'\" te
.\" Copyright (c) 2013 by Turbo Fredriksson <turbo@bayour.com>. All rights reserved.
+.\" Copyright (c) 2017 Datto Inc.
.\" 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
.\" CDDL HEADER, with the fields enclosed by brackets "[]" replaced with your
.\" own identifying information:
.\" Portions Copyright [yyyy] [name of copyright owner]
-.TH ZFS-MODULE-PARAMETERS 5 "Nov 16, 2013"
+.TH ZFS-MODULE-PARAMETERS 5 "Oct 28, 2017"
.SH NAME
zfs\-module\-parameters \- ZFS module parameters
.SH DESCRIPTION
.sp
.LP
+.sp
+.ne 2
+.na
+\fBignore_hole_birth\fR (int)
+.ad
+.RS 12n
+When set, the hole_birth optimization will not be used, and all holes will
+always be sent on zfs send. Useful if you suspect your datasets are affected
+by a bug in hole_birth.
+.sp
+Use \fB1\fR for on (default) and \fB0\fR for off.
+.RE
+
.sp
.ne 2
.na
\fBl2arc_feed_again\fR (int)
.ad
.RS 12n
-Turbo L2ARC warmup
+Turbo L2ARC warm-up. When the L2ARC is cold the fill interval will be set as
+fast as possible.
.sp
Use \fB1\fR for yes (default) and \fB0\fR to disable.
.RE
\fBl2arc_feed_min_ms\fR (ulong)
.ad
.RS 12n
-Min feed interval in milliseconds
+Min feed interval in milliseconds. Requires \fBl2arc_feed_again=1\fR and only
+applicable in related situations.
.sp
Default value: \fB200\fR.
.RE
\fBl2arc_headroom\fR (ulong)
.ad
.RS 12n
-Number of max device writes to precache
+How far through the ARC lists to search for L2ARC cacheable content, expressed
+as a multiplier of \fBl2arc_write_max\fR
.sp
Default value: \fB2\fR.
.RE
\fBl2arc_headroom_boost\fR (ulong)
.ad
.RS 12n
-Compressed l2arc_headroom multiplier
+Scales \fBl2arc_headroom\fR by this percentage when L2ARC contents are being
+successfully compressed before writing. A value of 100 disables this feature.
.sp
Default value: \fB200\fR.
.RE
\fBl2arc_noprefetch\fR (int)
.ad
.RS 12n
-Skip caching prefetched buffers
+Do not write buffers to L2ARC if they were prefetched but not used by
+applications
.sp
Use \fB1\fR for yes (default) and \fB0\fR to disable.
.RE
\fBl2arc_write_boost\fR (ulong)
.ad
.RS 12n
-Extra write bytes during device warmup
+Cold L2ARC devices will have \fBl2arc_write_max\fR increased by this amount
+while they remain cold.
.sp
Default value: \fB8,388,608\fR.
.RE
.sp
.ne 2
.na
-\fBmetaslab_debug\fR (int)
+\fBmetaslab_aliquot\fR (ulong)
.ad
.RS 12n
-Keep space maps in core to verify frees
+Metaslab granularity, in bytes. This is roughly similar to what would be
+referred to as the "stripe size" in traditional RAID arrays. In normal
+operation, ZFS will try to write this amount of data to a top-level vdev
+before moving on to the next one.
.sp
-Use \fB1\fR for yes and \fB0\fR for no (default).
+Default value: \fB524,288\fR.
.RE
.sp
.ne 2
.na
-\fBspa_config_path\fR (charp)
+\fBmetaslab_bias_enabled\fR (int)
.ad
.RS 12n
-SPA config file
+Enable metaslab group biasing based on its vdev's over- or under-utilization
+relative to the pool.
.sp
-Default value: \fB/etc/zfs/zpool.cache\fR.
+Use \fB1\fR for yes (default) and \fB0\fR for no.
.RE
.sp
.ne 2
.na
-\fBzfetch_array_rd_sz\fR (ulong)
+\fBzfs_metaslab_segment_weight_enabled\fR (int)
.ad
.RS 12n
-Number of bytes in a array_read
+Enable/disable segment-based metaslab selection.
.sp
-Default value: \fB1,048,576\fR.
+Use \fB1\fR for yes (default) and \fB0\fR for no.
.RE
.sp
.ne 2
.na
-\fBzfetch_block_cap\fR (uint)
+\fBzfs_metaslab_switch_threshold\fR (int)
.ad
.RS 12n
-Max number of blocks to fetch at a time
+When using segment-based metaslab selection, continue allocating
+from the active metaslab until \fBzfs_metaslab_switch_threshold\fR
+worth of buckets have been exhausted.
.sp
-Default value: \fB256\fR.
+Default value: \fB2\fR.
.RE
.sp
.ne 2
.na
-\fBzfetch_max_streams\fR (uint)
+\fBmetaslab_debug_load\fR (int)
.ad
.RS 12n
-Max number of streams per zfetch
+Load all metaslabs during pool import.
.sp
-Default value: \fB8\fR.
+Use \fB1\fR for yes and \fB0\fR for no (default).
.RE
.sp
.ne 2
.na
-\fBzfetch_min_sec_reap\fR (uint)
+\fBmetaslab_debug_unload\fR (int)
.ad
.RS 12n
-Min time before stream reclaim
+Prevent metaslabs from being unloaded.
.sp
-Default value: \fB2\fR.
+Use \fB1\fR for yes and \fB0\fR for no (default).
.RE
.sp
.ne 2
.na
-\fBzfs_arc_grow_retry\fR (int)
+\fBmetaslab_fragmentation_factor_enabled\fR (int)
.ad
.RS 12n
-Seconds before growing arc size
+Enable use of the fragmentation metric in computing metaslab weights.
.sp
-Default value: \fB5\fR.
+Use \fB1\fR for yes (default) and \fB0\fR for no.
.RE
.sp
.ne 2
.na
-\fBzfs_arc_max\fR (ulong)
+\fBmetaslabs_per_vdev\fR (int)
.ad
.RS 12n
-Max arc size
+When a vdev is added, it will be divided into approximately (but no more than) this number of metaslabs.
.sp
-Default value: \fB0\fR.
+Default value: \fB200\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_arc_memory_throttle_disable\fR (int)
+\fBmetaslab_preload_enabled\fR (int)
.ad
.RS 12n
-Disable memory throttle
+Enable metaslab group preloading.
.sp
-Use \fB1\fR for yes (default) and \fB0\fR to disable.
+Use \fB1\fR for yes (default) and \fB0\fR for no.
.RE
.sp
.ne 2
.na
-\fBzfs_arc_meta_limit\fR (ulong)
+\fBmetaslab_lba_weighting_enabled\fR (int)
.ad
.RS 12n
-Meta limit for arc size
+Give more weight to metaslabs with lower LBAs, assuming they have
+greater bandwidth as is typically the case on a modern constant
+angular velocity disk drive.
.sp
-Default value: \fB0\fR.
+Use \fB1\fR for yes (default) and \fB0\fR for no.
.RE
.sp
.ne 2
.na
-\fBzfs_arc_meta_prune\fR (int)
+\fBspa_config_path\fR (charp)
+.ad
+.RS 12n
+SPA config file
+.sp
+Default value: \fB/etc/zfs/zpool.cache\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBspa_asize_inflation\fR (int)
.ad
.RS 12n
-Bytes of meta data to prune
+Multiplication factor used to estimate actual disk consumption from the
+size of data being written. The default value is a worst case estimate,
+but lower values may be valid for a given pool depending on its
+configuration. Pool administrators who understand the factors involved
+may wish to specify a more realistic inflation factor, particularly if
+they operate close to quota or capacity limits.
.sp
-Default value: \fB1,048,576\fR.
+Default value: \fB24\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_arc_min\fR (ulong)
+\fBspa_load_verify_data\fR (int)
.ad
.RS 12n
-Min arc size
+Whether to traverse data blocks during an "extreme rewind" (\fB-X\fR)
+import. Use 0 to disable and 1 to enable.
+
+An extreme rewind import normally performs a full traversal of all
+blocks in the pool for verification. If this parameter is set to 0,
+the traversal skips non-metadata blocks. It can be toggled once the
+import has started to stop or start the traversal of non-metadata blocks.
.sp
-Default value: \fB100\fR.
+Default value: \fB1\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_arc_min_prefetch_lifespan\fR (int)
+\fBspa_load_verify_metadata\fR (int)
.ad
.RS 12n
-Min life of prefetch block
+Whether to traverse blocks during an "extreme rewind" (\fB-X\fR)
+pool import. Use 0 to disable and 1 to enable.
+
+An extreme rewind import normally performs a full traversal of all
+blocks in the pool for verification. If this parameter is set to 0,
+the traversal is not performed. It can be toggled once the import has
+started to stop or start the traversal.
.sp
-Default value: \fB100\fR.
+Default value: \fB1\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_arc_p_min_shift\fR (int)
+\fBspa_load_verify_maxinflight\fR (int)
.ad
.RS 12n
-arc_c shift to calc min/max arc_p
+Maximum concurrent I/Os during the traversal performed during an "extreme
+rewind" (\fB-X\fR) pool import.
.sp
-Default value: \fB4\fR.
