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ZFS(4) Kernel Interfaces Manual ZFS(4)

NAME
zfs -- tuning of the ZFS kernel module

DESCRIPTION
The ZFS module supports these parameters:

dbuf_cache_max_bytes=UINT64_MAXB (u64)
Maximum size in bytes of the dbuf cache. The target size is
determined by the MIN versus 1/2^dbuf_cache_shift (1/32nd) of
the target ARC size. The behavior of the dbuf cache and its
associated settings can be observed via the
/proc/spl/kstat/zfs/dbufstats kstat.

dbuf_metadata_cache_max_bytes=UINT64_MAXB (u64)
Maximum size in bytes of the metadata dbuf cache. The target
size is determined by the MIN versus
1/2^dbuf_metadata_cache_shift (1/64th) of the target ARC size.
The behavior of the metadata dbuf cache and its associated set-
tings can be observed via the /proc/spl/kstat/zfs/dbufstats
kstat.

dbuf_cache_hiwater_pct=10% (uint)
The percentage over dbuf_cache_max_bytes when dbufs must be
evicted directly.

dbuf_cache_lowater_pct=10% (uint)
The percentage below dbuf_cache_max_bytes when the evict thread
stops evicting dbufs.

dbuf_cache_shift=5 (uint)
Set the size of the dbuf cache (dbuf_cache_max_bytes) to a log2
fraction of the target ARC size.

dbuf_metadata_cache_shift=6 (uint)
Set the size of the dbuf metadata cache
(dbuf_metadata_cache_max_bytes) to a log2 fraction of the tar-
get ARC size.

dbuf_mutex_cache_shift=0 (uint)
Set the size of the mutex array for the dbuf cache. When set
to 0 the array is dynamically sized based on total system mem-
ory.

dmu_object_alloc_chunk_shift=7 (128) (uint)
dnode slots allocated in a single operation as a power of 2.
The default value minimizes lock contention for the bulk opera-
tion performed.

dmu_ddt_copies=3 (uint)
Controls the number of copies stored for DeDup Table (DDT) ob-
jects. Reducing the number of copies to 1 from the previous
default of 3 can reduce the write inflation caused by dedupli-
cation. This assumes redundancy for this data is provided by
the vdev layer. If the DDT is damaged, space may be leaked
(not freed) when the DDT can not report the correct reference
count.

dmu_prefetch_max=134217728B (128 MiB) (uint)
Limit the amount we can prefetch with one call to this amount
in bytes. This helps to limit the amount of memory that can be
used by prefetching.

l2arc_feed_again=1|0 (int)
Turbo L2ARC warm-up. When the L2ARC is cold the fill interval
will be set as fast as possible.

l2arc_feed_min_ms=200 (u64)
Min feed interval in milliseconds. Requires l2arc_feed_again=1
and only applicable in related situations.

l2arc_feed_secs=1 (u64)
Seconds between L2ARC writing.

l2arc_headroom=8 (u64)
How far through the ARC lists to search for L2ARC cacheable
content, expressed as a multiplier of l2arc_write_max. ARC
persistence across reboots can be achieved with persistent
L2ARC by setting this parameter to 0, allowing the full length
of ARC lists to be searched for cacheable content.

l2arc_headroom_boost=200% (u64)
Scales l2arc_headroom by this percentage when L2ARC contents
are being successfully compressed before writing. A value of
100 disables this feature.

l2arc_exclude_special=0|1 (int)
Controls whether buffers present on special vdevs are eligible
for caching into L2ARC. If set to 1, exclude dbufs on special
vdevs from being cached to L2ARC.

l2arc_mfuonly=0|1|2 (int)
Controls whether only MFU metadata and data are cached from ARC
into L2ARC. This may be desired to avoid wasting space on
L2ARC when reading/writing large amounts of data that are not
expected to be accessed more than once.

The default is 0, meaning both MRU and MFU data and metadata
are cached. When turning off this feature (setting it to 0),
some MRU buffers will still be present in ARC and eventually
cached on L2ARC. If l2arc_noprefetch=0, some prefetched
buffers will be cached to L2ARC, and those might later transi-
tion to MRU, in which case the l2arc_mru_asize arcstat will not
be 0.

Setting it to 1 means to L2 cache only MFU data and metadata.

Setting it to 2 means to L2 cache all metadata (MRU+MFU) but
only MFU data (i.e. MRU data are not cached). This can be the
right setting to cache as much metadata as possible even when
having high data turnover.

Regardless of l2arc_noprefetch, some MFU buffers might be
evicted from ARC, accessed later on as prefetches and transi-
tion to MRU as prefetches. If accessed again they are counted
as MRU and the l2arc_mru_asize arcstat will not be 0.

The ARC status of L2ARC buffers when they were first cached in
L2ARC can be seen in the l2arc_mru_asize, l2arc_mfu_asize, and
l2arc_prefetch_asize arcstats when importing the pool or onlin-
ing a cache device if persistent L2ARC is enabled.

The evict_l2_eligible_mru arcstat does not take into account if
this option is enabled as the information provided by the
evict_l2_eligible_m[rf]u arcstats can be used to decide if tog-
gling this option is appropriate for the current workload.

l2arc_meta_percent=33% (uint)
Percent of ARC size allowed for L2ARC-only headers. Since
L2ARC buffers are not evicted on memory pressure, too many
headers on a system with an irrationally large L2ARC can render
it slow or unusable. This parameter limits L2ARC writes and
rebuilds to achieve the target.

l2arc_trim_ahead=0% (u64)
Trims ahead of the current write size (l2arc_write_max) on
L2ARC devices by this percentage of write size if we have
filled the device. If set to 100 we TRIM twice the space re-
quired to accommodate upcoming writes. A minimum of 64 MiB
will be trimmed. It also enables TRIM of the whole L2ARC de-
vice upon creation or addition to an existing pool or if the
header of the device is invalid upon importing a pool or onlin-
ing a cache device. A value of 0 disables TRIM on L2ARC alto-
gether and is the default as it can put significant stress on
the underlying storage devices. This will vary depending of
how well the specific device handles these commands.

l2arc_noprefetch=1|0 (int)
Do not write buffers to L2ARC if they were prefetched but not
used by applications. In case there are prefetched buffers in
L2ARC and this option is later set, we do not read the
prefetched buffers from L2ARC. Unsetting this option is useful
for caching sequential reads from the disks to L2ARC and serve
those reads from L2ARC later on. This may be beneficial in
case the L2ARC device is significantly faster in sequential
reads than the disks of the pool.

Use 1 to disable and 0 to enable caching/reading prefetches
to/from L2ARC.

l2arc_norw=0|1 (int)
No reads during writes.

l2arc_write_boost=33554432B (32 MiB) (u64)
Cold L2ARC devices will have l2arc_write_max increased by this
amount while they remain cold.

l2arc_write_max=33554432B (32 MiB) (u64)
Max write bytes per interval.

l2arc_rebuild_enabled=1|0 (int)
Rebuild the L2ARC when importing a pool (persistent L2ARC).
This can be disabled if there are problems importing a pool or
attaching an L2ARC device (e.g. the L2ARC device is slow in
reading stored log metadata, or the metadata has become somehow
fragmented/unusable).

l2arc_rebuild_blocks_min_l2size=1073741824B (1 GiB) (u64)
Minimum size of an L2ARC device required in order to write log
blocks in it. The log blocks are used upon importing the pool
to rebuild the persistent L2ARC.

