Illumos 5960, 5925

[mirror_zfs.git] / module / zfs / dmu_traverse.c

/*
 * CDDL HEADER START
 *
 * The contents of this file are subject to the terms of the
 * Common Development and Distribution License (the "License").
 * 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.
 * See the License for the specific language governing permissions
 * and limitations under the License.
 *
 * When distributing Covered Code, include this CDDL HEADER in each
 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
 * If applicable, add the following below this CDDL HEADER, with the
 * fields enclosed by brackets "[]" replaced with your own identifying
 * information: Portions Copyright [yyyy] [name of copyright owner]
 *
 * CDDL HEADER END
 */
/*
 * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
 * Copyright (c) 2012, 2014 by Delphix. All rights reserved.
 */

#include <sys/zfs_context.h>
#include <sys/dmu_objset.h>
#include <sys/dmu_traverse.h>
#include <sys/dsl_dataset.h>
#include <sys/dsl_dir.h>
#include <sys/dsl_pool.h>
#include <sys/dnode.h>
#include <sys/spa.h>
#include <sys/zio.h>
#include <sys/dmu_impl.h>
#include <sys/sa.h>
#include <sys/sa_impl.h>
#include <sys/callb.h>
#include <sys/zfeature.h>

int32_t zfs_pd_bytes_max = 50 * 1024 * 1024;    /* 50MB */

typedef struct prefetch_data {
        kmutex_t pd_mtx;
        kcondvar_t pd_cv;
        int32_t pd_bytes_fetched;
        int pd_flags;
        boolean_t pd_cancel;
        boolean_t pd_exited;
} prefetch_data_t;

typedef struct traverse_data {
        spa_t *td_spa;
        uint64_t td_objset;
        blkptr_t *td_rootbp;
        uint64_t td_min_txg;
        zbookmark_phys_t *td_resume;
        int td_flags;
        prefetch_data_t *td_pfd;
        boolean_t td_paused;
        uint64_t td_hole_birth_enabled_txg;
        blkptr_cb_t *td_func;
        void *td_arg;
} traverse_data_t;

static int traverse_dnode(traverse_data_t *td, const dnode_phys_t *dnp,
    uint64_t objset, uint64_t object);
static void prefetch_dnode_metadata(traverse_data_t *td, const dnode_phys_t *,
    uint64_t objset, uint64_t object);

static int
traverse_zil_block(zilog_t *zilog, blkptr_t *bp, void *arg, uint64_t claim_txg)
{
        traverse_data_t *td = arg;
        zbookmark_phys_t zb;

        if (BP_IS_HOLE(bp))
                return (0);

        if (claim_txg == 0 && bp->blk_birth >= spa_first_txg(td->td_spa))
                return (0);

        SET_BOOKMARK(&zb, td->td_objset, ZB_ZIL_OBJECT, ZB_ZIL_LEVEL,
            bp->blk_cksum.zc_word[ZIL_ZC_SEQ]);

        (void) td->td_func(td->td_spa, zilog, bp, &zb, NULL, td->td_arg);

        return (0);
}

static int
traverse_zil_record(zilog_t *zilog, lr_t *lrc, void *arg, uint64_t claim_txg)
{
        traverse_data_t *td = arg;

        if (lrc->lrc_txtype == TX_WRITE) {
                lr_write_t *lr = (lr_write_t *)lrc;
                blkptr_t *bp = &lr->lr_blkptr;
                zbookmark_phys_t zb;

                if (BP_IS_HOLE(bp))
                        return (0);

                if (claim_txg == 0 || bp->blk_birth < claim_txg)
                        return (0);

                SET_BOOKMARK(&zb, td->td_objset, lr->lr_foid,
                    ZB_ZIL_LEVEL, lr->lr_offset / BP_GET_LSIZE(bp));

