zfs/man/man5/zfs-module-parameters.5

'\" te
.\" Copyright (c) 2013 by Turbo Fredriksson <turbo@bayour.com>. All rights reserved.
.\" 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]
.TH ZFS-MODULE-PARAMETERS 5 "Nov 16, 2013"
.SH NAME
zfs\-module\-parameters \- ZFS module parameters
.SH DESCRIPTION
.sp
.LP
Description of the different parameters to the ZFS module.

.SS "Module parameters"
.sp
.LP

.sp
.ne 2
.na
\fBl2arc_feed_again\fR (int)
.ad
.RS 12n
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

.sp
.ne 2
.na
\fBl2arc_feed_min_ms\fR (ulong)
.ad
.RS 12n
Min feed interval in milliseconds. Requires \fBl2arc_feed_again=1\fR and only
applicable in related situations.
.sp
Default value: \fB200\fR.
.RE

.sp
.ne 2
.na
\fBl2arc_feed_secs\fR (ulong)
.ad
.RS 12n
Seconds between L2ARC writing
.sp
Default value: \fB1\fR.
.RE

.sp
.ne 2
.na
\fBl2arc_headroom\fR (ulong)
.ad
.RS 12n
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

.sp
.ne 2
.na
\fBl2arc_headroom_boost\fR (ulong)
.ad
.RS 12n
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

.sp
.ne 2
.na
\fBl2arc_max_block_size\fR (ulong)
.ad
.RS 12n
The maximum block size which may be written to an L2ARC device, after
compression and other factors.  This setting is used to prevent a small
number of large blocks from pushing a larger number of small blocks out
of the cache.
.sp
Default value: \fB16,777,216\fR.
.RE

.sp
.ne 2
.na
\fBl2arc_nocompress\fR (int)
.ad
.RS 12n
Skip compressing L2ARC buffers
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE

.sp
.ne 2
.na
\fBl2arc_noprefetch\fR (int)
.ad
.RS 12n
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

.sp
.ne 2
.na
\fBl2arc_norw\fR (int)
.ad
.RS 12n
No reads during writes
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE

.sp
.ne 2
.na
\fBl2arc_write_boost\fR (ulong)
.ad
.RS 12n
Cold L2ARC devices will have \fBl2arc_write_nax\fR increased by this amount
while they remain cold.
.sp
Default value: \fB8,388,608\fR.
.RE

.sp
.ne 2
.na
\fBl2arc_write_max\fR (ulong)
.ad
.RS 12n
Max write bytes per interval
.sp
Default value: \fB8,388,608\fR.
.RE

.sp
.ne 2
.na
\fBmetaslab_aliquot\fR (ulong)
.ad
.RS 12n
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
Default value: \fB524,288\fR.
.RE

.sp
.ne 2
.na
\fBmetaslab_bias_enabled\fR (int)
.ad
.RS 12n
Enable metaslab group biasing based on its vdev's over- or under-utilization
relative to the pool.
.sp
Use \fB1\fR for yes (default) and \fB0\fR for no.
.RE

.sp
.ne 2
.na
\fBmetaslab_debug_load\fR (int)
.ad
.RS 12n
Load all metaslabs during pool import.
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE

.sp
.ne 2
.na
\fBmetaslab_debug_unload\fR (int)
.ad
.RS 12n
Prevent metaslabs from being unloaded.
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE

.sp
.ne 2
.na
\fBmetaslab_fragmentation_factor_enabled\fR (int)
.ad
.RS 12n
Enable use of the fragmentation metric in computing metaslab weights.
.sp
Use \fB1\fR for yes (default) and \fB0\fR for no.
.RE

.sp
.ne 2
.na
\fBmetaslabs_per_vdev\fR (int)
.ad
.RS 12n
When a vdev is added, it will be divided into approximately (but no more than) this number of metaslabs.
.sp
Default value: \fB200\fR.
.RE

.sp
.ne 2
.na
\fBmetaslab_preload_enabled\fR (int)
.ad
.RS 12n
Enable metaslab group preloading.
.sp
Use \fB1\fR for yes (default) and \fB0\fR for no.
.RE

.sp
.ne 2
.na
\fBmetaslab_lba_weighting_enabled\fR (int)
.ad
.RS 12n
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
Use \fB1\fR for yes (default) and \fB0\fR for no.
.RE

.sp
.ne 2
.na
\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
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: \fB24\fR.
.RE