For L2ARC devices less than 1 GiB, the amount of data
l2arc_evict() evicts is significant compared to the amount of
restored L2ARC data. In this case, do not write log blocks in
L2ARC in order not to waste space.

metaslab_aliquot=2097152B (2 MiB) (u64)
Metaslab group's per child vdev allocation granularity, in
bytes. This is roughly similar to what would be referred to as
the "stripe size" in traditional RAID arrays. In normal opera-
tion, ZFS will try to write this amount of data to each child
of a top-level vdev before moving on to the next top-level
vdev.

metaslab_bias_enabled=1|0 (int)
Enable metaslab groups biasing based on their over- or under-
utilization relative to the metaslab class average. If dis-
abled, each metaslab group will receive allocations propor-
tional to its capacity.

metaslab_perf_bias=1|0|2 (int)
Controls metaslab groups biasing based on their write perfor-
mance. Setting to 0 makes all metaslab groups receive fixed
amounts of allocations. Setting to 2 allows faster metaslab
groups to allocate more. Setting to 1 equals to 2 if the pool
is write-bound or 0 otherwise. That is, if the pool is limited
by write throughput, then allocate more from faster metaslab
groups, but if not, try to evenly distribute the allocations.

metaslab_force_ganging=16777217B (16 MiB + 1 B) (u64)
Make some blocks above a certain size be gang blocks. This op-
tion is used by the test suite to facilitate testing.

metaslab_force_ganging_pct=3% (uint)
For blocks that could be forced to be a gang block (due to
metaslab_force_ganging), force this many of them to be gang
blocks.

brt_zap_prefetch=1|0 (int)
Controls prefetching BRT records for blocks which are going to
be cloned.

brt_zap_default_bs=13 (8 KiB) (int)
Default BRT ZAP data block size as a power of 2. Note that
changing this after creating a BRT on the pool will not affect
existing BRTs, only newly created ones.

brt_zap_default_ibs=13 (8 KiB) (int)
Default BRT ZAP indirect block size as a power of 2. Note that
changing this after creating a BRT on the pool will not affect
existing BRTs, only newly created ones.

ddt_zap_default_bs=15 (32 KiB) (int)
Default DDT ZAP data block size as a power of 2. Note that
changing this after creating a DDT on the pool will not affect
existing DDTs, only newly created ones.

ddt_zap_default_ibs=15 (32 KiB) (int)
Default DDT ZAP indirect block size as a power of 2. Note that
changing this after creating a DDT on the pool will not affect
existing DDTs, only newly created ones.

zfs_default_bs=9 (512 B) (int)
Default dnode block size as a power of 2.

zfs_default_ibs=17 (128 KiB) (int)
Default dnode indirect block size as a power of 2.

zfs_dio_enabled=1|0 (int)
Enable Direct I/O. If this setting is 0, then all I/O requests
will be directed through the ARC acting as though the dataset
property direct was set to disabled.

zfs_dio_strict=0|1 (int)
Strictly enforce alignment for Direct I/O requests, returning
EINVAL if not page-aligned instead of silently falling back to
uncached I/O.

zfs_history_output_max=1048576B (1 MiB) (u64)
When attempting to log an output nvlist of an ioctl in the on-
disk history, the output will not be stored if it is larger
than this size (in bytes). This must be less than
DMU_MAX_ACCESS (64 MiB). This applies primarily to
zfs_ioc_channel_program() (cf. zfs-program(8)).

zfs_keep_log_spacemaps_at_export=0|1 (int)
Prevent log spacemaps from being destroyed during pool exports
and destroys.

zfs_metaslab_segment_weight_enabled=1|0 (int)
Enable/disable segment-based metaslab selection.

zfs_metaslab_switch_threshold=2 (int)
When using segment-based metaslab selection, continue allocat-
ing from the active metaslab until this option's worth of buck-
ets have been exhausted.

metaslab_debug_load=0|1 (int)
Load all metaslabs during pool import.

metaslab_debug_unload=0|1 (int)
Prevent metaslabs from being unloaded.

metaslab_fragmentation_factor_enabled=1|0 (int)
Enable use of the fragmentation metric in computing metaslab
weights.

metaslab_df_max_search=16777216B (16 MiB) (uint)
Maximum distance to search forward from the last offset. With-
out this limit, fragmented pools can see >100`000 iterations
and metaslab_block_picker() becomes the performance limiting
factor on high-performance storage.

With the default setting of 16 MiB, we typically see less than
500 iterations, even with very fragmented ashift=9 pools. The
maximum number of iterations possible is metaslab_df_max_search
/ 2^(ashift+1). With the default setting of 16 MiB this is
16*1024 (with ashift=9) or 2*1024 (with ashift=12).

metaslab_df_use_largest_segment=0|1 (int)
If not searching forward (due to metaslab_df_max_search,
metaslab_df_free_pct, or metaslab_df_alloc_threshold), this
tunable controls which segment is used. If set, we will use
the largest free segment. If unset, we will use a segment of
at least the requested size.

zfs_metaslab_max_size_cache_sec=3600s (1 hour) (u64)
When we unload a metaslab, we cache the size of the largest
free chunk. We use that cached size to determine whether or
not to load a metaslab for a given allocation. As more frees
accumulate in that metaslab while it's unloaded, the cached max
size becomes less and less accurate. After a number of seconds
controlled by this tunable, we stop considering the cached max
size and start considering only the histogram instead.

zfs_metaslab_mem_limit=25% (uint)
When we are loading a new metaslab, we check the amount of mem-
ory being used to store metaslab range trees. If it is over a
threshold, we attempt to unload the least recently used
metaslab to prevent the system from clogging all of its memory
with range trees. This tunable sets the percentage of total
system memory that is the threshold.

zfs_metaslab_try_hard_before_gang=0|1 (int)
If unset, we will first try normal allocation.
If that fails then we will do a gang allocation.
If that fails then we will do a "try hard" gang allocation.
If that fails then we will have a multi-layer gang block.