                (void) td->td_func(td->td_spa, zilog, bp, &zb, NULL,
                    td->td_arg);
        }
        return (0);
}

static void
traverse_zil(traverse_data_t *td, zil_header_t *zh)
{
        uint64_t claim_txg = zh->zh_claim_txg;
        zilog_t *zilog;

        /*
         * We only want to visit blocks that have been claimed but not yet
         * replayed; plus, in read-only mode, blocks that are already stable.
         */
        if (claim_txg == 0 && spa_writeable(td->td_spa))
                return;

        zilog = zil_alloc(spa_get_dsl(td->td_spa)->dp_meta_objset, zh);

        (void) zil_parse(zilog, traverse_zil_block, traverse_zil_record, td,
            claim_txg);

        zil_free(zilog);
}

typedef enum resume_skip {
        RESUME_SKIP_ALL,
        RESUME_SKIP_NONE,
        RESUME_SKIP_CHILDREN
} resume_skip_t;

/*
 * Returns RESUME_SKIP_ALL if td indicates that we are resuming a traversal and
 * the block indicated by zb does not need to be visited at all. Returns
 * RESUME_SKIP_CHILDREN if we are resuming a post traversal and we reach the
 * resume point. This indicates that this block should be visited but not its
 * children (since they must have been visited in a previous traversal).
 * Otherwise returns RESUME_SKIP_NONE.
 */
static resume_skip_t
resume_skip_check(traverse_data_t *td, const dnode_phys_t *dnp,
    const zbookmark_phys_t *zb)
{
        if (td->td_resume != NULL && !ZB_IS_ZERO(td->td_resume)) {
                /*
                 * If we already visited this bp & everything below,
                 * don't bother doing it again.
                 */
                if (zbookmark_subtree_completed(dnp, zb, td->td_resume))
                        return (RESUME_SKIP_ALL);

                /*
                 * If we found the block we're trying to resume from, zero
                 * the bookmark out to indicate that we have resumed.
                 */
                if (bcmp(zb, td->td_resume, sizeof (*zb)) == 0) {
                        bzero(td->td_resume, sizeof (*zb));
                        if (td->td_flags & TRAVERSE_POST)
                                return (RESUME_SKIP_CHILDREN);
                }
        }
        return (RESUME_SKIP_NONE);
}

static void
traverse_prefetch_metadata(traverse_data_t *td,
    const blkptr_t *bp, const zbookmark_phys_t *zb)
{
        arc_flags_t flags = ARC_FLAG_NOWAIT | ARC_FLAG_PREFETCH;

        if (!(td->td_flags & TRAVERSE_PREFETCH_METADATA))
                return;
        /*
         * If we are in the process of resuming, don't prefetch, because
         * some children will not be needed (and in fact may have already
         * been freed).
         */
        if (td->td_resume != NULL && !ZB_IS_ZERO(td->td_resume))
                return;
        if (BP_IS_HOLE(bp) || bp->blk_birth <= td->td_min_txg)
                return;
        if (BP_GET_LEVEL(bp) == 0 && BP_GET_TYPE(bp) != DMU_OT_DNODE)
                return;

        (void) arc_read(NULL, td->td_spa, bp, NULL, NULL,
            ZIO_PRIORITY_ASYNC_READ, ZIO_FLAG_CANFAIL, &flags, zb);
}

static boolean_t
prefetch_needed(prefetch_data_t *pfd, const blkptr_t *bp)
{
        ASSERT(pfd->pd_flags & TRAVERSE_PREFETCH_DATA);
        if (BP_IS_HOLE(bp) || BP_IS_EMBEDDED(bp) ||
            BP_GET_TYPE(bp) == DMU_OT_INTENT_LOG)
                return (B_FALSE);
        return (B_TRUE);
}

static int
traverse_visitbp(traverse_data_t *td, const dnode_phys_t *dnp,
    const blkptr_t *bp, const zbookmark_phys_t *zb)
{
        int err = 0;
        arc_buf_t *buf = NULL;
        prefetch_data_t *pd = td->td_pfd;