+Default value: \fB10000\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_arc_shrink_shift\fR (int)
+\fBspa_slop_shift\fR (int)
.ad
.RS 12n
-log2(fraction of arc to reclaim)
+Normally, we don't allow the last 3.2% (1/(2^spa_slop_shift)) of space
+in the pool to be consumed. This ensures that we don't run the pool
+completely out of space, due to unaccounted changes (e.g. to the MOS).
+It also limits the worst-case time to allocate space. If we have
+less than this amount of free space, most ZPL operations (e.g. write,
+create) will return ENOSPC.
.sp
Default value: \fB5\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_autoimport_disable\fR (int)
+\fBzfetch_array_rd_sz\fR (ulong)
.ad
.RS 12n
-Disable pool import at module load
+If prefetching is enabled, disable prefetching for reads larger than this size.
.sp
-Use \fB1\fR for yes and \fB0\fR for no (default).
+Default value: \fB1,048,576\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_dbuf_state_index\fR (int)
+\fBzfetch_max_distance\fR (uint)
.ad
.RS 12n
-Calculate arc header index
+Max bytes to prefetch per stream (default 8MB).
.sp
-Default value: \fB0\fR.
+Default value: \fB8,388,608\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_deadman_enabled\fR (int)
+\fBzfetch_max_streams\fR (uint)
.ad
.RS 12n
-Enable deadman timer
+Max number of streams per zfetch (prefetch streams per file).
.sp
-Use \fB1\fR for yes (default) and \fB0\fR to disable.
+Default value: \fB8\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_deadman_synctime\fR (ulong)
+\fBzfetch_min_sec_reap\fR (uint)
.ad
.RS 12n
-Expire in units of zfs_txg_synctime_ms
+Min time before an active prefetch stream can be reclaimed
.sp
-Default value: \fB1,000\fR.
+Default value: \fB2\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_dedup_prefetch\fR (int)
+\fBzfs_arc_dnode_limit\fR (ulong)
.ad
.RS 12n
-Enable prefetching dedup-ed blks
+When the number of bytes consumed by dnodes in the ARC exceeds this number of
+bytes, try to unpin some of it in response to demand for non-metadata. This
+value acts as a ceiling to the amount of dnode metadata, and defaults to 0 which
+indicates that a percent which is based on \fBzfs_arc_dnode_limit_percent\fR of
+the ARC meta buffers that may be used for dnodes.
+
+See also \fBzfs_arc_meta_prune\fR which serves a similar purpose but is used
+when the amount of metadata in the ARC exceeds \fBzfs_arc_meta_limit\fR rather
+than in response to overall demand for non-metadata.
+
.sp
-Use \fB1\fR for yes (default) and \fB0\fR to disable.
+Default value: \fB0\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_disable_dup_eviction\fR (int)
+\fBzfs_arc_dnode_limit_percent\fR (ulong)
.ad
.RS 12n
-Disable duplicate buffer eviction
+Percentage that can be consumed by dnodes of ARC meta buffers.
.sp
-Use \fB1\fR for yes and \fB0\fR for no (default).
+See also \fBzfs_arc_dnode_limit\fR which serves a similar purpose but has a
+higher priority if set to nonzero value.
+.sp
+Default value: \fB10\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_expire_snapshot\fR (int)
+\fBzfs_arc_dnode_reduce_percent\fR (ulong)
.ad
.RS 12n
-Seconds to expire .zfs/snapshot
+Percentage of ARC dnodes to try to scan in response to demand for non-metadata
+when the number of bytes consumed by dnodes exceeds \fBzfs_arc_dnode_limit\fR.
+
.sp
-Default value: \fB300\fR.
+Default value: \fB10% of the number of dnodes in the ARC\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_flags\fR (int)
+\fBzfs_arc_average_blocksize\fR (int)
.ad
.RS 12n
-Set additional debugging flags
+The ARC's buffer hash table is sized based on the assumption of an average
+block size of \fBzfs_arc_average_blocksize\fR (default 8K). This works out
+to roughly 1MB of hash table per 1GB of physical memory with 8-byte pointers.
+For configurations with a known larger average block size this value can be
+increased to reduce the memory footprint.
+
.sp
-Default value: \fB1\fR.
+Default value: \fB8192\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_free_min_time_ms\fR (int)
+\fBzfs_arc_evict_batch_limit\fR (int)
.ad
.RS 12n
-Min millisecs to free per txg
+Number ARC headers to evict per sub-list before proceeding to another sub-list.
+This batch-style operation prevents entire sub-lists from being evicted at once
+but comes at a cost of additional unlocking and locking.
.sp
-Default value: \fB1,000\fR.
+Default value: \fB10\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_immediate_write_sz\fR (long)
+\fBzfs_arc_grow_retry\fR (int)
.ad
.RS 12n
-Largest data block to write to zil
+If set to a non zero value, it will replace the arc_grow_retry value with this value.
+The arc_grow_retry value (default 5) is the number of seconds the ARC will wait before
+trying to resume growth after a memory pressure event.
.sp
-Default value: \fB32,768\fR.
+Default value: \fB0\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_mdcomp_disable\fR (int)
+\fBzfs_arc_lotsfree_percent\fR (int)
.ad
.RS 12n
-Disable meta data compression
+Throttle I/O when free system memory drops below this percentage of total
+system memory. Setting this value to 0 will disable the throttle.
.sp
-Use \fB1\fR for yes and \fB0\fR for no (default).
+Default value: \fB10\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_no_scrub_io\fR (int)
+\fBzfs_arc_max\fR (ulong)
.ad
.RS 12n
-Set for no scrub I/O
+Max arc size of ARC in bytes. If set to 0 then it will consume 1/2 of system
+RAM. This value must be at least 67108864 (64 megabytes).
.sp
-Use \fB1\fR for yes and \fB0\fR for no (default).
+This value can be changed dynamically with some caveats. It cannot be set back
+to 0 while running and reducing it below the current ARC size will not cause
+the ARC to shrink without memory pressure to induce shrinking.
+.sp
+Default value: \fB0\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_no_scrub_prefetch\fR (int)
+\fBzfs_arc_meta_adjust_restarts\fR (ulong)
.ad
.RS 12n
-Set for no scrub prefetching
+The number of restart passes to make while scanning the ARC attempting
+the free buffers in order to stay below the \fBzfs_arc_meta_limit\fR.
+This value should not need to be tuned but is available to facilitate
+performance analysis.
.sp
-Use \fB1\fR for yes and \fB0\fR for no (default).
+Default value: \fB4096\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_no_write_throttle\fR (int)
+\fBzfs_arc_meta_limit\fR (ulong)
.ad
.RS 12n
-Disable write throttling
+The maximum allowed size in bytes that meta data buffers are allowed to
+consume in the ARC. When this limit is reached meta data buffers will
+be reclaimed even if the overall arc_c_max has not been reached. This
+value defaults to 0 which indicates that a percent which is based on
+\fBzfs_arc_meta_limit_percent\fR of the ARC may be used for meta data.
.sp
-Use \fB1\fR for yes and \fB0\fR for no (default).
+This value my be changed dynamically except that it cannot be set back to 0
+for a specific percent of the ARC; it must be set to an explicit value.
+.sp
+Default value: \fB0\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_nocacheflush\fR (int)
+\fBzfs_arc_meta_limit_percent\fR (ulong)
.ad
.RS 12n
-Disable cache flushes
+Percentage of ARC buffers that can be used for meta data.
+
+See also \fBzfs_arc_meta_limit\fR which serves a similar purpose but has a
+higher priority if set to nonzero value.
+
.sp
-Use \fB1\fR for yes and \fB0\fR for no (default).
+Default value: \fB75\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_nopwrite_enabled\fR (int)
+\fBzfs_arc_meta_min\fR (ulong)
.ad
.RS 12n
-Enable NOP writes
+The minimum allowed size in bytes that meta data buffers may consume in
+the ARC. This value defaults to 0 which disables a floor on the amount
+of the ARC devoted meta data.
.sp
-Use \fB1\fR for yes (default) and \fB0\fR to disable.
+Default value: \fB0\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_pd_blks_max\fR (int)
+\fBzfs_arc_meta_prune\fR (int)
.ad
.RS 12n
-Max number of blocks to prefetch
+The number of dentries and inodes to be scanned looking for entries
+which can be dropped. This may be required when the ARC reaches the
+\fBzfs_arc_meta_limit\fR because dentries and inodes can pin buffers
+in the ARC. Increasing this value will cause to dentry and inode caches
+to be pruned more aggressively. Setting this value to 0 will disable
+pruning the inode and dentry caches.
.sp
-Default value: \fB100\fR.
+Default value: \fB10,000\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_prefetch_disable\fR (int)
+\fBzfs_arc_meta_strategy\fR (int)
.ad
.RS 12n
-Disable all ZFS prefetching
+Define the strategy for ARC meta data buffer eviction (meta reclaim strategy).
+A value of 0 (META_ONLY) will evict only the ARC meta data buffers.
+A value of 1 (BALANCED) indicates that additional data buffers may be evicted if
+that is required to in order to evict the required number of meta data buffers.
.sp
-Use \fB1\fR for yes and \fB0\fR for no (default).
+Default value: \fB1\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_read_chunk_size\fR (long)
+\fBzfs_arc_min\fR (ulong)
.ad
.RS 12n
-Bytes to read per chunk
+Min arc size of ARC in bytes. If set to 0 then arc_c_min will default to
+consuming the larger of 32M or 1/32 of total system memory.