If set, we will first try normal allocation.
If that fails then we will do a "try hard" allocation.
If that fails we will do a gang allocation.
If that fails we will do a "try hard" gang allocation.
If that fails then we will have a multi-layer gang block.

zfs_metaslab_find_max_tries=100 (uint)
When not trying hard, we only consider this number of the best
metaslabs. This improves performance, especially when there
are many metaslabs per vdev and the allocation can't actually
be satisfied (so we would otherwise iterate all metaslabs).

zfs_vdev_default_ms_count=200 (uint)
When a vdev is added, target this number of metaslabs per top-
level vdev.

zfs_vdev_default_ms_shift=29 (512 MiB) (uint)
Default lower limit for metaslab size.

zfs_vdev_max_ms_shift=34 (16 GiB) (uint)
Default upper limit for metaslab size.

zfs_vdev_max_auto_ashift=14 (uint)
Maximum ashift used when optimizing for logical -> physical
sector size on new top-level vdevs. May be increased up to
ASHIFT_MAX (16), but this may negatively impact pool space ef-
ficiency.

zfs_vdev_direct_write_verify=Linux 1 | FreeBSD 0 (uint)
If non-zero, then a Direct I/O write's checksum will be veri-
fied every time the write is issued and before it is committed
to the block pointer. In the event the checksum is not valid
then the I/O operation will return EIO. This module parameter
can be used to detect if the contents of the users buffer have
changed in the process of doing a Direct I/O write. It can
also help to identify if reported checksum errors are tied to
Direct I/O writes. Each verify error causes a dio_verify_wr
zevent. Direct Write I/O checksum verify errors can be seen
with zpool status -d. The default value for this is 1 on
Linux, but is 0 for FreeBSD because user pages can be placed
under write protection in FreeBSD before the Direct I/O write
is issued.

zfs_vdev_min_auto_ashift=ASHIFT_MIN (9) (uint)
Minimum ashift used when creating new top-level vdevs.

zfs_vdev_min_ms_count=16 (uint)
Minimum number of metaslabs to create in a top-level vdev.

vdev_validate_skip=0|1 (int)
Skip label validation steps during pool import. Changing is
not recommended unless you know what you're doing and are re-
covering a damaged label.

zfs_vdev_ms_count_limit=131072 (128k) (uint)
Practical upper limit of total metaslabs per top-level vdev.

metaslab_preload_enabled=1|0 (int)
Enable metaslab group preloading.

metaslab_preload_limit=10 (uint)
Maximum number of metaslabs per group to preload

metaslab_preload_pct=50 (uint)
Percentage of CPUs to run a metaslab preload taskq

metaslab_lba_weighting_enabled=1|0 (int)
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.

metaslab_unload_delay=32 (uint)
After a metaslab is used, we keep it loaded for this many TXGs,
to attempt to reduce unnecessary reloading. Note that both
this many TXGs and metaslab_unload_delay_ms milliseconds must
pass before unloading will occur.

metaslab_unload_delay_ms=600000ms (10 min) (uint)
After a metaslab is used, we keep it loaded for this many mil-
liseconds, to attempt to reduce unnecessary reloading. Note,
that both this many milliseconds and metaslab_unload_delay TXGs
must pass before unloading will occur.

reference_history=3 (uint)
Maximum reference holders being tracked when reference_track-
ing_enable is active.

raidz_expand_max_copy_bytes=160MB (ulong)
Max amount of memory to use for RAID-Z expansion I/O. This
limits how much I/O can be outstanding at once.

raidz_expand_max_reflow_bytes=0 (ulong)
For testing, pause RAID-Z expansion when reflow amount reaches
this value.

raidz_io_aggregate_rows=4 (ulong)
For expanded RAID-Z, aggregate reads that have more rows than
this.

reference_history=3 (int)
Maximum reference holders being tracked when reference_track-
ing_enable is active.

reference_tracking_enable=0|1 (int)
Track reference holders to refcount_t objects (debug builds
only).

send_holes_without_birth_time=1|0 (int)
When set, the hole_birth optimization will not be used, and all
holes will always be sent during a zfs send. This is useful if
you suspect your datasets are affected by a bug in hole_birth.

spa_config_path=/etc/zfs/zpool.cache (charp)
SPA config file.

spa_asize_inflation=24 (uint)
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 re-
alistic inflation factor, particularly if they operate close to
quota or capacity limits.

spa_load_print_vdev_tree=0|1 (int)
Whether to print the vdev tree in the debugging message buffer
during pool import.

spa_load_verify_data=1|0 (int)
Whether to traverse data blocks during an "extreme rewind" (-X)
import.

An extreme rewind import normally performs a full traversal of
all blocks in the pool for verification. If this parameter is
unset, the traversal skips non-metadata blocks. It can be tog-
gled once the import has started to stop or start the traversal
of non-metadata blocks.

spa_load_verify_metadata=1|0 (int)
Whether to traverse blocks during an "extreme rewind" (-X) pool
import.

An extreme rewind import normally performs a full traversal of
all blocks in the pool for verification. If this parameter is
unset, the traversal is not performed. It can be toggled once
the import has started to stop or start the traversal.

spa_load_verify_shift=4 (1/16th) (uint)
Sets the maximum number of bytes to consume during pool import
to the log2 fraction of the target ARC size.

spa_slop_shift=5 (1/32nd) (int)
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.

spa_num_allocators=4 (int)
Determines the number of block allocators to use per spa in-
stance. Capped by the number of actual CPUs in the system via
spa_cpus_per_allocator.

Note that setting this value too high could result in perfor-
mance degradation and/or excess fragmentation. Set value only
applies to pools imported/created after that.

spa_cpus_per_allocator=4 (int)
Determines the minimum number of CPUs in a system for block al-
locator per spa instance. Set value only applies to pools im-
ported/created after that.

spa_upgrade_errlog_limit=0 (uint)
Limits the number of on-disk error log entries that will be
converted to the new format when enabling the head_errlog fea-
ture. The default is to convert all log entries.

vdev_read_sit_out_secs=600s (10 min) (ulong)
When a slow disk outlier is detected it is placed in a sit out
state. While sitting out the disk will not participate in nor-
mal reads, instead its data will be reconstructed as needed
from parity. Scrub operations will always read from a disk,
even if it's sitting out. A number of disks in a RAID-Z or
dRAID vdev may sit out at the same time, up to the number of
parity devices. Writes will still be issued to a disk which is
sitting out to maintain full redundancy. Defaults to 600 sec-
onds and a value of zero disables disk sit-outs in general, in-
cluding slow disk outlier detection.

vdev_raidz_outlier_check_interval_ms=1000ms (1 sec) (ulong)
How often each RAID-Z and dRAID vdev will check for slow disk
outliers. Increasing this interval will reduce the sensitivity
of detection (since all I/Os since the last check are included
in the statistics), but will slow the response to a disk devel-
oping a problem. Defaults to once per second; setting ex-
tremely small values may cause negative performance effects.

vdev_raidz_outlier_insensitivity=50 (uint)
When performing slow outlier checks for RAID-Z and dRAID vdevs,
this value is used to determine how far out an outlier must be
before it counts as an event worth consdering. This is phrased
as "insensitivity" because larger values result in fewer detec-
tions. Smaller values will result in more aggressive sitting
out of disks that may have problems, but may significantly in-
crease the rate of spurious sit-outs.

To provide a more technical definition of this parameter, this
is the multiple of the inter-quartile range (IQR) that is being
used in a Tukey's Fence detection algorithm. This is much
higher than a normal Tukey's Fence k-value, because the distri-
bution under consideration is probably an extreme-value distri-
bution, rather than a more typical Gaussian distribution.

vdev_removal_max_span=32768B (32 KiB) (uint)
During top-level vdev removal, chunks of data are copied from
the vdev which may include free space in order to trade band-
width for IOPS. This parameter determines the maximum span of
free space, in bytes, which will be included as "unnecessary"
data in a chunk of copied data.