        switch (resume_skip_check(td, dnp, zb)) {
        case RESUME_SKIP_ALL:
                return (0);
        case RESUME_SKIP_CHILDREN:
                goto post;
        case RESUME_SKIP_NONE:
                break;
        default:
                ASSERT(0);
        }

        if (bp->blk_birth == 0) {
                /*
                 * Since this block has a birth time of 0 it must be a
                 * hole created before the SPA_FEATURE_HOLE_BIRTH
                 * feature was enabled.  If SPA_FEATURE_HOLE_BIRTH
                 * was enabled before the min_txg for this traveral we
                 * know the hole must have been created before the
                 * min_txg for this traveral, so we can skip it. If
                 * SPA_FEATURE_HOLE_BIRTH was enabled after the min_txg
                 * for this traveral we cannot tell if the hole was
                 * created before or after the min_txg for this
                 * traversal, so we cannot skip it.
                 */
                if (td->td_hole_birth_enabled_txg < td->td_min_txg)
                        return (0);
        } else if (bp->blk_birth <= td->td_min_txg) {
                return (0);
        }

        if (pd != NULL && !pd->pd_exited && prefetch_needed(pd, bp)) {
                uint64_t size = BP_GET_LSIZE(bp);
                mutex_enter(&pd->pd_mtx);
                ASSERT(pd->pd_bytes_fetched >= 0);
                while (pd->pd_bytes_fetched < size && !pd->pd_exited)
                        cv_wait_sig(&pd->pd_cv, &pd->pd_mtx);
                pd->pd_bytes_fetched -= size;
                cv_broadcast(&pd->pd_cv);
                mutex_exit(&pd->pd_mtx);
        }

        if (BP_IS_HOLE(bp)) {
                err = td->td_func(td->td_spa, NULL, bp, zb, dnp, td->td_arg);
                if (err != 0)
                        goto post;
                return (0);
        }

        if (td->td_flags & TRAVERSE_PRE) {
                err = td->td_func(td->td_spa, NULL, bp, zb, dnp,
                    td->td_arg);
                if (err == TRAVERSE_VISIT_NO_CHILDREN)
                        return (0);
                if (err != 0)
                        goto post;
        }

        if (BP_GET_LEVEL(bp) > 0) {
                uint32_t flags = ARC_FLAG_WAIT;
                int32_t i;
                int32_t epb = BP_GET_LSIZE(bp) >> SPA_BLKPTRSHIFT;
                zbookmark_phys_t *czb;

                err = arc_read(NULL, td->td_spa, bp, arc_getbuf_func, &buf,
                    ZIO_PRIORITY_ASYNC_READ, ZIO_FLAG_CANFAIL, &flags, zb);
                if (err != 0)
                        goto post;

                czb = kmem_alloc(sizeof (zbookmark_phys_t), KM_SLEEP);

                for (i = 0; i < epb; i++) {
                        SET_BOOKMARK(czb, zb->zb_objset, zb->zb_object,
                            zb->zb_level - 1,
                            zb->zb_blkid * epb + i);
                        traverse_prefetch_metadata(td,
                            &((blkptr_t *)buf->b_data)[i], czb);
                }

                /* recursively visitbp() blocks below this */
                for (i = 0; i < epb; i++) {
                        SET_BOOKMARK(czb, zb->zb_objset, zb->zb_object,
                            zb->zb_level - 1,
                            zb->zb_blkid * epb + i);
                        err = traverse_visitbp(td, dnp,
                            &((blkptr_t *)buf->b_data)[i], czb);
                        if (err != 0)
                                break;
                }

                kmem_free(czb, sizeof (zbookmark_phys_t));