.sp
-Default value: \fB1,048,576\fR.
+Default value: \fB0\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_read_history\fR (int)
+\fBzfs_arc_min_prefetch_ms\fR (int)
.ad
.RS 12n
-Historic statistics for the last N reads
+Minimum time prefetched blocks are locked in the ARC, specified in ms.
+A value of \fB0\fR will default to 1 second.
.sp
Default value: \fB0\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_read_history_hits\fR (int)
+\fBzfs_arc_min_prescient_prefetch_ms\fR (int)
.ad
.RS 12n
-Include cache hits in read history
+Minimum time "prescient prefetched" blocks are locked in the ARC, specified
+in ms. These blocks are meant to be prefetched fairly aggresively ahead of
+the code that may use them. A value of \fB0\fR will default to 6 seconds.
.sp
-Use \fB1\fR for yes and \fB0\fR for no (default).
+Default value: \fB0\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_recover\fR (int)
+\fBzfs_multilist_num_sublists\fR (int)
.ad
.RS 12n
-Set to attempt to recover from fatal errors. This should only be used as a
-last resort, as it typically results in leaked space, or worse.
+To allow more fine-grained locking, each ARC state contains a series
+of lists for both data and meta data objects. Locking is performed at
+the level of these "sub-lists". This parameters controls the number of
+sub-lists per ARC state, and also applies to other uses of the
+multilist data structure.
.sp
-Use \fB1\fR for yes and \fB0\fR for no (default).
+Default value: \fB4\fR or the number of online CPUs, whichever is greater
.RE
.sp
.ne 2
.na
-\fBzfs_resilver_delay\fR (int)
+\fBzfs_arc_overflow_shift\fR (int)
.ad
.RS 12n
-Number of ticks to delay resilver
+The ARC size is considered to be overflowing if it exceeds the current
+ARC target size (arc_c) by a threshold determined by this parameter.
+The threshold is calculated as a fraction of arc_c using the formula
+"arc_c >> \fBzfs_arc_overflow_shift\fR".
+
+The default value of 8 causes the ARC to be considered to be overflowing
+if it exceeds the target size by 1/256th (0.3%) of the target size.
+
+When the ARC is overflowing, new buffer allocations are stalled until
+the reclaim thread catches up and the overflow condition no longer exists.
.sp
-Default value: \fB2\fR.
+Default value: \fB8\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_resilver_min_time_ms\fR (int)
+
+\fBzfs_arc_p_min_shift\fR (int)
.ad
.RS 12n
-Min millisecs to resilver per txg
+If set to a non zero value, this will update arc_p_min_shift (default 4)
+with the new value.
+arc_p_min_shift is used to shift of arc_c for calculating both min and max
+max arc_p
.sp
-Default value: \fB3,000\fR.
+Default value: \fB0\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_scan_idle\fR (int)
+\fBzfs_arc_p_aggressive_disable\fR (int)
.ad
.RS 12n
-Idle window in clock ticks
+Disable aggressive arc_p growth
.sp
-Default value: \fB50\fR.
+Use \fB1\fR for yes (default) and \fB0\fR to disable.
.RE
.sp
.ne 2
.na
-\fBzfs_scan_min_time_ms\fR (int)
+\fBzfs_arc_p_dampener_disable\fR (int)
.ad
.RS 12n
-Min millisecs to scrub per txg
+Disable arc_p adapt dampener
.sp
-Default value: \fB1,000\fR.
+Use \fB1\fR for yes (default) and \fB0\fR to disable.
.RE
.sp
.ne 2
.na
-\fBzfs_scrub_delay\fR (int)
+\fBzfs_arc_shrink_shift\fR (int)
.ad
.RS 12n
-Number of ticks to delay scrub
+If set to a non zero value, this will update arc_shrink_shift (default 7)
+with the new value.
.sp
-Default value: \fB4\fR.
+Default value: \fB0\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_sync_pass_deferred_free\fR (int)
+\fBzfs_arc_pc_percent\fR (uint)
.ad
.RS 12n
-Defer frees starting in this pass
+Percent of pagecache to reclaim arc to
+
+This tunable allows ZFS arc to play more nicely with the kernel's LRU
+pagecache. It can guarantee that the arc size won't collapse under scanning
+pressure on the pagecache, yet still allows arc to be reclaimed down to
+zfs_arc_min if necessary. This value is specified as percent of pagecache
+size (as measured by NR_FILE_PAGES) where that percent may exceed 100. This
+only operates during memory pressure/reclaim.
.sp
-Default value: \fB2\fR.
+Default value: \fB0\fR (disabled).
.RE
.sp
.ne 2
.na
-\fBzfs_sync_pass_dont_compress\fR (int)
+\fBzfs_arc_sys_free\fR (ulong)
.ad
.RS 12n
-Don't compress starting in this pass
+The target number of bytes the ARC should leave as free memory on the system.
+Defaults to the larger of 1/64 of physical memory or 512K. Setting this
+option to a non-zero value will override the default.
.sp
-Default value: \fB5\fR.
+Default value: \fB0\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_sync_pass_rewrite\fR (int)
+\fBzfs_autoimport_disable\fR (int)
.ad
.RS 12n
-Rewrite new bps starting in this pass
+Disable pool import at module load by ignoring the cache file (typically \fB/etc/zfs/zpool.cache\fR).
.sp
-Default value: \fB2\fR.
+Use \fB1\fR for yes (default) and \fB0\fR for no.
.RE
.sp
.ne 2
.na
-\fBzfs_top_maxinflight\fR (int)
+\fBzfs_dbgmsg_enable\fR (int)
.ad
.RS 12n
-Max I/Os per top-level
+Internally ZFS keeps a small log to facilitate debugging. By default the log
+is disabled, to enable it set this option to 1. The contents of the log can
+be accessed by reading the /proc/spl/kstat/zfs/dbgmsg file. Writing 0 to
+this proc file clears the log.
.sp
-Default value: \fB32\fR.
+Default value: \fB0\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_txg_history\fR (int)
+\fBzfs_dbgmsg_maxsize\fR (int)
.ad
.RS 12n
-Historic statistics for the last N txgs
+The maximum size in bytes of the internal ZFS debug log.
+.sp
+Default value: \fB4M\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_dbuf_state_index\fR (int)
+.ad
+.RS 12n
+This feature is currently unused. It is normally used for controlling what
+reporting is available under /proc/spl/kstat/zfs.
.sp
Default value: \fB0\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_txg_synctime_ms\fR (int)
+\fBzfs_deadman_enabled\fR (int)
.ad
.RS 12n
-Target milliseconds between txg sync
+When a pool sync operation takes longer than \fBzfs_deadman_synctime_ms\fR
+milliseconds, a "slow spa_sync" message is logged to the debug log
+(see \fBzfs_dbgmsg_enable\fR). If \fBzfs_deadman_enabled\fR is set,
+all pending IO operations are also checked and if any haven't completed
+within \fBzfs_deadman_synctime_ms\fR milliseconds, a "SLOW IO" message
+is logged to the debug log and a "delay" system event with the details of
+the hung IO is posted.
.sp
-Default value: \fB1,000\fR.
+Use \fB1\fR (default) to enable the slow IO check and \fB0\fR to disable.
.RE
.sp
.ne 2
.na
-\fBzfs_txg_timeout\fR (int)
+\fBzfs_deadman_checktime_ms\fR (int)
.ad
.RS 12n
-Max seconds worth of delta per txg
+Once a pool sync operation has taken longer than
+\fBzfs_deadman_synctime_ms\fR milliseconds, continue to check for slow
+operations every \fBzfs_deadman_checktime_ms\fR milliseconds.
.sp
-Default value: \fB5\fR.
+Default value: \fB5,000\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_vdev_aggregation_limit\fR (int)
+\fBzfs_deadman_synctime_ms\fR (ulong)
.ad
.RS 12n
-Max vdev I/O aggregation size
+Interval in milliseconds after which the deadman is triggered and also
+the interval after which an IO operation is considered to be "hung"
+if \fBzfs_deadman_enabled\fR is set.
+
+See \fBzfs_deadman_enabled\fR.
.sp
-Default value: \fB131,072\fR.
+Default value: \fB1,000,000\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_vdev_cache_bshift\fR (int)
+\fBzfs_dedup_prefetch\fR (int)
.ad
.RS 12n
-Shift size to inflate reads too
+Enable prefetching dedup-ed blks
.sp
-Default value: \fB16\fR.
+Use \fB1\fR for yes and \fB0\fR to disable (default).
.RE
.sp
.ne 2
.na
-\fBzfs_vdev_cache_max\fR (int)
+\fBzfs_delay_min_dirty_percent\fR (int)
.ad
.RS 12n
-Inflate reads small than max
+Start to delay each transaction once there is this amount of dirty data,
+expressed as a percentage of \fBzfs_dirty_data_max\fR.
+This value should be >= zfs_vdev_async_write_active_max_dirty_percent.
+See the section "ZFS TRANSACTION DELAY".
+.sp
+Default value: \fB60\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_vdev_cache_size\fR (int)
+\fBzfs_delay_scale\fR (int)
.ad
.RS 12n
-Total size of the per-disk cache
+This controls how quickly the transaction delay approaches infinity.
+Larger values cause longer delays for a given amount of dirty data.
.sp
-Default value: \fB0\fR.
