs_, free_bpobj_enabled, INT, ZMOD_RW,
+       "Enable processing of the free_bpobj");
+
+ZFS_MODULE_PARAM(zfs, zfs_, scan_blkstats, INT, ZMOD_RW,
+       "Enable block statistics calculation during scrub");
+
+ZFS_MODULE_PARAM(zfs, zfs_, scan_mem_lim_fact, UINT, ZMOD_RW,
+       "Fraction of RAM for scan hard limit");
+
+ZFS_MODULE_PARAM(zfs, zfs_, scan_issue_strategy, UINT, ZMOD_RW,
+       "IO issuing strategy during scrubbing. 0 = default, 1 = LBA, 2 = size");
+
+ZFS_MODULE_PARAM(zfs, zfs_, scan_legacy, INT, ZMOD_RW,
+       "Scrub using legacy non-sequential method");
+
+ZFS_MODULE_PARAM(zfs, zfs_, scan_checkpoint_intval, UINT, ZMOD_RW,
+       "Scan progress on-disk checkpointing interval");
+
+ZFS_MODULE_PARAM(zfs, zfs_, scan_max_ext_gap, U64, ZMOD_RW,
+       "Max gap in bytes between sequential scrub / resilver I/Os");
+
+ZFS_MODULE_PARAM(zfs, zfs_, scan_mem_lim_soft_fact, UINT, ZMOD_RW,
+       "Fraction of hard limit used as soft limit");
+
+ZFS_MODULE_PARAM(zfs, zfs_, scan_strict_mem_lim, INT, ZMOD_RW,
+       "Tunable to attempt to reduce lock contention");
+
+ZFS_MODULE_PARAM(zfs, zfs_, scan_fill_weight, UINT, ZMOD_RW,
+       "Tunable to adjust bias towards more filled segments during scans");
+
+ZFS_MODULE_PARAM(zfs, zfs_, scan_report_txgs, UINT, ZMOD_RW,
+       "Tunable to report resilver performance over the last N txgs");
+
+ZFS_MODULE_PARAM(zfs, zfs_, resilver_disable_defer, INT, ZMOD_RW,
+       "Process all resilvers immediately");