.sp
.ne 2
.na
\fBspa_load_verify_data\fR (int)
.ad
.RS 12n
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: \fB1\fR.
.RE

.sp
.ne 2
.na
\fBspa_load_verify_metadata\fR (int)
.ad
.RS 12n
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: \fB1\fR.
.RE

.sp
.ne 2
.na
\fBspa_load_verify_maxinflight\fR (int)
.ad
.RS 12n
Maximum concurrent I/Os during the traversal performed during an "extreme
rewind" (\fB-X\fR) pool import.
.sp
Default value: \fB10000\fR.
.RE

.sp
.ne 2
.na
\fBspa_slop_shift\fR (int)
.ad
.RS 12n
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
\fBzfetch_array_rd_sz\fR (ulong)
.ad
.RS 12n
If prefetching is enabled, disable prefetching for reads larger than this size.
.sp
Default value: \fB1,048,576\fR.
.RE

.sp
.ne 2
.na
\fBzfetch_max_distance\fR (uint)
.ad
.RS 12n
Max bytes to prefetch per stream (default 8MB).
.sp
Default value: \fB8,388,608\fR.
.RE

.sp
.ne 2
.na
\fBzfetch_max_streams\fR (uint)
.ad
.RS 12n
Max number of streams per zfetch (prefetch streams per file).
.sp
Default value: \fB8\fR.
.RE

.sp
.ne 2
.na
\fBzfetch_min_sec_reap\fR (uint)
.ad
.RS 12n
Min time before an active prefetch stream can be reclaimed
.sp
Default value: \fB2\fR.
.RE

.sp
.ne 2
.na
\fBzfs_arc_average_blocksize\fR (int)
.ad
.RS 12n
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: \fB8192\fR.
.RE

.sp
.ne 2
.na
\fBzfs_arc_evict_batch_limit\fR (int)
.ad
.RS 12n
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: \fB10\fR.
.RE

.sp
.ne 2
.na
\fBzfs_arc_grow_retry\fR (int)
.ad
.RS 12n
After a memory pressure event the ARC will wait this many seconds before trying
to resume growth
.sp
Default value: \fB5\fR.
.RE

.sp
.ne 2
.na
\fBzfs_arc_lotsfree_percent\fR (int)
.ad
.RS 12n
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
Default value: \fB10\fR.
.RE

.sp
.ne 2
.na
\fBzfs_arc_max\fR (ulong)
.ad
.RS 12n
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
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_arc_meta_limit\fR (ulong)
.ad
.RS 12n
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 3/4 of the ARC may be used
for meta data.
.sp
This value my be changed dynamically except that it cannot be set back to 0
for 3/4 of the ARC; it must be set to an explicit value.
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzfs_arc_meta_min\fR (ulong)
.ad
.RS 12n
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
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzfs_arc_meta_prune\fR (int)
.ad
.RS 12n
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: \fB10,000\fR.
.RE

.sp
.ne 2
.na
\fBzfs_arc_meta_adjust_restarts\fR (ulong)
.ad
.RS 12n
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
Default value: \fB4096\fR.
.RE

.sp
.ne 2
.na
\fBzfs_arc_min\fR (ulong)
.ad
.RS 12n
Min arc size
.sp
Default value: \fB100\fR.
.RE

.sp
.ne 2
.na
\fBzfs_arc_min_prefetch_lifespan\fR (int)
.ad
.RS 12n
Minimum time prefetched blocks are locked in the ARC, specified in jiffies.
A value of 0 will default to 1 second.
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzfs_arc_num_sublists_per_state\fR (int)
.ad
.RS 12n
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.
.sp
Default value: \fR1\fB or the number of online CPUs, whichever is greater
.RE

.sp
.ne 2
.na
\fBzfs_arc_overflow_shift\fR (int)
.ad
.RS 12n
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: \fB8\fR.
.RE

.sp
.ne 2
.na

\fBzfs_arc_p_min_shift\fR (int)
.ad
.RS 12n
arc_c shift to calc min/max arc_p
.sp
Default value: \fB4\fR.
.RE

.sp
.ne 2
.na
\fBzfs_arc_p_aggressive_disable\fR (int)
.ad
.RS 12n
Disable aggressive arc_p growth
.sp
Use \fB1\fR for yes (default) and \fB0\fR to disable.
.RE

.sp
.ne 2
.na
\fBzfs_arc_p_dampener_disable\fR (int)
.ad
.RS 12n
Disable arc_p adapt dampener
.sp
Use \fB1\fR for yes (default) and \fB0\fR to disable.
.RE