+For the smoothest delay, this value should be about 1 billion divided
+by the maximum number of operations per second. This will smoothly
+handle between 10x and 1/10th this number.
+.sp
+See the section "ZFS TRANSACTION DELAY".
+.sp
+Note: \fBzfs_delay_scale\fR * \fBzfs_dirty_data_max\fR must be < 2^64.
+.sp
+Default value: \fB500,000\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_vdev_max_pending\fR (int)
+\fBzfs_delete_blocks\fR (ulong)
.ad
.RS 12n
-Max pending per-vdev I/Os
+This is the used to define a large file for the purposes of delete. Files
+containing more than \fBzfs_delete_blocks\fR will be deleted asynchronously
+while smaller files are deleted synchronously. Decreasing this value will
+reduce the time spent in an unlink(2) system call at the expense of a longer
+delay before the freed space is available.
.sp
-Default value: \fB10\fR.
+Default value: \fB20,480\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_vdev_min_pending\fR (int)
+\fBzfs_dirty_data_max\fR (int)
.ad
.RS 12n
-Min pending per-vdev I/Os
+Determines the dirty space limit in bytes. Once this limit is exceeded, new
+writes are halted until space frees up. This parameter takes precedence
+over \fBzfs_dirty_data_max_percent\fR.
+See the section "ZFS TRANSACTION DELAY".
.sp
-Default value: \fB4\fR.
+Default value: 10 percent of all memory, capped at \fBzfs_dirty_data_max_max\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_vdev_mirror_switch_us\fR (int)
+\fBzfs_dirty_data_max_max\fR (int)
.ad
.RS 12n
-Switch mirrors every N usecs
+Maximum allowable value of \fBzfs_dirty_data_max\fR, expressed in bytes.
+This limit is only enforced at module load time, and will be ignored if
+\fBzfs_dirty_data_max\fR is later changed. This parameter takes
+precedence over \fBzfs_dirty_data_max_max_percent\fR. See the section
+"ZFS TRANSACTION DELAY".
.sp
-Default value: \fB10,000\fR.
+Default value: 25% of physical RAM.
.RE
.sp
.ne 2
.na
-\fBzfs_vdev_ramp_rate\fR (int)
+\fBzfs_dirty_data_max_max_percent\fR (int)
.ad
.RS 12n
-Exponential I/O issue ramp-up rate
+Maximum allowable value of \fBzfs_dirty_data_max\fR, expressed as a
+percentage of physical RAM. This limit is only enforced at module load
+time, and will be ignored if \fBzfs_dirty_data_max\fR is later changed.
+The parameter \fBzfs_dirty_data_max_max\fR takes precedence over this
+one. See the section "ZFS TRANSACTION DELAY".
.sp
-Default value: \fB2\fR.
+Default value: \fB25\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_vdev_read_gap_limit\fR (int)
+\fBzfs_dirty_data_max_percent\fR (int)
.ad
.RS 12n
-Aggregate read I/O over gap
+Determines the dirty space limit, expressed as a percentage of all
+memory. Once this limit is exceeded, new writes are halted until space frees
+up. The parameter \fBzfs_dirty_data_max\fR takes precedence over this
+one. See the section "ZFS TRANSACTION DELAY".
.sp
-Default value: \fB32,768\fR.
+Default value: 10%, subject to \fBzfs_dirty_data_max_max\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_vdev_scheduler\fR (charp)
+\fBzfs_dirty_data_sync\fR (int)
.ad
.RS 12n
-I/O scheduler
+Start syncing out a transaction group if there is at least this much dirty data.
.sp
-Default value: \fBnoop\fR.
+Default value: \fB67,108,864\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_vdev_time_shift\fR (int)
+\fBzfs_fletcher_4_impl\fR (string)
.ad
.RS 12n
-Deadline time shift for vdev I/O
+Select a fletcher 4 implementation.
+.sp
+Supported selectors are: \fBfastest\fR, \fBscalar\fR, \fBsse2\fR, \fBssse3\fR,
+\fBavx2\fR, \fBavx512f\fR, and \fBaarch64_neon\fR.
+All of the selectors except \fBfastest\fR and \fBscalar\fR require instruction
+set extensions to be available and will only appear if ZFS detects that they are
+present at runtime. If multiple implementations of fletcher 4 are available,
+the \fBfastest\fR will be chosen using a micro benchmark. Selecting \fBscalar\fR
+results in the original, CPU based calculation, being used. Selecting any option
+other than \fBfastest\fR and \fBscalar\fR results in vector instructions from
+the respective CPU instruction set being used.
.sp
-Default value: \fB29\fR (each bucket is 0.537 seconds).
+Default value: \fBfastest\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_vdev_write_gap_limit\fR (int)
+\fBzfs_free_bpobj_enabled\fR (int)
.ad
.RS 12n
-Aggregate write I/O over gap
+Enable/disable the processing of the free_bpobj object.
.sp
-Default value: \fB4,096\fR.
+Default value: \fB1\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_write_limit_inflated\fR (ulong)
+\fBzfs_free_max_blocks\fR (ulong)
.ad
.RS 12n
-Inflated txg write limit
+Maximum number of blocks freed in a single txg.
.sp
-Default value: \fB0\fR.
+Default value: \fB100,000\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_write_limit_max\fR (ulong)
+\fBzfs_vdev_async_read_max_active\fR (int)
.ad
.RS 12n
-Max txg write limit
+Maximum asynchronous read I/Os active to each device.
+See the section "ZFS I/O SCHEDULER".
.sp
-Default value: \fB0\fR.
+Default value: \fB3\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_write_limit_min\fR (ulong)
+\fBzfs_vdev_async_read_min_active\fR (int)
.ad
.RS 12n
-Min txg write limit
+Minimum asynchronous read I/Os active to each device.
+See the section "ZFS I/O SCHEDULER".
.sp
-Default value: \fB33,554,432\fR.
+Default value: \fB1\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_write_limit_override\fR (ulong)
+\fBzfs_vdev_async_write_active_max_dirty_percent\fR (int)
.ad
.RS 12n
-Override txg write limit
+When the pool has more than
+\fBzfs_vdev_async_write_active_max_dirty_percent\fR dirty data, use
+\fBzfs_vdev_async_write_max_active\fR to limit active async writes. If
+the dirty data is between min and max, the active I/O limit is linearly
+interpolated. See the section "ZFS I/O SCHEDULER".
.sp
-Default value: \fB0\fR.
+Default value: \fB60\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_write_limit_shift\fR (int)
+\fBzfs_vdev_async_write_active_min_dirty_percent\fR (int)
.ad
.RS 12n
-log2(fraction of memory) per txg
+When the pool has less than
+\fBzfs_vdev_async_write_active_min_dirty_percent\fR dirty data, use
+\fBzfs_vdev_async_write_min_active\fR to limit active async writes. If
+the dirty data is between min and max, the active I/O limit is linearly
+interpolated. See the section "ZFS I/O SCHEDULER".
.sp
-Default value: \fB3\fR.
+Default value: \fB30\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_zevent_cols\fR (int)
+\fBzfs_vdev_async_write_max_active\fR (int)
.ad
.RS 12n
-Max event column width
+Maximum asynchronous write I/Os active to each device.
+See the section "ZFS I/O SCHEDULER".
.sp
-Default value: \fB80\fR.
+Default value: \fB10\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_zevent_console\fR (int)
+\fBzfs_vdev_async_write_min_active\fR (int)
.ad
.RS 12n
-Log events to the console
+Minimum asynchronous write I/Os active to each device.
+See the section "ZFS I/O SCHEDULER".
.sp
-Use \fB1\fR for yes and \fB0\fR for no (default).
+Lower values are associated with better latency on rotational media but poorer
+resilver performance. The default value of 2 was chosen as a compromise. A
+value of 3 has been shown to improve resilver performance further at a cost of
+further increasing latency.
+.sp
+Default value: \fB2\fR.
.RE
.sp
.ne 2
.na
-\fBzfs_zevent_len_max\fR (int)
+\fBzfs_vdev_max_active\fR (int)
.ad
.RS 12n
-Max event queue length
+The maximum number of I/Os active to each device. Ideally, this will be >=
+the sum of each queue's max_active. It must be at least the sum of each
+queue's min_active. See the section "ZFS I/O SCHEDULER".
.sp
-Default value: \fB0\fR.
+Default value: \fB1,000\fR.
.RE
.sp
.ne 2
.na
-\fBzil_replay_disable\fR (int)
+\fBzfs_vdev_scrub_max_active\fR (int)
.ad
.RS 12n
-Disable intent logging replay
+Maximum scrub I/Os active to each device.
+See the section "ZFS I/O SCHEDULER".
.sp
-Use \fB1\fR for yes and \fB0\fR for no (default).
+Default value: \fB2\fR.
.RE
.sp
.ne 2
.na
-\fBzil_slog_limit\fR (ulong)
+\fBzfs_vdev_scrub_min_active\fR (int)
.ad
.RS 12n
-Max commit bytes to separate log device
+Minimum scrub I/Os active to each device.
+See the section "ZFS I/O SCHEDULER".
.sp
-Default value: \fB1,048,576\fR.
+Default value: \fB1\fR.
.RE
.sp
.ne 2
.na
-\fBzio_bulk_flags\fR (int)
+\fBzfs_vdev_sync_read_max_active\fR (int)
.ad
.RS 12n
-Additional flags to pass to bulk buffers
+Maximum synchronous read I/Os active to each device.