        } else if (BP_GET_TYPE(bp) == DMU_OT_DNODE) {
                uint32_t flags = ARC_FLAG_WAIT;
                int32_t i;
                int32_t epb = BP_GET_LSIZE(bp) >> DNODE_SHIFT;
                dnode_phys_t *cdnp;

                err = arc_read(NULL, td->td_spa, bp, arc_getbuf_func, &buf,
                    ZIO_PRIORITY_ASYNC_READ, ZIO_FLAG_CANFAIL, &flags, zb);
                if (err != 0)
                        goto post;
                cdnp = buf->b_data;

                for (i = 0; i < epb; i++) {
                        prefetch_dnode_metadata(td, &cdnp[i], zb->zb_objset,
                            zb->zb_blkid * epb + i);
                }

                /* recursively visitbp() blocks below this */
                for (i = 0; i < epb; i++) {
                        err = traverse_dnode(td, &cdnp[i], zb->zb_objset,
                            zb->zb_blkid * epb + i);
                        if (err != 0)
                                break;
                }
        } else if (BP_GET_TYPE(bp) == DMU_OT_OBJSET) {
                arc_flags_t flags = ARC_FLAG_WAIT;
                objset_phys_t *osp;
                dnode_phys_t *mdnp, *gdnp, *udnp;

                err = arc_read(NULL, td->td_spa, bp, arc_getbuf_func, &buf,
                    ZIO_PRIORITY_ASYNC_READ, ZIO_FLAG_CANFAIL, &flags, zb);
                if (err != 0)
                        goto post;

                osp = buf->b_data;
                mdnp = &osp->os_meta_dnode;
                gdnp = &osp->os_groupused_dnode;
                udnp = &osp->os_userused_dnode;

                prefetch_dnode_metadata(td, mdnp, zb->zb_objset,
                    DMU_META_DNODE_OBJECT);
                if (arc_buf_size(buf) >= sizeof (objset_phys_t)) {
                        prefetch_dnode_metadata(td, gdnp, zb->zb_objset,
                            DMU_GROUPUSED_OBJECT);
                        prefetch_dnode_metadata(td, udnp, zb->zb_objset,
                            DMU_USERUSED_OBJECT);
                }

                err = traverse_dnode(td, mdnp, zb->zb_objset,
                    DMU_META_DNODE_OBJECT);
                if (err == 0 && arc_buf_size(buf) >= sizeof (objset_phys_t)) {
                        err = traverse_dnode(td, gdnp, zb->zb_objset,
                            DMU_GROUPUSED_OBJECT);
                }
                if (err == 0 && arc_buf_size(buf) >= sizeof (objset_phys_t)) {
                        err = traverse_dnode(td, udnp, zb->zb_objset,
                            DMU_USERUSED_OBJECT);
                }
        }

        if (buf)
                (void) arc_buf_remove_ref(buf, &buf);

post:
        if (err == 0 && (td->td_flags & TRAVERSE_POST))
                err = td->td_func(td->td_spa, NULL, bp, zb, dnp, td->td_arg);

        if ((td->td_flags & TRAVERSE_HARD) && (err == EIO || err == ECKSUM)) {
                /*
                 * Ignore this disk error as requested by the HARD flag,
                 * and continue traversal.
                 */
                err = 0;
        }

        /*
         * If we are stopping here, set td_resume.
         */
        if (td->td_resume != NULL && err != 0 && !td->td_paused) {
                td->td_resume->zb_objset = zb->zb_objset;
                td->td_resume->zb_object = zb->zb_object;
                td->td_resume->zb_level = 0;
                /*
                 * If we have stopped on an indirect block (e.g. due to
                 * i/o error), we have not visited anything below it.
                 * Set the bookmark to the first level-0 block that we need
                 * to visit.  This way, the resuming code does not need to
                 * deal with resuming from indirect blocks.
                 */
                td->td_resume->zb_blkid = zb->zb_blkid <<
                    (zb->zb_level * (dnp->dn_indblkshift - SPA_BLKPTRSHIFT));
                td->td_paused = B_TRUE;
        }

