zfs-module-parameters.5

'\" te
.\" Copyright (c) 2013 by Turbo Fredriksson <turbo@bayour.com>. All rights reserved.
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.\" and Distribution License (the "License").  You may not use this file except
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.\"
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.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 warmup
.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
.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
Number of max device writes to precache
.sp
Default value: \fB2\fR.
.RE

.sp
.ne 2
.na
\fBl2arc_headroom_boost\fR (ulong)
.ad
.RS 12n
Compressed l2arc_headroom multiplier
.sp
Default value: \fB200\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
Skip caching prefetched buffers
.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
Extra write bytes during device warmup
.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_debug\fR (int)
.ad
.RS 12n
Keep space maps in core to verify frees
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.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: 24
.RE

.sp
.ne 2
.na
\fBzfetch_array_rd_sz\fR (ulong)
.ad
.RS 12n
Number of bytes in a array_read
.sp
Default value: \fB1,048,576\fR.
.RE

.sp
.ne 2
.na
\fBzfetch_block_cap\fR (uint)
.ad
.RS 12n
Max number of blocks to fetch at a time
.sp
Default value: \fB256\fR.
.RE

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

.sp
.ne 2
.na
\fBzfetch_min_sec_reap\fR (uint)
.ad
.RS 12n
Min time before stream reclaim
.sp
Default value: \fB2\fR.
.RE

.sp
.ne 2
.na
\fBzfs_arc_grow_retry\fR (int)
.ad
.RS 12n
Seconds before growing arc size
.sp
Default value: \fB5\fR.
.RE

.sp
.ne 2
.na
\fBzfs_arc_max\fR (ulong)
.ad
.RS 12n
Max arc size
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzfs_arc_memory_throttle_disable\fR (int)
.ad
.RS 12n
Disable memory throttle
.sp
Use \fB1\fR for yes (default) and \fB0\fR to disable.
.RE

.sp
.ne 2
.na
\fBzfs_arc_meta_limit\fR (ulong)
.ad
.RS 12n
Meta limit for arc size
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzfs_arc_meta_prune\fR (int)
.ad
.RS 12n
Bytes of meta data to prune
.sp
Default value: \fB1,048,576\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
Min life of prefetch block
.sp
Default value: \fB100\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_shrink_shift\fR (int)
.ad
.RS 12n
log2(fraction of arc to reclaim)
.sp
Default value: \fB5\fR.
.RE

.sp
.ne 2
.na
\fBzfs_autoimport_disable\fR (int)
.ad
.RS 12n
Disable pool import at module load
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE

.sp
.ne 2
.na
\fBzfs_dbuf_state_index\fR (int)
.ad
.RS 12n
Calculate arc header index
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzfs_deadman_enabled\fR (int)
.ad
.RS 12n
Enable deadman timer
.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 (default) and \fB0\fR to disable.
.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_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: 25
.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_vdev_async_read_max_active\fR (int)
.ad
.RS 12n
Maxium 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
Maxium 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
Maxium 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
Maxium 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
Maxium 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_flags\fR (int)
.ad
.RS 12n
Set additional debugging flags
.sp
Default value: \fB1\fR.
.RE

.sp
.ne 2
.na
\fBzfs_free_min_time_ms\fR (int)
.ad
.RS 12n
Min millisecs to free 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
.sp
Default value: \fB32,768\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_no_scrub_io\fR (int)
.ad
.RS 12n
Set for no scrub I/O
.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 for no scrub prefetching
.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 flushes
.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_blks_max\fR (int)
.ad
.RS 12n
Max number of blocks to prefetch
.sp
Default value: \fB100\fR.
.RE

.sp
.ne 2
.na
\fBzfs_prefetch_disable\fR (int)
.ad
.RS 12n
Disable all ZFS prefetching
.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
.sp
Default value: \fB0\fR.
.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 resilver
.sp
Default value: \fB2\fR.
.RE

.sp
.ne 2
.na
\fBzfs_resilver_min_time_ms\fR (int)
.ad
.RS 12n
Min millisecs to resilver per txg
.sp
Default value: \fB3,000\fR.
.RE

.sp
.ne 2
.na
\fBzfs_scan_idle\fR (int)
.ad
.RS 12n
Idle window in clock ticks
.sp
Default value: \fB50\fR.
.RE

.sp
.ne 2
.na
\fBzfs_scan_min_time_ms\fR (int)
.ad
.RS 12n
Min millisecs to scrub per txg
.sp
Default value: \fB1,000\fR.
.RE

.sp
.ne 2
.na
\fBzfs_scrub_delay\fR (int)
.ad
.RS 12n
Number of ticks to delay scrub
.sp
Default value: \fB4\fR.
.RE

.sp
.ne 2
.na
\fBzfs_send_corrupt_data\fR (int)
.ad
.RS 12n
Allow to send 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
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 bps starting in this pass
.sp
Default value: \fB2\fR.
.RE

.sp
.ne 2
.na
\fBzfs_top_maxinflight\fR (int)
.ad
.RS 12n
Max I/Os per top-level
.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
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzfs_txg_timeout\fR (int)
.ad
.RS 12n
Max seconds worth of delta per txg
.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.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_cache_max\fR (int)
.ad
.RS 12n
Inflate reads small than max
.RE

.sp
.ne 2
.na
\fBzfs_vdev_cache_size\fR (int)
.ad
.RS 12n
Total size of the per-disk cache
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_mirror_switch_us\fR (int)
.ad
.RS 12n
Switch mirrors every N usecs
.sp
Default value: \fB10,000\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_read_gap_limit\fR (int)
.ad
.RS 12n
Aggregate read I/O over gap
.sp
Default value: \fB32,768\fR.
.RE

.sp
.ne 2
.na
\fBzfs_vdev_scheduler\fR (charp)
.ad
.RS 12n
I/O 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
Max event column 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
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzil_replay_disable\fR (int)
.ad
.RS 12n
Disable intent logging replay
.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_bulk_flags\fR (int)
.ad
.RS 12n
Additional flags to pass to bulk buffers
.sp
Default value: \fB0\fR.
.RE

.sp
.ne 2
.na
\fBzio_delay_max\fR (int)
.ad
.RS 12n
Max zio millisec delay before posting event
.sp
Default value: \fB30,000\fR.
.RE

.sp
.ne 2
.na
\fBzio_injection_enabled\fR (int)
.ad
.RS 12n
Enable fault injection
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.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
\fBzvol_inhibit_dev\fR (uint)
.ad
.RS 12n
Do not create zvol device nodes
.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 device
.sp
Default value: \fB230\fR.
.RE

.sp
.ne 2
.na
\fBzvol_max_discard_blocks\fR (ulong)
.ad
.RS 12n
Max number of blocks to discard at once
.sp
Default value: \fB16,384\fR.
.RE

.sp
.ne 2
.na
\fBzvol_threads\fR (uint)
.ad
.RS 12n
Number of threads for zvol device
.sp
Default value: \fB32\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.