+See the section "ZFS I/O SCHEDULER".
.sp
-Default value: \fB0\fR.
+Default value: \fB10\fR.
.RE
.sp
.ne 2
.na
-\fBzio_delay_max\fR (int)
+\fBzfs_vdev_sync_read_min_active\fR (int)
.ad
.RS 12n
-Max zio millisec delay before posting event
+Minimum synchronous read I/Os active to each device.
+See the section "ZFS I/O SCHEDULER".
.sp
-Default value: \fB30,000\fR.
+Default value: \fB10\fR.
.RE
.sp
.ne 2
.na
-\fBzio_injection_enabled\fR (int)
+\fBzfs_vdev_sync_write_max_active\fR (int)
.ad
.RS 12n
-Enable fault injection
+Maximum synchronous write I/Os active to each device.
+See the section "ZFS I/O SCHEDULER".
.sp
-Use \fB1\fR for yes and \fB0\fR for no (default).
+Default value: \fB10\fR.
.RE
.sp
.ne 2
.na
-\fBzio_requeue_io_start_cut_in_line\fR (int)
+\fBzfs_vdev_sync_write_min_active\fR (int)
.ad
.RS 12n
-Prioritize requeued I/O
+Minimum synchronous write I/Os active to each device.
+See the section "ZFS I/O SCHEDULER".
.sp
-Default value: \fB0\fR.
+Default value: \fB10\fR.
.RE
.sp
.ne 2
.na
-\fBzvol_inhibit_dev\fR (uint)
+\fBzfs_vdev_queue_depth_pct\fR (int)
+.ad
+.RS 12n
+Maximum number of queued allocations per top-level vdev expressed as
+a percentage of \fBzfs_vdev_async_write_max_active\fR which allows the
+system to detect devices that are more capable of handling allocations
+and to allocate more blocks to those devices. It allows for dynamic
+allocation distribution when devices are imbalanced as fuller devices
+will tend to be slower than empty devices.
+
+See also \fBzio_dva_throttle_enabled\fR.
+.sp
+Default value: \fB1000\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_disable_dup_eviction\fR (int)
.ad
.RS 12n
-Do not create zvol device nodes
+Disable duplicate buffer eviction
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE
.sp
.ne 2
.na
-\fBzvol_major\fR (uint)
+\fBzfs_expire_snapshot\fR (int)
.ad
.RS 12n
-Major number for zvol device
+Seconds to expire .zfs/snapshot
.sp
-Default value: \fB230\fR.
+Default value: \fB300\fR.
.RE
.sp
.ne 2
.na
-\fBzvol_max_discard_blocks\fR (ulong)
+\fBzfs_admin_snapshot\fR (int)
.ad
.RS 12n
-Max number of blocks to discard at once
+Allow the creation, removal, or renaming of entries in the .zfs/snapshot
+directory to cause the creation, destruction, or renaming of snapshots.
+When enabled this functionality works both locally and over NFS exports
+which have the 'no_root_squash' option set. This functionality is disabled
+by default.
.sp
-Default value: \fB16,384\fR.
+Use \fB1\fR for yes and \fB0\fR for no (default).
.RE
.sp
.ne 2
.na
-\fBzvol_threads\fR (uint)
+\fBzfs_flags\fR (int)
.ad
.RS 12n
-Number of threads for zvol device
+Set additional debugging flags. The following flags may be bitwise-or'd
+together.
+.sp
+.TS
+box;
+rB lB
+lB lB
+r l.
+Value Symbolic Name
+ Description
+_
+1 ZFS_DEBUG_DPRINTF
+ Enable dprintf entries in the debug log.
+_
+2 ZFS_DEBUG_DBUF_VERIFY *
+ Enable extra dbuf verifications.
+_
+4 ZFS_DEBUG_DNODE_VERIFY *
+ Enable extra dnode verifications.
+_
+8 ZFS_DEBUG_SNAPNAMES
+ Enable snapshot name verification.
+_
+16 ZFS_DEBUG_MODIFY
+ Check for illegally modified ARC buffers.
+_
+32 ZFS_DEBUG_SPA
+ Enable spa_dbgmsg entries in the debug log.
+_
+64 ZFS_DEBUG_ZIO_FREE
+ Enable verification of block frees.
+_
+128 ZFS_DEBUG_HISTOGRAM_VERIFY
+ Enable extra spacemap histogram verifications.
+_
+256 ZFS_DEBUG_METASLAB_VERIFY
+ Verify space accounting on disk matches in-core range_trees.
+_
+512 ZFS_DEBUG_SET_ERROR
+ Enable SET_ERROR and dprintf entries in the debug log.
+.TE
+.sp
+* Requires debug build.
.sp
-Default value: \fB32\fR.
+Default value: \fB0\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_free_leak_on_eio\fR (int)
+.ad
+.RS 12n
+If destroy encounters an EIO while reading metadata (e.g. indirect
+blocks), space referenced by the missing metadata can not be freed.
+Normally this causes the background destroy to become "stalled", as
+it is unable to make forward progress. While in this stalled state,
+all remaining space to free from the error-encountering filesystem is
+"temporarily leaked". Set this flag to cause it to ignore the EIO,
+permanently leak the space from indirect blocks that can not be read,
+and continue to free everything else that it can.
+
+The default, "stalling" behavior is useful if the storage partially
+fails (i.e. some but not all i/os fail), and then later recovers. In
+this case, we will be able to continue pool operations while it is
+partially failed, and when it recovers, we can continue to free the
+space, with no leaks. However, note that this case is actually
+fairly rare.
+
+Typically pools either (a) fail completely (but perhaps temporarily,
+e.g. a top-level vdev going offline), or (b) have localized,
+permanent errors (e.g. disk returns the wrong data due to bit flip or
+firmware bug). In case (a), this setting does not matter because the
+pool will be suspended and the sync thread will not be able to make
+forward progress regardless. In case (b), because the error is
+permanent, the best we can do is leak the minimum amount of space,
+which is what setting this flag will do. Therefore, it is reasonable
+for this flag to normally be set, but we chose the more conservative
+approach of not setting it, so that there is no possibility of
+leaking space in the "partial temporary" failure case.
+.sp
+Default value: \fB0\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_free_min_time_ms\fR (int)
+.ad
+.RS 12n
+During a \fBzfs destroy\fR operation using \fBfeature@async_destroy\fR a minimum
+of this much time will be spent working on freeing blocks per txg.
+.sp
+Default value: \fB1,000\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_immediate_write_sz\fR (long)
+.ad
+.RS 12n
+Largest data block to write to zil. Larger blocks will be treated as if the
+dataset being written to had the property setting \fBlogbias=throughput\fR.
+.sp
+Default value: \fB32,768\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_max_recordsize\fR (int)
+.ad
+.RS 12n
+We currently support block sizes from 512 bytes to 16MB. The benefits of
+larger blocks, and thus larger IO, need to be weighed against the cost of
+COWing a giant block to modify one byte. Additionally, very large blocks
+can have an impact on i/o latency, and also potentially on the memory
+allocator. Therefore, we do not allow the recordsize to be set larger than
+zfs_max_recordsize (default 1MB). Larger blocks can be created by changing
+this tunable, and pools with larger blocks can always be imported and used,
+regardless of this setting.
+.sp
+Default value: \fB1,048,576\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_mdcomp_disable\fR (int)
+.ad
+.RS 12n
+Disable meta data compression
+.sp
+Use \fB1\fR for yes and \fB0\fR for no (default).
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_metaslab_fragmentation_threshold\fR (int)
+.ad
+.RS 12n
+Allow metaslabs to keep their active state as long as their fragmentation
+percentage is less than or equal to this value. An active metaslab that
+exceeds this threshold will no longer keep its active status allowing
+better metaslabs to be selected.
+.sp
+Default value: \fB70\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_mg_fragmentation_threshold\fR (int)
+.ad
+.RS 12n
+Metaslab groups are considered eligible for allocations if their
+fragmentation metric (measured as a percentage) is less than or equal to
+this value. If a metaslab group exceeds this threshold then it will be
+skipped unless all metaslab groups within the metaslab class have also
+crossed this threshold.
+.sp
+Default value: \fB85\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_mg_noalloc_threshold\fR (int)
+.ad
+.RS 12n
+Defines a threshold at which metaslab groups should be eligible for
+allocations. The value is expressed as a percentage of free space
+beyond which a metaslab group is always eligible for allocations.
+If a metaslab group's free space is less than or equal to the
+threshold, the allocator will avoid allocating to that group
+unless all groups in the pool have reached the threshold. Once all
+groups have reached the threshold, all groups are allowed to accept
+allocations. The default value of 0 disables the feature and causes
+all metaslab groups to be eligible for allocations.
+
+This parameter allows one to deal with pools having heavily imbalanced
+vdevs such as would be the case when a new vdev has been added.
+Setting the threshold to a non-zero percentage will stop allocations
+from being made to vdevs that aren't filled to the specified percentage
+and allow lesser filled vdevs to acquire more allocations than they
+otherwise would under the old \fBzfs_mg_alloc_failures\fR facility.