        return (err);
}

static void
prefetch_dnode_metadata(traverse_data_t *td, const dnode_phys_t *dnp,
    uint64_t objset, uint64_t object)
{
        int j;
        zbookmark_phys_t czb;

        for (j = 0; j < dnp->dn_nblkptr; j++) {
                SET_BOOKMARK(&czb, objset, object, dnp->dn_nlevels - 1, j);
                traverse_prefetch_metadata(td, &dnp->dn_blkptr[j], &czb);
        }

        if (dnp->dn_flags & DNODE_FLAG_SPILL_BLKPTR) {
                SET_BOOKMARK(&czb, objset, object, 0, DMU_SPILL_BLKID);
                traverse_prefetch_metadata(td, &dnp->dn_spill, &czb);
        }
}

static int
traverse_dnode(traverse_data_t *td, const dnode_phys_t *dnp,
    uint64_t objset, uint64_t object)
{
        int j, err = 0;
        zbookmark_phys_t czb;

        if (td->td_flags & TRAVERSE_PRE) {
                SET_BOOKMARK(&czb, objset, object, ZB_DNODE_LEVEL,
                    ZB_DNODE_BLKID);
                err = td->td_func(td->td_spa, NULL, NULL, &czb, dnp,
                    td->td_arg);
                if (err == TRAVERSE_VISIT_NO_CHILDREN)
                        return (0);
                if (err != 0)
                        return (err);
        }

        for (j = 0; j < dnp->dn_nblkptr; j++) {
                SET_BOOKMARK(&czb, objset, object, dnp->dn_nlevels - 1, j);
                err = traverse_visitbp(td, dnp, &dnp->dn_blkptr[j], &czb);
                if (err != 0)
                        break;
        }

        if (err == 0 && (dnp->dn_flags & DNODE_FLAG_SPILL_BLKPTR)) {
                SET_BOOKMARK(&czb, objset, object, 0, DMU_SPILL_BLKID);
                err = traverse_visitbp(td, dnp, &dnp->dn_spill, &czb);
        }

        if (err == 0 && (td->td_flags & TRAVERSE_POST)) {
                SET_BOOKMARK(&czb, objset, object, ZB_DNODE_LEVEL,
                    ZB_DNODE_BLKID);
                err = td->td_func(td->td_spa, NULL, NULL, &czb, dnp,
                    td->td_arg);
                if (err == TRAVERSE_VISIT_NO_CHILDREN)
                        return (0);
                if (err != 0)
                        return (err);
        }
        return (err);
}

/* ARGSUSED */
static int
traverse_prefetcher(spa_t *spa, zilog_t *zilog, const blkptr_t *bp,
    const zbookmark_phys_t *zb, const dnode_phys_t *dnp, void *arg)
{
        prefetch_data_t *pfd = arg;
        arc_flags_t aflags = ARC_FLAG_NOWAIT | ARC_FLAG_PREFETCH;

        ASSERT(pfd->pd_bytes_fetched >= 0);
        if (bp == NULL)
                return (0);
        if (pfd->pd_cancel)
                return (SET_ERROR(EINTR));

        if (!prefetch_needed(pfd, bp))
                return (0);

        mutex_enter(&pfd->pd_mtx);
        while (!pfd->pd_cancel && pfd->pd_bytes_fetched >= zfs_pd_bytes_max)
                cv_wait_sig(&pfd->pd_cv, &pfd->pd_mtx);
        pfd->pd_bytes_fetched += BP_GET_LSIZE(bp);
        cv_broadcast(&pfd->pd_cv);
        mutex_exit(&pfd->pd_mtx);

        (void) arc_read(NULL, spa, bp, NULL, NULL, ZIO_PRIORITY_ASYNC_READ,
            ZIO_FLAG_CANFAIL | ZIO_FLAG_SPECULATIVE, &aflags, zb);

        return (0);
}

static void
traverse_prefetch_thread(void *arg)
{
        traverse_data_t *td_main = arg;
        traverse_data_t td = *td_main;
        zbookmark_phys_t czb;
        fstrans_cookie_t cookie = spl_fstrans_mark();

        td.td_func = traverse_prefetcher;
        td.td_arg = td_main->td_pfd;
        td.td_pfd = NULL;