The default value here was chosen to align with
zfs_vdev_read_gap_limit, which is a similar concept when doing
regular reads (but there's no reason it has to be the same).

vdev_file_logical_ashift=9 (512 B) (u64)
Logical ashift for file-based devices.

vdev_file_physical_ashift=9 (512 B) (u64)
Physical ashift for file-based devices.

zap_iterate_prefetch=1|0 (int)
If set, when we start iterating over a ZAP object, prefetch the
entire object (all leaf blocks). However, this is limited by
dmu_prefetch_max.

zap_micro_max_size=131072B (128 KiB) (int)
Maximum micro ZAP size. A "micro" ZAP is upgraded to a "fat"
ZAP once it grows beyond the specified size. Sizes higher than
128KiB will be clamped to 128KiB unless the large_microzap fea-
ture is enabled.

zap_shrink_enabled=1|0 (int)
If set, adjacent empty ZAP blocks will be collapsed, reducing
disk space.

zfetch_min_distance=4194304B (4 MiB) (uint)
Min bytes to prefetch per stream. Prefetch distance starts
from the demand access size and quickly grows to this value,
doubling on each hit. After that it may grow further by 1/8
per hit, but only if some prefetch since last time haven't com-
pleted in time to satisfy demand request, i.e. prefetch depth
didn't cover the read latency or the pool got saturated.

zfetch_max_distance=67108864B (64 MiB) (uint)
Max bytes to prefetch per stream.

zfetch_max_idistance=67108864B (64 MiB) (uint)
Max bytes to prefetch indirects for per stream.

zfetch_max_reorder=16777216B (16 MiB) (uint)
Requests within this byte distance from the current prefetch
stream position are considered parts of the stream, reordered
due to parallel processing. Such requests do not advance the
stream position immediately unless zfetch_hole_shift fill
threshold is reached, but saved to fill holes in the stream
later.

zfetch_max_streams=8 (uint)
Max number of streams per zfetch (prefetch streams per file).

zfetch_min_sec_reap=1 (uint)
Min time before inactive prefetch stream can be reclaimed

zfetch_max_sec_reap=2 (uint)
Max time before inactive prefetch stream can be deleted

zfs_abd_scatter_enabled=1|0 (int)
Enables ARC from using scatter/gather lists and forces all al-
locations to be linear in kernel memory. Disabling can improve
performance in some code paths at the expense of fragmented
kernel memory.

zfs_abd_scatter_max_order=MAX_ORDER-1 (uint)
Maximum number of consecutive memory pages allocated in a sin-
gle block for scatter/gather lists.

The value of MAX_ORDER depends on kernel configuration.

zfs_abd_scatter_min_size=1536B (1.5 KiB) (uint)
This is the minimum allocation size that will use scatter
(page-based) ABDs. Smaller allocations will use linear ABDs.

zfs_arc_dnode_limit=0B (u64)
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 zfs_arc_dnode_limit_percent of
the ARC meta buffers that may be used for dnodes.

zfs_arc_dnode_limit_percent=10% (u64)
Percentage that can be consumed by dnodes of ARC meta buffers.

See also zfs_arc_dnode_limit, which serves a similar purpose
but has a higher priority if nonzero.

zfs_arc_dnode_reduce_percent=10% (u64)
Percentage of ARC dnodes to try to scan in response to demand
for non-metadata when the number of bytes consumed by dnodes
exceeds zfs_arc_dnode_limit.

zfs_arc_average_blocksize=8192B (8 KiB) (uint)
The ARC's buffer hash table is sized based on the assumption of
an average block size of this value. This works out to roughly
1 MiB of hash table per 1 GiB 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 foot-
print.

zfs_arc_eviction_pct=200% (uint)
When arc_is_overflowing(), arc_get_data_impl() waits for this
percent of the requested amount of data to be evicted. For ex-
ample, by default, for every 2 KiB that's evicted, 1 KiB of it
may be "reused" by a new allocation. Since this is above 100%,
it ensures that progress is made towards getting arc_size under
arc_c. Since this is finite, it ensures that allocations can
still happen, even during the potentially long time that
arc_size is more than arc_c.

zfs_arc_evict_batch_limit=10 (uint)
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 ad-
ditional unlocking and locking.

zfs_arc_evict_threads=0 (int)
Sets the number of ARC eviction threads to be used.

If set greater than 0, ZFS will dedicate up to that many
threads to ARC eviction. Each thread will process one sub-list
at a time, until the eviction target is reached or all sub-
lists have been processed. When set to 0, ZFS will compute a
reasonable number of eviction threads based on the number of
CPUs.
+-----------------------+
| CPUs Threads |
+-----------------------+
| 1-4 1 |
| 5-8 2 |
| 9-15 3 |
| 16-31 4 |
| 32-63 6 |
| 64-95 8 |
| 96-127 9 |
| 128-160 11 |
| 160-191 12 |
| 192-223 13 |
| 224-255 14 |
| 256+ 16 |
+-----------------------+

More threads may improve the responsiveness of ZFS to memory
pressure. This can be important for performance when eviction
from the ARC becomes a bottleneck for reads and writes.

This parameter can only be set at module load time.

zfs_arc_grow_retry=0s (uint)
If set to a non zero value, it will replace the arc_grow_retry
value with this value. The arc_grow_retry value (default 5s)
is the number of seconds the ARC will wait before trying to re-
sume growth after a memory pressure event.

zfs_arc_lotsfree_percent=10% (int)
Throttle I/O when free system memory drops below this percent-
age of total system memory. Setting this value to 0 will dis-
able the throttle.

zfs_arc_max=0B (u64)
Max size of ARC in bytes. If 0, then the max size of ARC is
determined by the amount of system memory installed. The
larger of all_system_memory - 1 GiB and 5/8 x all_system_memory
will be used as the limit. This value must be at least
67108864B (64 MiB).

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.

zfs_arc_meta_balance=500 (uint)
Balance between metadata and data on ghost hits. Values above
100 increase metadata caching by proportionally reducing effect
of ghost data hits on target data/metadata rate.

zfs_arc_min=0B (u64)
Min size of ARC in bytes. If set to 0, arc_c_min will default
to consuming the larger of 32 MiB and all_system_memory / 32.

zfs_arc_min_prefetch_ms=0ms(1s) (uint)
Minimum time prefetched blocks are locked in the ARC.

zfs_arc_min_prescient_prefetch_ms=0ms(6s) (uint)
Minimum time "prescient prefetched" blocks are locked in the
ARC. These blocks are meant to be prefetched fairly aggres-
sively ahead of the code that may use them.

zfs_arc_prune_task_threads=1 (int)
Number of arc_prune threads. FreeBSD does not need more than
one. Linux may theoretically use one per mount point up to
number of CPUs, but that was not proven to be useful.

zfs_max_missing_tvds=0 (int)
Number of missing top-level vdevs which will be allowed during
pool import (only in read-only mode).

zfs_max_nvlist_src_size= 0 (u64)
Maximum size in bytes allowed to be passed as
zc_nvlist_src_size for ioctls on /dev/zfs. This prevents a
user from causing the kernel to allocate an excessive amount of
memory. When the limit is exceeded, the ioctl fails with
EINVAL and a description of the error is sent to the zfs-dbgmsg
log. This parameter should not need to be touched under normal
circumstances. If 0, equivalent to a quarter of the user-wired
memory limit under FreeBSD and to 134217728B (128 MiB) under
Linux.

zfs_multilist_num_sublists=0 (uint)
To allow more fine-grained locking, each ARC state contains a
series of lists for both data and metadata objects. Locking is
performed at the level of these "sub-lists". This parameters
controls the number of sub-lists per ARC state, and also ap-
plies to other uses of the multilist data structure.