+.sp
+Default value: \fB0\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_multihost_history\fR (int)
+.ad
+.RS 12n
+Historical statistics for the last N multihost updates will be available in
+\fB/proc/spl/kstat/zfs/<pool>/multihost\fR
+.sp
+Default value: \fB0\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_multihost_interval\fR (ulong)
+.ad
+.RS 12n
+Used to control the frequency of multihost writes which are performed when the
+\fBmultihost\fR pool property is on. This is one factor used to determine
+the length of the activity check during import.
+.sp
+The multihost write period is \fBzfs_multihost_interval / leaf-vdevs\fR milliseconds.
+This means that on average a multihost write will be issued for each leaf vdev every
+\fBzfs_multihost_interval\fR milliseconds. In practice, the observed period can
+vary with the I/O load and this observed value is the delay which is stored in
+the uberblock.
+.sp
+On import the activity check waits a minimum amount of time determined by
+\fBzfs_multihost_interval * zfs_multihost_import_intervals\fR. The activity
+check time may be further extended if the value of mmp delay found in the best
+uberblock indicates actual multihost updates happened at longer intervals than
+\fBzfs_multihost_interval\fR. A minimum value of \fB100ms\fR is enforced.
+.sp
+Default value: \fB1000\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_multihost_import_intervals\fR (uint)
+.ad
+.RS 12n
+Used to control the duration of the activity test on import. Smaller values of
+\fBzfs_multihost_import_intervals\fR will reduce the import time but increase
+the risk of failing to detect an active pool. The total activity check time is
+never allowed to drop below one second. A value of 0 is ignored and treated as
+if it was set to 1
+.sp
+Default value: \fB10\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_multihost_fail_intervals\fR (uint)
+.ad
+.RS 12n
+Controls the behavior of the pool when multihost write failures are detected.
+.sp
+When \fBzfs_multihost_fail_intervals = 0\fR then multihost write failures are ignored.
+The failures will still be reported to the ZED which depending on its
+configuration may take action such as suspending the pool or offlining a device.
+.sp
+When \fBzfs_multihost_fail_intervals > 0\fR then sequential multihost write failures
+will cause the pool to be suspended. This occurs when
+\fBzfs_multihost_fail_intervals * zfs_multihost_interval\fR milliseconds have
+passed since the last successful multihost write. This guarantees the activity test
+will see multihost writes if the pool is imported.
+.sp
+Default value: \fB5\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_no_scrub_io\fR (int)
+.ad
+.RS 12n
+Set for no scrub I/O. This results in scrubs not actually scrubbing data and
+simply doing a metadata crawl of the pool instead.
+.sp
+Use \fB1\fR for yes and \fB0\fR for no (default).
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_no_scrub_prefetch\fR (int)
+.ad
+.RS 12n
+Set to disable block prefetching for scrubs.
+.sp
+Use \fB1\fR for yes and \fB0\fR for no (default).
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_nocacheflush\fR (int)
+.ad
+.RS 12n
+Disable cache flush operations on disks when writing. Beware, this may cause
+corruption if disks re-order writes.
+.sp
+Use \fB1\fR for yes and \fB0\fR for no (default).
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_nopwrite_enabled\fR (int)
+.ad
+.RS 12n
+Enable NOP writes
+.sp
+Use \fB1\fR for yes (default) and \fB0\fR to disable.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_dmu_offset_next_sync\fR (int)
+.ad
+.RS 12n
+Enable forcing txg sync to find holes. When enabled forces ZFS to act
+like prior versions when SEEK_HOLE or SEEK_DATA flags are used, which
+when a dnode is dirty causes txg's to be synced so that this data can be
+found.
+.sp
+Use \fB1\fR for yes and \fB0\fR to disable (default).
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_pd_bytes_max\fR (int)
+.ad
+.RS 12n
+The number of bytes which should be prefetched during a pool traversal
+(eg: \fBzfs send\fR or other data crawling operations)
+.sp
+Default value: \fB52,428,800\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_per_txg_dirty_frees_percent \fR (ulong)
+.ad
+.RS 12n
+Tunable to control percentage of dirtied blocks from frees in one TXG.
+After this threshold is crossed, additional dirty blocks from frees
+wait until the next TXG.
+A value of zero will disable this throttle.
+.sp
+Default value: \fB30\fR and \fB0\fR to disable.
+.RE
+
+
+
+.sp
+.ne 2
+.na
+\fBzfs_prefetch_disable\fR (int)
+.ad
+.RS 12n
+This tunable disables predictive prefetch. Note that it leaves "prescient"
+prefetch (e.g. prefetch for zfs send) intact. Unlike predictive prefetch,
+prescient prefetch never issues i/os that end up not being needed, so it
+can't hurt performance.
+.sp
+Use \fB1\fR for yes and \fB0\fR for no (default).
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_read_chunk_size\fR (long)
+.ad
+.RS 12n
+Bytes to read per chunk
+.sp
+Default value: \fB1,048,576\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_read_history\fR (int)
+.ad
+.RS 12n
+Historical statistics for the last N reads will be available in
+\fB/proc/spl/kstat/zfs/<pool>/reads\fR
+.sp
+Default value: \fB0\fR (no data is kept).
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_read_history_hits\fR (int)
+.ad
+.RS 12n
+Include cache hits in read history
+.sp
+Use \fB1\fR for yes and \fB0\fR for no (default).
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_recover\fR (int)
+.ad
+.RS 12n
+Set to attempt to recover from fatal errors. This should only be used as a
+last resort, as it typically results in leaked space, or worse.
+.sp
+Use \fB1\fR for yes and \fB0\fR for no (default).
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_resilver_min_time_ms\fR (int)
+.ad
+.RS 12n
+Resilvers are processed by the sync thread. While resilvering it will spend
+at least this much time working on a resilver between txg flushes.
+.sp
+Default value: \fB3,000\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_scrub_min_time_ms\fR (int)
+.ad
+.RS 12n
+Scrubs are processed by the sync thread. While scrubbing it will spend
+at least this much time working on a scrub between txg flushes.
+.sp
+Default value: \fB1,000\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_scan_checkpoint_intval\fR (int)
+.ad
+.RS 12n
+To preserve progress across reboots the sequential scan algorithm periodically
+needs to stop metadata scanning and issue all the verifications I/Os to disk.
+The frequency of this flushing is determined by the
+\fBfBzfs_scan_checkpoint_intval\fR tunable.
+.sp
+Default value: \fB7200\fR seconds (every 2 hours).
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_scan_fill_weight\fR (int)
+.ad
+.RS 12n
+This tunable affects how scrub and resilver I/O segments are ordered. A higher
+number indicates that we care more about how filled in a segment is, while a
+lower number indicates we care more about the size of the extent without
+considering the gaps within a segment. This value is only tunable upon module
+insertion. Changing the value afterwards will have no affect on scrub or
+resilver performance.
+.sp
+Default value: \fB3\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_scan_issue_strategy\fR (int)
+.ad
+.RS 12n
+Determines the order that data will be verified while scrubbing or resilvering.
+If set to \fB1\fR, data will be verified as sequentially as possible, given the
+amount of memory reserved for scrubbing (see \fBzfs_scan_mem_lim_fact\fR). This
+may improve scrub performance if the pool's data is very fragmented. If set to
+\fB2\fR, the largest mostly-contiguous chunk of found data will be verified
+first. By deferring scrubbing of small segments, we may later find adjacent data
+to coalesce and increase the segment size. If set to \fB0\fR, zfs will use
+strategy \fB1\fR during normal verification and strategy \fB2\fR while taking a
+checkpoint.
+.sp
+Default value: \fB0\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_scan_legacy\fR (int)
+.ad
+.RS 12n
+A value of 0 indicates that scrubs and resilvers will gather metadata in
+memory before issuing sequential I/O. A value of 1 indicates that the legacy
+algorithm will be used where I/O is initiated as soon as it is discovered.
+Changing this value to 0 will not affect scrubs or resilvers that are already
+in progress.
+.sp
+Default value: \fB0\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_scan_max_ext_gap\fR (int)
+.ad
+.RS 12n
+Indicates the largest gap in bytes between scrub / resilver I/Os that will still
+be considered sequential for sorting purposes. Changing this value will not
+affect scrubs or resilvers that are already in progress.
+.sp
+Default value: \fB2097152 (2 MB)\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_scan_mem_lim_fact\fR (int)
+.ad
+.RS 12n
+Maximum fraction of RAM used for I/O sorting by sequential scan algorithm.
+This tunable determines the hard limit for I/O sorting memory usage.
+When the hard limit is reached we stop scanning metadata and start issuing
+data verification I/O. This is done until we get below the soft limit.
+.sp
+Default value: \fB20\fR which is 5% of RAM (1/20).
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_scan_mem_lim_soft_fact\fR (int)
+.ad
+.RS 12n
+The fraction of the hard limit used to determined the soft limit for I/O sorting
+by the sequential scan algorithm. When we cross this limit from bellow no action
+is taken. When we cross this limit from above it is because we are issuing
+verification I/O. In this case (unless the metadata scan is done) we stop
+issuing verification I/O and start scanning metadata again until we get to the
+hard limit.
+.sp
+Default value: \fB20\fR which is 5% of the hard limit (1/20).
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_scan_vdev_limit\fR (int)
+.ad
+.RS 12n
+Maximum amount of data that can be concurrently issued at once for scrubs and
+resilvers per leaf device, given in bytes.
+.sp
+Default value: \fB41943040\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_send_corrupt_data\fR (int)
+.ad
+.RS 12n
+Allow sending of corrupt data (ignore read/checksum errors when sending data)
+.sp
+Use \fB1\fR for yes and \fB0\fR for no (default).