        SET_BOOKMARK(&czb, td.td_objset,
            ZB_ROOT_OBJECT, ZB_ROOT_LEVEL, ZB_ROOT_BLKID);
        (void) traverse_visitbp(&td, NULL, td.td_rootbp, &czb);

        mutex_enter(&td_main->td_pfd->pd_mtx);
        td_main->td_pfd->pd_exited = B_TRUE;
        cv_broadcast(&td_main->td_pfd->pd_cv);
        mutex_exit(&td_main->td_pfd->pd_mtx);
        spl_fstrans_unmark(cookie);
}

/*
 * NB: dataset must not be changing on-disk (eg, is a snapshot or we are
 * in syncing context).
 */
static int
traverse_impl(spa_t *spa, dsl_dataset_t *ds, uint64_t objset, blkptr_t *rootbp,
    uint64_t txg_start, zbookmark_phys_t *resume, int flags,
    blkptr_cb_t func, void *arg)
{
        traverse_data_t *td;
        prefetch_data_t *pd;
        zbookmark_phys_t *czb;
        int err;

        ASSERT(ds == NULL || objset == ds->ds_object);
        ASSERT(!(flags & TRAVERSE_PRE) || !(flags & TRAVERSE_POST));

        /*
         * The data prefetching mechanism (the prefetch thread) is incompatible
         * with resuming from a bookmark.
         */
        ASSERT(resume == NULL || !(flags & TRAVERSE_PREFETCH_DATA));

        td = kmem_alloc(sizeof (traverse_data_t), KM_SLEEP);
        pd = kmem_zalloc(sizeof (prefetch_data_t), KM_SLEEP);
        czb = kmem_alloc(sizeof (zbookmark_phys_t), KM_SLEEP);

        td->td_spa = spa;
        td->td_objset = objset;
        td->td_rootbp = rootbp;
        td->td_min_txg = txg_start;
        td->td_resume = resume;
        td->td_func = func;
        td->td_arg = arg;
        td->td_pfd = pd;
        td->td_flags = flags;
        td->td_paused = B_FALSE;

        if (spa_feature_is_active(spa, SPA_FEATURE_HOLE_BIRTH)) {
                VERIFY(spa_feature_enabled_txg(spa,
                    SPA_FEATURE_HOLE_BIRTH, &td->td_hole_birth_enabled_txg));
        } else {
                td->td_hole_birth_enabled_txg = 0;
        }

        pd->pd_flags = flags;
        mutex_init(&pd->pd_mtx, NULL, MUTEX_DEFAULT, NULL);
        cv_init(&pd->pd_cv, NULL, CV_DEFAULT, NULL);

        SET_BOOKMARK(czb, td->td_objset,
            ZB_ROOT_OBJECT, ZB_ROOT_LEVEL, ZB_ROOT_BLKID);

        /* See comment on ZIL traversal in dsl_scan_visitds. */
        if (ds != NULL && !ds->ds_is_snapshot && !BP_IS_HOLE(rootbp)) {
                uint32_t flags = ARC_FLAG_WAIT;
                objset_phys_t *osp;
                arc_buf_t *buf;

                err = arc_read(NULL, td->td_spa, rootbp,
                    arc_getbuf_func, &buf,
                    ZIO_PRIORITY_ASYNC_READ, ZIO_FLAG_CANFAIL, &flags, czb);
                if (err != 0)
                        return (err);

                osp = buf->b_data;
                traverse_zil(td, &osp->os_zil_header);
                (void) arc_buf_remove_ref(buf, &buf);
        }