If 0, equivalent to the greater of the number of online CPUs
and 4.

zfs_arc_overflow_shift=8 (int)
The ARC size is considered to be overflowing if it exceeds the
current ARC target size (arc_c) by thresholds determined by
this parameter. Exceeding by (arc_c >> zfs_arc_overflow_shift)
/ 2 starts ARC reclamation process. If that appears insuffi-
cient, exceeding by (arc_c >> zfs_arc_overflow_shift) x 1.5
blocks new buffer allocation until the reclaim thread catches
up. Started reclamation process continues till ARC size re-
turns below the target size.

The default value of 8 causes the ARC to start reclamation if
it exceeds the target size by 0.2% of the target size, and
block allocations by 0.6%.

zfs_arc_shrink_shift=0 (uint)
If nonzero, this will update arc_shrink_shift (default 7) with
the new value.

zfs_arc_pc_percent=0% (off) (uint)
Percent of pagecache to reclaim ARC to.

This tunable allows the 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 the ARC to be reclaimed down to zfs_arc_min if
necessary. This value is specified as percent of pagecache
size (as measured by NR_ACTIVE_FILE + NR_INACTIVE_FILE), where
that percent may exceed 100. This only operates during memory
pressure/reclaim.

zfs_arc_shrinker_limit=0 (int)
This is a limit on how many pages the ARC shrinker makes avail-
able for eviction in response to one page allocation attempt.
Note that in practice, the kernel's shrinker can ask us to
evict up to about four times this for one allocation attempt.
To reduce OOM risk, this limit is applied for kswapd reclaims
only.

For example a value of 10000 (in practice, 160 MiB per
allocation attempt with 4 KiB pages) limits the amount of time
spent attempting to reclaim ARC memory to less than 100 ms per
allocation attempt, even with a small average compressed block
size of ~8 KiB.

The parameter can be set to 0 (zero) to disable the limit, and
only applies on Linux.

zfs_arc_shrinker_seeks=2 (int)
Relative cost of ARC eviction on Linux, AKA number of seeks
needed to restore evicted page. Bigger values make ARC more
precious and evictions smaller, comparing to other kernel sub-
systems. Value of 4 means parity with page cache.

zfs_arc_sys_free=0B (u64)
The target number of bytes the ARC should leave as free memory
on the system. If zero, equivalent to the bigger of 512 KiB
and all_system_memory/64.

zfs_checksum_events_per_second=20/s (uint)
Rate limit checksum events to this many per second. Note that
this should not be set below the ZED thresholds (currently 10
checksums over 10 seconds) or else the daemon may not trigger
any action.

zfs_commit_timeout_pct=10% (uint)
This controls the amount of time that a ZIL block (lwb) will
remain "open" when it isn't "full", and it has a thread waiting
for it to be committed to stable storage. The timeout is
scaled based on a percentage of the last lwb latency to avoid
significantly impacting the latency of each individual transac-
tion record (itx).

zfs_condense_indirect_commit_entry_delay_ms=0ms (int)
Vdev indirection layer (used for device removal) sleeps for
this many milliseconds during mapping generation. Intended for
use with the test suite to throttle vdev removal speed.

zfs_condense_indirect_obsolete_pct=25% (uint)
Minimum percent of obsolete bytes in vdev mapping required to
attempt to condense (see zfs_condense_indirect_vdevs_enable).
Intended for use with the test suite to facilitate triggering
condensing as needed.

zfs_condense_indirect_vdevs_enable=1|0 (int)
Enable condensing indirect vdev mappings. When set, attempt to
condense indirect vdev mappings if the mapping uses more than
zfs_condense_min_mapping_bytes bytes of memory and if the obso-
lete space map object uses more than
zfs_condense_max_obsolete_bytes bytes on-disk. The condensing
process is an attempt to save memory by removing obsolete map-
pings.

zfs_condense_max_obsolete_bytes=1073741824B (1 GiB) (u64)
Only attempt to condense indirect vdev mappings if the on-disk
size of the obsolete space map object is greater than this num-
ber of bytes (see zfs_condense_indirect_vdevs_enable).

zfs_condense_min_mapping_bytes=131072B (128 KiB) (u64)
Minimum size vdev mapping to attempt to condense (see
zfs_condense_indirect_vdevs_enable).

zfs_dbgmsg_enable=1|0 (int)
Internally ZFS keeps a small log to facilitate debugging. The
log is enabled by default, and can be disabled by unsetting
this option. The contents of the log can be accessed by read-
ing /proc/spl/kstat/zfs/dbgmsg. Writing 0 to the file clears
the log.

This setting does not influence debug prints due to zfs_flags.

zfs_dbgmsg_maxsize=4194304B (4 MiB) (uint)
Maximum size of the internal ZFS debug log.

zfs_dbuf_state_index=0 (int)
Historically used for controlling what reporting was available
under /proc/spl/kstat/zfs. No effect.

zfs_deadman_checktime_ms=60000ms (1 min) (u64)
Check time in milliseconds. This defines the frequency at
which we check for hung I/O requests and potentially invoke the
zfs_deadman_failmode behavior.

zfs_deadman_enabled=1|0 (int)
When a pool sync operation takes longer than
zfs_deadman_synctime_ms, or when an individual I/O operation
takes longer than zfs_deadman_ziotime_ms, then the operation is
considered to be "hung". If zfs_deadman_enabled is set, then
the deadman behavior is invoked as described by
zfs_deadman_failmode. By default, the deadman is enabled and
set to wait which results in "hung" I/O operations only being
logged. The deadman is automatically disabled when a pool gets
suspended.

zfs_deadman_events_per_second=1/s (int)
Rate limit deadman zevents (which report hung I/O operations)
to this many per second.

zfs_deadman_failmode=wait (charp)
Controls the failure behavior when the deadman detects a "hung"
I/O operation. Valid values are:
wait Wait for a "hung" operation to complete. For
each "hung" operation a "deadman" event will be
posted describing that operation.
continue Attempt to recover from a "hung" operation by re-
dispatching it to the I/O pipeline if possible.
panic Panic the system. This can be used to facilitate
automatic fail-over to a properly configured
fail-over partner.

zfs_deadman_synctime_ms=600000ms (10 min) (u64)
Interval in milliseconds after which the deadman is triggered
and also the interval after which a pool sync operation is con-
sidered to be "hung". Once this limit is exceeded the deadman
will be invoked every zfs_deadman_checktime_ms milliseconds un-
til the pool sync completes.

zfs_deadman_ziotime_ms=300000ms (5 min) (u64)
Interval in milliseconds after which the deadman is triggered
and an individual I/O operation is considered to be "hung". As
long as the operation remains "hung", the deadman will be in-
voked every zfs_deadman_checktime_ms milliseconds until the op-
eration completes.

zfs_dedup_prefetch=0|1 (int)
Enable prefetching dedup-ed blocks which are going to be freed.

zfs_dedup_log_flush_min_time_ms=1000(uint)
Minimum time to spend on dedup log flush each transaction.