.sp
.ne 2
.na
\fBzfs_arc_shrink_shift\fR (int)
.ad
.RS 12n
log2(fraction of arc to reclaim)
.sp
Default value: \fB5\fR.
.RE

.sp
.ne 2
.na
\fBzfs_arc_sys_free\fR (ulong)
.ad
.RS 12n
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: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzfs_autoimport_disable\fR (int)
.ad
.RS 12n
Disable pool import at module load by ignoring the cache file (typically \fB/etc/zfs/zpool.cache\fR).
.sp
Use \fB1\fR for yes (default) and \fB0\fR for no.
.RE

.sp
.ne 2
.na
\fBzfs_dbgmsg_enable\fR (int)
.ad
.RS 12n
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: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzfs_dbgmsg_maxsize\fR (int)
.ad
.RS 12n
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_deadman_enabled\fR (int)
.ad
.RS 12n
Enable deadman timer. See description below.
.sp
Use \fB1\fR for yes (default) and \fB0\fR to disable.
.RE

.sp
.ne 2
.na
\fBzfs_deadman_synctime_ms\fR (ulong)
.ad
.RS 12n
Expiration time in milliseconds. This value has two meanings. First it is
used to determine when the spa_deadman() logic should fire. By default the
spa_deadman() will fire if spa_sync() has not completed in 1000 seconds.
Secondly, the value determines if an I/O is considered "hung". Any I/O that
has not completed in zfs_deadman_synctime_ms is considered "hung" resulting
in a zevent being logged.
.sp
Default value: \fB1,000,000\fR.
.RE

.sp
.ne 2
.na
\fBzfs_dedup_prefetch\fR (int)
.ad
.RS 12n
Enable prefetching dedup-ed blks
.sp
Use \fB1\fR for yes and \fB0\fR to disable (default).
.RE

.sp
.ne 2
.na
\fBzfs_delay_min_dirty_percent\fR (int)
.ad
.RS 12n
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_delay_scale\fR (int)
.ad
.RS 12n
This controls how quickly the transaction delay approaches infinity.
Larger values cause longer delays for a given amount of dirty data.
.sp
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_delete_blocks\fR (ulong)
.ad
.RS 12n
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: \fB20,480\fR.
.RE

.sp
.ne 2
.na
\fBzfs_dirty_data_max\fR (int)
.ad
.RS 12n
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: 10 percent of all memory, capped at \fBzfs_dirty_data_max_max\fR.
.RE

.sp
.ne 2
.na
\fBzfs_dirty_data_max_max\fR (int)
.ad
.RS 12n
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: 25% of physical RAM.
.RE

.sp
.ne 2
.na
\fBzfs_dirty_data_max_max_percent\fR (int)
.ad
.RS 12n
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: \fN25\fR.
.RE

.sp
.ne 2
.na
\fBzfs_dirty_data_max_percent\fR (int)
.ad
.RS 12n
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: 10%, subject to \fBzfs_dirty_data_max_max\fR.
.RE

.sp
.ne 2
.na
\fBzfs_dirty_data_sync\fR (int)
.ad
.RS 12n
Start syncing out a transaction group if there is at least this much dirty data.
.sp
Default value: \fB67,108,864\fR.
.RE

.sp
.ne 2
.na
\fBzfs_free_bpobj_enabled\fR (int)
.ad
.RS 12n
Enable/disable the processing of the free_bpobj object.
.sp
Default value: \fB1\fR.
.RE