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_sync_pass_deferred_free\fR (int)
+.ad
+.RS 12n
+Flushing of data to disk is done in passes. Defer frees starting in this pass
+.sp
+Default value: \fB2\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_sync_pass_dont_compress\fR (int)
+.ad
+.RS 12n
+Don't compress starting in this pass
+.sp
+Default value: \fB5\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_sync_pass_rewrite\fR (int)
+.ad
+.RS 12n
+Rewrite new block pointers starting in this pass
+.sp
+Default value: \fB2\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_sync_taskq_batch_pct\fR (int)
+.ad
+.RS 12n
+This controls the number of threads used by the dp_sync_taskq. The default
+value of 75% will create a maximum of one thread per cpu.
+.sp
+Default value: \fB75\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_txg_history\fR (int)
+.ad
+.RS 12n
+Historical statistics for the last N txgs will be available in
+\fB/proc/spl/kstat/zfs/<pool>/txgs\fR
+.sp
+Default value: \fB0\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_txg_timeout\fR (int)
+.ad
+.RS 12n
+Flush dirty data to disk at least every N seconds (maximum txg duration)
+.sp
+Default value: \fB5\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_vdev_aggregation_limit\fR (int)
+.ad
+.RS 12n
+Max vdev I/O aggregation size
+.sp
+Default value: \fB131,072\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_vdev_cache_bshift\fR (int)
+.ad
+.RS 12n
+Shift size to inflate reads too
+.sp
+Default value: \fB16\fR (effectively 65536).
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_vdev_cache_max\fR (int)
+.ad
+.RS 12n
+Inflate reads smaller than this value to meet the \fBzfs_vdev_cache_bshift\fR
+size (default 64k).
+.sp
+Default value: \fB16384\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_vdev_cache_size\fR (int)
+.ad
+.RS 12n
+Total size of the per-disk cache in bytes.
+.sp
+Currently this feature is disabled as it has been found to not be helpful
+for performance and in some cases harmful.
+.sp
+Default value: \fB0\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_vdev_mirror_rotating_inc\fR (int)
+.ad
+.RS 12n
+A number by which the balancing algorithm increments the load calculation for
+the purpose of selecting the least busy mirror member when an I/O immediately
+follows its predecessor on rotational vdevs for the purpose of making decisions
+based on load.
+.sp
+Default value: \fB0\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_vdev_mirror_rotating_seek_inc\fR (int)
+.ad
+.RS 12n
+A number by which the balancing algorithm increments the load calculation for
+the purpose of selecting the least busy mirror member when an I/O lacks
+locality as defined by the zfs_vdev_mirror_rotating_seek_offset. I/Os within
+this that are not immediately following the previous I/O are incremented by
+half.
+.sp
+Default value: \fB5\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_vdev_mirror_rotating_seek_offset\fR (int)
+.ad
+.RS 12n
+The maximum distance for the last queued I/O in which the balancing algorithm
+considers an I/O to have locality.
+See the section "ZFS I/O SCHEDULER".
+.sp
+Default value: \fB1048576\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_vdev_mirror_non_rotating_inc\fR (int)
+.ad
+.RS 12n
+A number by which the balancing algorithm increments the load calculation for
+the purpose of selecting the least busy mirror member on non-rotational vdevs
+when I/Os do not immediately follow one another.
+.sp
+Default value: \fB0\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_vdev_mirror_non_rotating_seek_inc\fR (int)
+.ad
+.RS 12n
+A number by which the balancing algorithm increments the load calculation for
+the purpose of selecting the least busy mirror member when an I/O lacks
+locality as defined by the zfs_vdev_mirror_rotating_seek_offset. I/Os within
+this that are not immediately following the previous I/O are incremented by
+half.
+.sp
+Default value: \fB1\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_vdev_read_gap_limit\fR (int)
+.ad
+.RS 12n
+Aggregate read I/O operations if the gap on-disk between them is within this
+threshold.
+.sp
+Default value: \fB32,768\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_vdev_scheduler\fR (charp)
+.ad
+.RS 12n
+Set the Linux I/O scheduler on whole disk vdevs to this scheduler. Valid options
+are noop, cfq, bfq & deadline
+.sp
+Default value: \fBnoop\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_vdev_write_gap_limit\fR (int)
+.ad
+.RS 12n
+Aggregate write I/O over gap
+.sp
+Default value: \fB4,096\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_vdev_raidz_impl\fR (string)
+.ad
+.RS 12n
+Parameter for selecting raidz parity implementation to use.
+
+Options marked (always) below may be selected on module load as they are
+supported on all systems.
+The remaining options may only be set after the module is loaded, as they
+are available only if the implementations are compiled in and supported
+on the running system.
+
+Once the module is loaded, the content of
+/sys/module/zfs/parameters/zfs_vdev_raidz_impl will show available options
+with the currently selected one enclosed in [].
+Possible options are:
+ fastest - (always) implementation selected using built-in benchmark
+ original - (always) original raidz implementation
+ scalar - (always) scalar raidz implementation
+ sse2 - implementation using SSE2 instruction set (64bit x86 only)
+ ssse3 - implementation using SSSE3 instruction set (64bit x86 only)
+ avx2 - implementation using AVX2 instruction set (64bit x86 only)
+ avx512f - implementation using AVX512F instruction set (64bit x86 only)
+ avx512bw - implementation using AVX512F & AVX512BW instruction sets (64bit x86 only)
+ aarch64_neon - implementation using NEON (Aarch64/64 bit ARMv8 only)
+ aarch64_neonx2 - implementation using NEON with more unrolling (Aarch64/64 bit ARMv8 only)
+.sp
+Default value: \fBfastest\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_zevent_cols\fR (int)
+.ad
+.RS 12n
+When zevents are logged to the console use this as the word wrap width.
+.sp
+Default value: \fB80\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_zevent_console\fR (int)
+.ad
+.RS 12n
+Log events to the console
+.sp
+Use \fB1\fR for yes and \fB0\fR for no (default).
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_zevent_len_max\fR (int)
+.ad
+.RS 12n
+Max event queue length. A value of 0 will result in a calculated value which
+increases with the number of CPUs in the system (minimum 64 events). Events
+in the queue can be viewed with the \fBzpool events\fR command.
+.sp
+Default value: \fB0\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_zil_clean_taskq_maxalloc\fR (int)
+.ad
+.RS 12n
+The maximum number of taskq entries that are allowed to be cached. When this
+limit is exceeded itx's will be cleaned synchronously.
+.sp
+Default value: \fB1048576\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_zil_clean_taskq_minalloc\fR (int)
+.ad
+.RS 12n
+The number of taskq entries that are pre-populated when the taskq is first
+created and are immediately available for use.
+.sp
+Default value: \fB1024\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_zil_clean_taskq_nthr_pct\fR (int)
+.ad
+.RS 12n
+This controls the number of threads used by the dp_zil_clean_taskq. The default
+value of 100% will create a maximum of one thread per cpu.
+.sp
+Default value: \fB100\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzil_replay_disable\fR (int)
+.ad
+.RS 12n
+Disable intent logging replay. Can be disabled for recovery from corrupted
+ZIL
+.sp
+Use \fB1\fR for yes and \fB0\fR for no (default).
+.RE
+
+.sp
+.ne 2
+.na
+\fBzil_slog_bulk\fR (ulong)
+.ad
+.RS 12n
+Limit SLOG write size per commit executed with synchronous priority.
+Any writes above that will be executed with lower (asynchronous) priority
+to limit potential SLOG device abuse by single active ZIL writer.
+.sp
+Default value: \fB786,432\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzio_delay_max\fR (int)
+.ad
+.RS 12n
+A zevent will be logged if a ZIO operation takes more than N milliseconds to
+complete. Note that this is only a logging facility, not a timeout on
+operations.
+.sp
+Default value: \fB30,000\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzio_dva_throttle_enabled\fR (int)
+.ad
+.RS 12n
+Throttle block allocations in the ZIO pipeline. This allows for
+dynamic allocation distribution when devices are imbalanced.
+When enabled, the maximum number of pending allocations per top-level vdev
+is limited by \fBzfs_vdev_queue_depth_pct\fR.
+.sp
+Default value: \fB1\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzio_requeue_io_start_cut_in_line\fR (int)
+.ad
+.RS 12n
+Prioritize requeued I/O
+.sp
+Default value: \fB0\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzio_taskq_batch_pct\fR (uint)
+.ad
+.RS 12n
+Percentage of online CPUs (or CPU cores, etc) which will run a worker thread
+for IO. These workers are responsible for IO work such as compression and
+checksum calculations. Fractional number of CPUs will be rounded down.
+.sp
+The default value of 75 was chosen to avoid using all CPUs which can result in
+latency issues and inconsistent application performance, especially when high
+compression is enabled.
+.sp
+Default value: \fB75\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzvol_inhibit_dev\fR (uint)
+.ad
+.RS 12n
+Do not create zvol device nodes. This may slightly improve startup time on
+systems with a very large number of zvols.
+.sp
+Use \fB1\fR for yes and \fB0\fR for no (default).