At least this long will be spent flushing dedup log entries
each transaction, up to zfs_txg_timeout. This occurs even if
doing so would delay the transaction, that is, other IO com-
pletes under this time.

zfs_dedup_log_flush_entries_min=100(uint)
Flush at least this many entries each transaction.

OpenZFS will flush a fraction of the log every TXG, to keep the
size proportional to the ingest rate (see
zfs_dedup_log_flush_txgs). This sets the minimum for that es-
timate, which prevents the backlog from completely draining if
the ingest rate falls. Raising it can force OpenZFS to flush
more aggressively, reducing the backlog to zero more quickly,
but can make it less able to back off if log flushing would
compete with other IO too much.

zfs_dedup_log_flush_entries_max=UINT_MAX(uint)
Flush at most this many entries each transaction.

Mostly used for debugging purposes.

zfs_dedup_log_flush_txgs=100(uint)
Target number of TXGs to process the whole dedup log.

Every TXG, OpenZFS will process the inverse of this number
times the size of the DDT backlog. This will keep the backlog
at a size roughly equal to the ingest rate times this value.
This offers a balance between a more efficient DDT log, with
better aggregation, and shorter import times, which increase as
the size of the DDT log increases. Increasing this value will
result in a more efficient DDT log, but longer import times.

zfs_dedup_log_cap=UINT_MAX(uint)
Soft cap for the size of the current dedup log.

If the log is larger than this size, we increase the aggres-
siveness of the flushing to try to bring it back down to the
soft cap. Setting it will reduce import times, but will reduce
the efficiency of the DDT log, increasing the expected number
of IOs required to flush the same amount of data.

zfs_dedup_log_hard_cap=0|1 (uint)
Whether to treat the log cap as a firm cap or not.

When set to 0 (the default), the zfs_dedup_log_cap will in-
crease the maximum number of log entries we flush in a given
txg. This will bring the backlog size down towards the cap,
but not at the expense of making TXG syncs take longer. If
this is set to 1, the cap acts more like a hard cap than a soft
cap; it will also increase the minimum number of log entries we
flush per TXG. Enabling it will reduce worst-case import
times, at the cost of increased TXG sync times.

zfs_dedup_log_flush_flow_rate_txgs=10(uint)
Number of transactions to use to compute the flow rate.

OpenZFS will estimate number of entries changed (ingest rate),
number of entries flushed (flush rate) and time spent flushing
(flush time rate) and combining these into an overall "flow
rate". It will use an exponential weighted moving average over
some number of recent transactions to compute these rates.
This sets the number of transactions to compute these averages
over. Setting it higher can help to smooth out the flow rate
in the face of spiky workloads, but will take longer for the
flow rate to adjust to a sustained change in the ingress rate.

zfs_dedup_log_txg_max=8(uint)
Max transactions to before starting to flush dedup logs.

OpenZFS maintains two dedup logs, one receiving new changes,
one flushing. If there is nothing to flush, it will accumulate
changes for no more than this many transactions before switch-
ing the logs and starting to flush entries out.

zfs_dedup_log_mem_max=0(u64)
Max memory to use for dedup logs.

OpenZFS will spend no more than this much memory on maintaining
the in-memory dedup log. Flushing will begin when around half
this amount is being spent on logs. The default value of 0
will cause it to be set by zfs_dedup_log_mem_max_percent in-
stead.

zfs_dedup_log_mem_max_percent=1% (uint)
Max memory to use for dedup logs, as a percentage of total mem-
ory.

If zfs_dedup_log_mem_max is not set, it will be initialized as
a percentage of the total memory in the system.

zfs_delay_min_dirty_percent=60% (uint)
Start to delay each transaction once there is this amount of
dirty data, expressed as a percentage of zfs_dirty_data_max.
This value should be at least
zfs_vdev_async_write_active_max_dirty_percent. See "ZFS
TRANSACTION DELAY".

zfs_delay_scale=500000 (int)
This controls how quickly the transaction delay approaches in-
finity. Larger values cause longer delays for a given amount
of dirty data.

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 ten times and a tenth of this num-
ber. See "ZFS TRANSACTION DELAY".

zfs_delay_scale x zfs_dirty_data_max must be smaller than 2^64.

zfs_dio_write_verify_events_per_second=20/s (uint)
Rate limit Direct I/O write verify events to this many per sec-
ond.

zfs_disable_ivset_guid_check=0|1 (int)
Disables requirement for IVset GUIDs to be present and match
when doing a raw receive of encrypted datasets. Intended for
users whose pools were created with OpenZFS pre-release ver-
sions and now have compatibility issues.

zfs_key_max_salt_uses=400000000 (4*10^8) (ulong)
Maximum number of uses of a single salt value before generating
a new one for encrypted datasets. The default value is also
the maximum.

zfs_object_mutex_size=64 (uint)
Size of the znode hashtable used for holds.

Due to the need to hold locks on objects that may not exist
yet, kernel mutexes are not created per-object and instead a
hashtable is used where collisions will result in objects wait-
ing when there is not actually contention on the same object.

zfs_slow_io_events_per_second=20/s (int)
Rate limit delay zevents (which report slow I/O operations) to
this many per second.

zfs_unflushed_max_mem_amt=1073741824B (1 GiB) (u64)
Upper-bound limit for unflushed metadata changes to be held by
the log spacemap in memory, in bytes.

zfs_unflushed_max_mem_ppm=1000ppm (0.1%) (u64)
Part of overall system memory that ZFS allows to be used for
unflushed metadata changes by the log spacemap, in millionths.

zfs_unflushed_log_block_max=131072 (128k) (u64)
Describes the maximum number of log spacemap blocks allowed for
each pool. The default value means that the space in all the
log spacemaps can add up to no more than 131072 blocks (which
means 16 GiB of logical space before compression and ditto
blocks, assuming that blocksize is 128 KiB).

This tunable is important because it involves a trade-off be-
tween import time after an unclean export and the frequency of
flushing metaslabs. The higher this number is, the more log
blocks we allow when the pool is active which means that we
flush metaslabs less often and thus decrease the number of I/O
operations for spacemap updates per TXG. At the same time
though, that means that in the event of an unclean export,
there will be more log spacemap blocks for us to read, inducing
overhead in the import time of the pool. The lower the number,
the amount of flushing increases, destroying log blocks quicker
as they become obsolete faster, which leaves less blocks to be
read during import time after a crash.