.sp
.ne 2
.na
\fBzfs_free_max_blocks\fR (ulong)
.ad
.RS 12n
Maximum number of blocks freed in a single txg.
.sp
Default value: \fB100,000\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_async_read_max_active\fR (int)
.ad
.RS 12n
Maximum asynchronous read I/Os active to each device.
See the section "ZFS I/O SCHEDULER".
.sp
Default value: \fB3\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_async_read_min_active\fR (int)
.ad
.RS 12n
Minimum asynchronous read I/Os active to each device.
See the section "ZFS I/O SCHEDULER".
.sp
Default value: \fB1\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_async_write_active_max_dirty_percent\fR (int)
.ad
.RS 12n
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: \fB60\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_async_write_active_min_dirty_percent\fR (int)
.ad
.RS 12n
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: \fB30\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_async_write_max_active\fR (int)
.ad
.RS 12n
Maximum asynchronous write I/Os active to each device.
See the section "ZFS I/O SCHEDULER".
.sp
Default value: \fB10\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_async_write_min_active\fR (int)
.ad
.RS 12n
Minimum asynchronous write I/Os active to each device.
See the section "ZFS I/O SCHEDULER".
.sp
Default value: \fB1\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_max_active\fR (int)
.ad
.RS 12n
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: \fB1,000\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_scrub_max_active\fR (int)
.ad
.RS 12n
Maximum scrub I/Os active to each device.
See the section "ZFS I/O SCHEDULER".
.sp
Default value: \fB2\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_scrub_min_active\fR (int)
.ad
.RS 12n
Minimum scrub I/Os active to each device.
See the section "ZFS I/O SCHEDULER".
.sp
Default value: \fB1\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_sync_read_max_active\fR (int)
.ad
.RS 12n
Maximum synchronous read I/Os active to each device.
See the section "ZFS I/O SCHEDULER".
.sp
Default value: \fB10\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_sync_read_min_active\fR (int)
.ad
.RS 12n
Minimum synchronous read I/Os active to each device.
See the section "ZFS I/O SCHEDULER".
.sp
Default value: \fB10\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_sync_write_max_active\fR (int)
.ad
.RS 12n
Maximum synchronous write I/Os active to each device.
See the section "ZFS I/O SCHEDULER".
.sp
Default value: \fB10\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_sync_write_min_active\fR (int)
.ad
.RS 12n
Minimum synchronous write I/Os active to each device.
See the section "ZFS I/O SCHEDULER".
.sp
Default value: \fB10\fR.
.RE

.sp
.ne 2
.na
\fBzfs_disable_dup_eviction\fR (int)
.ad
.RS 12n
Disable duplicate buffer eviction
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE

.sp
.ne 2
.na
\fBzfs_expire_snapshot\fR (int)
.ad
.RS 12n
Seconds to expire .zfs/snapshot
.sp
Default value: \fB300\fR.
.RE

.sp
.ne 2
.na
\fBzfs_admin_snapshot\fR (int)
.ad
.RS 12n
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
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE

.sp
.ne 2
.na
\fBzfs_flags\fR (int)
.ad
.RS 12n
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.
.TE
.sp
* Requires debug build.
.sp
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 \fRzfs destroy\fB operation using \fRfeature@async_destroy\fB 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 \fRlogbias=throughput\fB.
.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 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_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_pd_bytes_max\fR (int)
.ad
.RS 12n
The number of bytes which should be prefetched during a pool traversal
(eg: \fRzfs send\fB or other data crawling operations)
.sp
Default value: \fB52,428,800\fR.
.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
Historic statistics for the last N reads will be available in
\fR/proc/spl/kstat/zfs/POOLNAME/reads\fB
.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_delay\fR (int)
.ad
.RS 12n
Number of ticks to delay prior to issuing a resilver I/O operation when
a non-resilver or non-scrub I/O operation has occurred within the past
\fBzfs_scan_idle\fR ticks.
.sp
Default value: \fB2\fR.
.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_scan_idle\fR (int)
.ad
.RS 12n
Idle window in clock ticks.  During a scrub or a resilver, if
a non-scrub or non-resilver I/O operation has occurred during this
window, the next scrub or resilver operation is delayed by, respectively
\fBzfs_scrub_delay\fR or \fBzfs_resilver_delay\fR ticks.
.sp
Default value: \fB50\fR.
.RE

.sp
.ne 2
.na
\fBzfs_scan_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_scrub_delay\fR (int)
.ad
.RS 12n
Number of ticks to delay prior to issuing a scrub I/O operation when
a non-scrub or non-resilver I/O operation has occurred within the past
\fBzfs_scan_idle\fR ticks.
.sp
Default value: \fB4\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_top_maxinflight\fR (int)
.ad
.RS 12n
Max concurrent I/Os per top-level vdev (mirrors or raidz arrays) allowed during
scrub or resilver operations.
.sp
Default value: \fB32\fR.
.RE

.sp
.ne 2
.na
\fBzfs_txg_history\fR (int)
.ad
.RS 12n
Historic statistics for the last N txgs will be available in
\fR/proc/spl/kstat/zfs/POOLNAME/txgs\fB
.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 small than this value to meet the \fBzfs_vdev_cache_bshift\fR
size.
.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
.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_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
\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_limit\fR (ulong)
.ad
.RS 12n
Max commit bytes to separate log device
.sp
Default value: \fB1,048,576\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_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

.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.