+.RE
+
+.sp
+.ne 2
+.na
+\fBzvol_major\fR (uint)
+.ad
+.RS 12n
+Major number for zvol block devices
+.sp
+Default value: \fB230\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzvol_max_discard_blocks\fR (ulong)
+.ad
+.RS 12n
+Discard (aka TRIM) operations done on zvols will be done in batches of this
+many blocks, where block size is determined by the \fBvolblocksize\fR property
+of a zvol.
+.sp
+Default value: \fB16,384\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzvol_prefetch_bytes\fR (uint)
+.ad
+.RS 12n
+When adding a zvol to the system prefetch \fBzvol_prefetch_bytes\fR
+from the start and end of the volume. Prefetching these regions
+of the volume is desirable because they are likely to be accessed
+immediately by \fBblkid(8)\fR or by the kernel scanning for a partition
+table.
+.sp
+Default value: \fB131,072\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzvol_request_sync\fR (uint)
+.ad
+.RS 12n
+When processing I/O requests for a zvol submit them synchronously. This
+effectively limits the queue depth to 1 for each I/O submitter. When set
+to 0 requests are handled asynchronously by a thread pool. The number of
+requests which can be handled concurrently is controller by \fBzvol_threads\fR.
+.sp
+Default value: \fB0\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzvol_threads\fR (uint)
+.ad
+.RS 12n
+Max number of threads which can handle zvol I/O requests concurrently.
+.sp
+Default value: \fB32\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzvol_volmode\fR (uint)
+.ad
+.RS 12n
+Defines zvol block devices behaviour when \fBvolmode\fR is set to \fBdefault\fR.
+Valid values are \fB1\fR (full), \fB2\fR (dev) and \fB3\fR (none).
+.sp
+Default value: \fB1\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_qat_disable\fR (int)
+.ad
+.RS 12n
+This tunable disables qat hardware acceleration for gzip compression.
+It is available only if qat acceleration is compiled in and qat driver
+is present.
+.sp
+Use \fB1\fR for yes and \fB0\fR for no (default).
.RE
+.SH ZFS I/O SCHEDULER
+ZFS issues I/O operations to leaf vdevs to satisfy and complete I/Os.
+The I/O scheduler determines when and in what order those operations are
+issued. The I/O scheduler divides operations into five I/O classes
+prioritized in the following order: sync read, sync write, async read,
+async write, and scrub/resilver. Each queue defines the minimum and
+maximum number of concurrent operations that may be issued to the
+device. In addition, the device has an aggregate maximum,
+\fBzfs_vdev_max_active\fR. Note that the sum of the per-queue minimums
+must not exceed the aggregate maximum. If the sum of the per-queue
+maximums exceeds the aggregate maximum, then the number of active I/Os
+may reach \fBzfs_vdev_max_active\fR, in which case no further I/Os will
+be issued regardless of whether all per-queue minimums have been met.
+.sp
+For many physical devices, throughput increases with the number of
+concurrent operations, but latency typically suffers. Further, physical
+devices typically have a limit at which more concurrent operations have no
+effect on throughput or can actually cause it to decrease.
+.sp
+The scheduler selects the next operation to issue by first looking for an
+I/O class whose minimum has not been satisfied. Once all are satisfied and
+the aggregate maximum has not been hit, the scheduler looks for classes
+whose maximum has not been satisfied. Iteration through the I/O classes is
+done in the order specified above. No further operations are issued if the
+aggregate maximum number of concurrent operations has been hit or if there
+are no operations queued for an I/O class that has not hit its maximum.
+Every time an I/O is queued or an operation completes, the I/O scheduler
+looks for new operations to issue.
+.sp
+In general, smaller max_active's will lead to lower latency of synchronous
+operations. Larger max_active's may lead to higher overall throughput,
+depending on underlying storage.
+.sp
+The ratio of the queues' max_actives determines the balance of performance
+between reads, writes, and scrubs. E.g., increasing
+\fBzfs_vdev_scrub_max_active\fR will cause the scrub or resilver to complete
+more quickly, but reads and writes to have higher latency and lower throughput.
+.sp
+All I/O classes have a fixed maximum number of outstanding operations
+except for the async write class. Asynchronous writes represent the data
+that is committed to stable storage during the syncing stage for
+transaction groups. Transaction groups enter the syncing state
+periodically so the number of queued async writes will quickly burst up
+and then bleed down to zero. Rather than servicing them as quickly as
+possible, the I/O scheduler changes the maximum number of active async
+write I/Os according to the amount of dirty data in the pool. Since
+both throughput and latency typically increase with the number of
+concurrent operations issued to physical devices, reducing the
+burstiness in the number of concurrent operations also stabilizes the
+response time of operations from other -- and in particular synchronous
+-- queues. In broad strokes, the I/O scheduler will issue more
+concurrent operations from the async write queue as there's more dirty
+data in the pool.
+.sp
+Async Writes
+.sp
+The number of concurrent operations issued for the async write I/O class
+follows a piece-wise linear function defined by a few adjustable points.
+.nf
+
+ | o---------| <-- zfs_vdev_async_write_max_active
+ ^ | /^ |
+ | | / | |
+active | / | |
+ I/O | / | |
+count | / | |
+ | / | |
+ |-------o | | <-- zfs_vdev_async_write_min_active
+ 0|_______^______|_________|
+ 0% | | 100% of zfs_dirty_data_max
+ | |
+ | `-- zfs_vdev_async_write_active_max_dirty_percent
+ `--------- zfs_vdev_async_write_active_min_dirty_percent
+
+.fi
+Until the amount of dirty data exceeds a minimum percentage of the dirty
+data allowed in the pool, the I/O scheduler will limit the number of
+concurrent operations to the minimum. As that threshold is crossed, the
+number of concurrent operations issued increases linearly to the maximum at
+the specified maximum percentage of the dirty data allowed in the pool.
+.sp
+Ideally, the amount of dirty data on a busy pool will stay in the sloped
+part of the function between \fBzfs_vdev_async_write_active_min_dirty_percent\fR
+and \fBzfs_vdev_async_write_active_max_dirty_percent\fR. If it exceeds the
+maximum percentage, this indicates that the rate of incoming data is
+greater than the rate that the backend storage can handle. In this case, we
+must further throttle incoming writes, as described in the next section.
+
+.SH ZFS TRANSACTION DELAY
+We delay transactions when we've determined that the backend storage
+isn't able to accommodate the rate of incoming writes.
+.sp
+If there is already a transaction waiting, we delay relative to when
+that transaction will finish waiting. This way the calculated delay time
+is independent of the number of threads concurrently executing
+transactions.
+.sp
+If we are the only waiter, wait relative to when the transaction
+started, rather than the current time. This credits the transaction for
+"time already served", e.g. reading indirect blocks.
+.sp
+The minimum time for a transaction to take is calculated as:
+.nf
+ min_time = zfs_delay_scale * (dirty - min) / (max - dirty)
+ min_time is then capped at 100 milliseconds.
+.fi
+.sp
+The delay has two degrees of freedom that can be adjusted via tunables. The
+percentage of dirty data at which we start to delay is defined by
+\fBzfs_delay_min_dirty_percent\fR. This should typically be at or above
+\fBzfs_vdev_async_write_active_max_dirty_percent\fR so that we only start to
+delay after writing at full speed has failed to keep up with the incoming write
+rate. The scale of the curve is defined by \fBzfs_delay_scale\fR. Roughly speaking,
+this variable determines the amount of delay at the midpoint of the curve.
+.sp
+.nf
+delay
+ 10ms +-------------------------------------------------------------*+
+ | *|
+ 9ms + *+
+ | *|
+ 8ms + *+
+ | * |
+ 7ms + * +
+ | * |
+ 6ms + * +
+ | * |
+ 5ms + * +
+ | * |
+ 4ms + * +
+ | * |
+ 3ms + * +
+ | * |
+ 2ms + (midpoint) * +
+ | | ** |
+ 1ms + v *** +
+ | zfs_delay_scale ----------> ******** |
+ 0 +-------------------------------------*********----------------+
+ 0% <- zfs_dirty_data_max -> 100%
+.fi
+.sp
+Note that since the delay is added to the outstanding time remaining on the
+most recent transaction, the delay is effectively the inverse of IOPS.
+Here the midpoint of 500us translates to 2000 IOPS. The shape of the curve
+was chosen such that small changes in the amount of accumulated dirty data
+in the first 3/4 of the curve yield relatively small differences in the
+amount of delay.
+.sp
+The effects can be easier to understand when the amount of delay is
+represented on a log scale:
+.sp
+.nf
+delay
+100ms +-------------------------------------------------------------++
+ + +
+ | |
+ + *+
+ 10ms + *+
+ + ** +
+ | (midpoint) ** |
+ + | ** +
+ 1ms + v **** +
+ + zfs_delay_scale ----------> ***** +
+ | **** |
+ + **** +
+100us + ** +
+ + * +
+ | * |
+ + * +
+ 10us + * +
+ + +
+ | |
+ + +
+ +--------------------------------------------------------------+
+ 0% <- zfs_dirty_data_max -> 100%
+.fi
+.sp
+Note here that only as the amount of dirty data approaches its limit does
+the delay start to increase rapidly. The goal of a properly tuned system
+should be to keep the amount of dirty data out of that range by first
+ensuring that the appropriate limits are set for the I/O scheduler to reach
+optimal throughput on the backend storage, and then by changing the value
+of \fBzfs_delay_scale\fR to increase the steepness of the curve.
projects / mirror_zfs.git / blobdiff