Each log spacemap block existing during pool import leads to
approximately one extra logical I/O issued. This is the reason
why this tunable is exposed in terms of blocks rather than
space used.

zfs_unflushed_log_block_min=1000 (u64)
If the number of metaslabs is small and our incoming rate is
high, we could get into a situation that we are flushing all
our metaslabs every TXG. Thus we always allow at least this
many log blocks.

zfs_unflushed_log_block_pct=400% (u64)
Tunable used to determine the number of blocks that can be used
for the spacemap log, expressed as a percentage of the total
number of unflushed metaslabs in the pool.

zfs_unflushed_log_txg_max=1000 (u64)
Tunable limiting maximum time in TXGs any metaslab may remain
unflushed. It effectively limits maximum number of unflushed
per-TXG spacemap logs that need to be read after unclean pool
export.

zfs_unlink_suspend_progress=0|1 (uint)
When enabled, files will not be asynchronously removed from the
list of pending unlinks and the space they consume will be
leaked. Once this option has been disabled and the dataset is
remounted, the pending unlinks will be processed and the freed
space returned to the pool. This option is used by the test
suite.

zfs_delete_blocks=20480 (ulong)
This is the used to define a large file for the purposes of
deletion. Files containing more than zfs_delete_blocks will be
deleted asynchronously, while smaller files are deleted syn-
chronously. 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. This only applies on
Linux.

zfs_dirty_data_max= (int)
Determines the dirty space limit in bytes. Once this limit is
exceeded, new writes are halted until space frees up. This pa-
rameter takes precedence over zfs_dirty_data_max_percent. See
"ZFS TRANSACTION DELAY".

Defaults to physical_ram/10, capped at zfs_dirty_data_max_max.

zfs_dirty_data_max_max= (int)
Maximum allowable value of zfs_dirty_data_max, expressed in
bytes. This limit is only enforced at module load time, and
will be ignored if zfs_dirty_data_max is later changed. This
parameter takes precedence over zfs_dirty_data_max_max_percent.
See "ZFS TRANSACTION DELAY".

Defaults to min(physical_ram/4, 4GiB), or min(physical_ram/4,
1GiB) for 32-bit systems.

zfs_dirty_data_max_max_percent=25% (uint)
Maximum allowable value of zfs_dirty_data_max, expressed as a
percentage of physical RAM. This limit is only enforced at
module load time, and will be ignored if zfs_dirty_data_max is
later changed. The parameter zfs_dirty_data_max_max takes
precedence over this one. See "ZFS TRANSACTION DELAY".

zfs_dirty_data_max_percent=10% (uint)
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 zfs_dirty_data_max takes
precedence over this one. See "ZFS TRANSACTION DELAY".

Subject to zfs_dirty_data_max_max.

zfs_dirty_data_sync_percent=20% (uint)
Start syncing out a transaction group if there's at least this
much dirty data (as a percentage of zfs_dirty_data_max). This
should be less than
zfs_vdev_async_write_active_min_dirty_percent.

zfs_wrlog_data_max= (int)
The upper limit of write-transaction ZIL log data size in
bytes. Write operations are throttled when approaching the
limit until log data is cleared out after transaction group
sync. Because of some overhead, it should be set at least 2
times the size of zfs_dirty_data_max to prevent harming normal
write throughput. It also should be smaller than the size of
the slog device if slog is present.

Defaults to zfs_dirty_data_max*2

zfs_fallocate_reserve_percent=110% (uint)
Since ZFS is a copy-on-write filesystem with snapshots, blocks
cannot be preallocated for a file in order to guarantee that
later writes will not run out of space. Instead, fallocate(2)
space preallocation only checks that sufficient space is cur-
rently available in the pool or the user's project quota allo-
cation, and then creates a sparse file of the requested size.
The requested space is multiplied by
zfs_fallocate_reserve_percent to allow additional space for in-
direct blocks and other internal metadata. Setting this to 0
disables support for fallocate(2) and causes it to return
EOPNOTSUPP.

zfs_fletcher_4_impl=fastest (string)
Select a fletcher 4 implementation.

Supported selectors are: fastest, scalar, sse2, ssse3, avx2,
avx512f, avx512bw, and aarch64_neon. All except fastest and
scalar require instruction set extensions to be available, and
will only appear if ZFS detects that they are present at run-
time. If multiple implementations of fletcher 4 are available,
the fastest will be chosen using a micro benchmark. Selecting
scalar results in the original CPU-based calculation being
used. Selecting any option other than fastest or scalar re-
sults in vector instructions from the respective CPU instruc-
tion set being used.

zfs_bclone_enabled=1|0 (int)
Enables access to the block cloning feature. If this setting
is 0, then even if feature@block_cloning is enabled, using
functions and system calls that attempt to clone blocks will
act as though the feature is disabled.

zfs_bclone_wait_dirty=1|0 (int)
When set to 1 the FICLONE and FICLONERANGE ioctls will wait for
any dirty data to be written to disk before proceeding. This
ensures that the clone operation reliably succeeds, even if a
file is modified and then immediately cloned. Note that for
small files this may be slower than simply copying the file.
When set to 0 the clone operation will immediately fail if it
encounters any dirty blocks. By default waiting is enabled.

zfs_blake3_impl=fastest (string)
Select a BLAKE3 implementation.

Supported selectors are: cycle, fastest, generic, sse2, sse41,
avx2, avx512. All except cycle, fastest and generic require
instruction set extensions to be available, and will only ap-
pear if ZFS detects that they are present at runtime. If mul-
tiple implementations of BLAKE3 are available, the fastest will
be chosen using a micro benchmark. You can see the benchmark
results by reading this kstat file:
/proc/spl/kstat/zfs/chksum_bench.

zfs_free_bpobj_enabled=1|0 (int)
Enable/disable the processing of the free_bpobj object.

zfs_async_block_max_blocks=UINT64_MAX (unlimited) (u64)
Maximum number of blocks freed in a single TXG.

zfs_max_async_dedup_frees=100000 (10^5) (u64)
Maximum number of dedup blocks freed in a single TXG.

zfs_vdev_async_read_max_active=3 (uint)
Maximum asynchronous read I/O operations active to each device.
See "ZFS I/O SCHEDULER".

zfs_vdev_async_read_min_active=1 (uint)
Minimum asynchronous read I/O operation active to each device.
See "ZFS I/O SCHEDULER".

zfs_vdev_async_write_active_max_dirty_percent=60% (uint)
When the pool has more than this much dirty data, use
zfs_vdev_async_write_max_active to limit active async writes.
If the dirty data is between the minimum and maximum, the ac-
tive I/O limit is linearly interpolated. See "ZFS I/O
SCHEDULER".

zfs_vdev_async_write_active_min_dirty_percent=30% (uint)
When the pool has less than this much dirty data, use
zfs_vdev_async_write_min_active to limit active async writes.
If the dirty data is between the minimum and maximum, the ac-
tive I/O limit is linearly interpolated. See "ZFS I/O
SCHEDULER".

zfs_vdev_async_write_max_active=10 (uint)
Maximum asynchronous write I/O operations active to each de-
vice. See "ZFS I/O SCHEDULER".

zfs_vdev_async_write_min_active=2 (uint)
Minimum asynchronous write I/O operations active to each de-
vice. See "ZFS I/O SCHEDULER".