        if (!(flags & TRAVERSE_PREFETCH_DATA) ||
            0 == taskq_dispatch(system_taskq, traverse_prefetch_thread,
            td, TQ_NOQUEUE))
                pd->pd_exited = B_TRUE;

        err = traverse_visitbp(td, NULL, rootbp, czb);

        mutex_enter(&pd->pd_mtx);
        pd->pd_cancel = B_TRUE;
        cv_broadcast(&pd->pd_cv);
        while (!pd->pd_exited)
                cv_wait_sig(&pd->pd_cv, &pd->pd_mtx);
        mutex_exit(&pd->pd_mtx);

        mutex_destroy(&pd->pd_mtx);
        cv_destroy(&pd->pd_cv);

        kmem_free(czb, sizeof (zbookmark_phys_t));
        kmem_free(pd, sizeof (struct prefetch_data));
        kmem_free(td, sizeof (struct traverse_data));

        return (err);
}

/*
 * NB: dataset must not be changing on-disk (eg, is a snapshot or we are
 * in syncing context).
 */
int
traverse_dataset(dsl_dataset_t *ds, uint64_t txg_start, int flags,
    blkptr_cb_t func, void *arg)
{
        return (traverse_impl(ds->ds_dir->dd_pool->dp_spa, ds, ds->ds_object,
            &dsl_dataset_phys(ds)->ds_bp, txg_start, NULL, flags, func, arg));
}

int
traverse_dataset_destroyed(spa_t *spa, blkptr_t *blkptr,
    uint64_t txg_start, zbookmark_phys_t *resume, int flags,
    blkptr_cb_t func, void *arg)
{
        return (traverse_impl(spa, NULL, ZB_DESTROYED_OBJSET,
            blkptr, txg_start, resume, flags, func, arg));
}

/*
 * NB: pool must not be changing on-disk (eg, from zdb or sync context).
 */
int
traverse_pool(spa_t *spa, uint64_t txg_start, int flags,
    blkptr_cb_t func, void *arg)
{
        int err;
        uint64_t obj;
        dsl_pool_t *dp = spa_get_dsl(spa);
        objset_t *mos = dp->dp_meta_objset;
        boolean_t hard = (flags & TRAVERSE_HARD);

        /* visit the MOS */
        err = traverse_impl(spa, NULL, 0, spa_get_rootblkptr(spa),
            txg_start, NULL, flags, func, arg);
        if (err != 0)
                return (err);

        /* visit each dataset */
        for (obj = 1; err == 0;
            err = dmu_object_next(mos, &obj, FALSE, txg_start)) {
                dmu_object_info_t doi;

                err = dmu_object_info(mos, obj, &doi);
                if (err != 0) {
                        if (hard)
                                continue;
                        break;
                }

                if (doi.doi_bonus_type == DMU_OT_DSL_DATASET) {
                        dsl_dataset_t *ds;
                        uint64_t txg = txg_start;

                        dsl_pool_config_enter(dp, FTAG);
                        err = dsl_dataset_hold_obj(dp, obj, FTAG, &ds);
                        dsl_pool_config_exit(dp, FTAG);
                        if (err != 0) {
                                if (hard)
                                        continue;
                                break;
                        }
                        if (dsl_dataset_phys(ds)->ds_prev_snap_txg > txg)
                                txg = dsl_dataset_phys(ds)->ds_prev_snap_txg;
                        err = traverse_dataset(ds, txg, flags, func, arg);
                        dsl_dataset_rele(ds, FTAG);
                        if (err != 0)
                                break;
                }
        }
        if (err == ESRCH)
                err = 0;
        return (err);
}

#if defined(_KERNEL) && defined(HAVE_SPL)
EXPORT_SYMBOL(traverse_dataset);
EXPORT_SYMBOL(traverse_pool);

module_param(zfs_pd_bytes_max, int, 0644);
MODULE_PARM_DESC(zfs_pd_bytes_max, "Max number of bytes to prefetch");
#endif