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 im-
prove resilver performance further at a cost of further in-
creasing latency.

zfs_vdev_initializing_max_active=1 (uint)
Maximum initializing I/O operations active to each device. See
"ZFS I/O SCHEDULER".

zfs_vdev_initializing_min_active=1 (uint)
Minimum initializing I/O operations active to each device. See
"ZFS I/O SCHEDULER".

zfs_vdev_max_active=1000 (uint)
The maximum number of I/O operations active to each device.
Ideally, this will be at least the sum of each queue's
max_active. See "ZFS I/O SCHEDULER".

zfs_vdev_open_timeout_ms=1000 (uint)
Timeout value to wait before determining a device is missing
during import. This is helpful for transient missing paths due
to links being briefly removed and recreated in response to
udev events.

zfs_vdev_rebuild_max_active=3 (uint)
Maximum sequential resilver I/O operations active to each de-
vice. See "ZFS I/O SCHEDULER".

zfs_vdev_rebuild_min_active=1 (uint)
Minimum sequential resilver I/O operations active to each de-
vice. See "ZFS I/O SCHEDULER".

zfs_vdev_removal_max_active=2 (uint)
Maximum removal I/O operations active to each device. See "ZFS
I/O SCHEDULER".

zfs_vdev_removal_min_active=1 (uint)
Minimum removal I/O operations active to each device. See "ZFS
I/O SCHEDULER".

zfs_vdev_scrub_max_active=2 (uint)
Maximum scrub I/O operations active to each device. See "ZFS
I/O SCHEDULER".

zfs_vdev_scrub_min_active=1 (uint)
Minimum scrub I/O operations active to each device. See "ZFS
I/O SCHEDULER".

zfs_vdev_sync_read_max_active=10 (uint)
Maximum synchronous read I/O operations active to each device.
See "ZFS I/O SCHEDULER".

zfs_vdev_sync_read_min_active=10 (uint)
Minimum synchronous read I/O operations active to each device.
See "ZFS I/O SCHEDULER".

zfs_vdev_sync_write_max_active=10 (uint)
Maximum synchronous write I/O operations active to each device.
See "ZFS I/O SCHEDULER".

zfs_vdev_sync_write_min_active=10 (uint)
Minimum synchronous write I/O operations active to each device.
See "ZFS I/O SCHEDULER".

zfs_vdev_trim_max_active=2 (uint)
Maximum trim/discard I/O operations active to each device. See
"ZFS I/O SCHEDULER".

zfs_vdev_trim_min_active=1 (uint)
Minimum trim/discard I/O operations active to each device. See
"ZFS I/O SCHEDULER".

zfs_vdev_nia_delay=5 (uint)
For non-interactive I/O (scrub, resilver, removal, initialize
and rebuild), the number of concurrently-active I/O operations
is limited to zfs_*_min_active, unless the vdev is "idle".
When there are no interactive I/O operations active (synchro-
nous or otherwise), and zfs_vdev_nia_delay operations have com-
pleted since the last interactive operation, then the vdev is
considered to be "idle", and the number of concurrently-active
non-interactive operations is increased to zfs_*_max_active.
See "ZFS I/O SCHEDULER".

zfs_vdev_nia_credit=5 (uint)
Some HDDs tend to prioritize sequential I/O so strongly, that
concurrent random I/O latency reaches several seconds. On some
HDDs this happens even if sequential I/O operations are submit-
ted one at a time, and so setting zfs_*_max_active= 1 does not
help. To prevent non-interactive I/O, like scrub, from monopo-
lizing the device, no more than zfs_vdev_nia_credit operations
can be sent while there are outstanding incomplete interactive
operations. This enforced wait ensures the HDD services the
interactive I/O within a reasonable amount of time. See "ZFS
I/O SCHEDULER".

zfs_vdev_failfast_mask=1 (uint)
Defines if the driver should retire on a given error type. The
following options may be bitwise-ored together:
+----------------------------------------------------------------+
| Value Name Description |
+----------------------------------------------------------------+
| 1 Device No driver retries on device errors |
| 2 Transport No driver retries on transport errors. |
| 4 Driver No driver retries on driver errors. |
+----------------------------------------------------------------+

zfs_vdev_disk_max_segs=0 (uint)
Maximum number of segments to add to a BIO (min 4). If this is
higher than the maximum allowed by the device queue or the ker-
nel itself, it will be clamped. Setting it to zero will cause
the kernel's ideal size to be used. This parameter only ap-
plies on Linux.

zfs_expire_snapshot=300s (int)
Time before expiring .zfs/snapshot.

zfs_admin_snapshot=0|1 (int)
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.

zfs_snapshot_no_setuid=0|1 (int)
Whether to disable setuid/setgid support for snapshot mounts
triggered by access to the .zfs/snapshot directory by setting
the nosuid mount option.

zfs_flags=0 (int)
Set additional debugging flags. The following flags may be
bitwise-ored together:
+------------------------------------------------------------------------------------------------------------+
| Value 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. |
| 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-memory range_trees. |
| 512 ZFS_DEBUG_SET_ERROR Enable SET_ERROR and dprintf entries in the debug log. |
| 1024 ZFS_DEBUG_INDIRECT_REMAP Verify split blocks created by device removal. |
| 2048 ZFS_DEBUG_TRIM Verify TRIM ranges are always within the allocatable range tree. |
| 4096 ZFS_DEBUG_LOG_SPACEMAP Verify that the log summary is consistent with the spacemap log |
| and enable zfs_dbgmsgs for metaslab loading and flushing. |
| 8192 ZFS_DEBUG_METASLAB_ALLOC Enable debugging messages when allocations fail. |
| 16384 ZFS_DEBUG_BRT Enable BRT-related debugging messages. |
| 32768 ZFS_DEBUG_RAIDZ_RECONSTRUCT Enabled debugging messages for raidz reconstruction. |
| 65536 ZFS_DEBUG_DDT Enable DDT-related debugging messages. |
+------------------------------------------------------------------------------------------------------------+
* Requires debug build.

zfs_btree_verify_intensity=0 (uint)
Enables btree verification. The following settings are cumula-
tive:
+---------------------------------------------------------------+
| Value Description |
| |
| 1 Verify height. |
| 2 Verify pointers from children to parent. |
| 3 Verify element counts. |
| 4 Verify element order. (expensive) |
| * 5 Verify unused memory is poisoned. (expensive) |
+---------------------------------------------------------------+
* Requires debug build.

zfs_free_leak_on_eio=0|1 (int)
If destroy encounters an EIO while reading metadata (e.g. indi-
rect blocks), space referenced by the missing metadata can not
be freed. Normally this causes the background destroy to be-
come "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 par-
tially fails (i.e. some but not all I/O operations fail), and
then later recovers. In this case, we will be able to continue
pool operations while it is partially failed, and when it re-
covers, we can continue to free the space, with no leaks.
Note, however, that this case is actually fairly rare.

Typically pools either
1. fail completely (but perhaps temporarily, e.g. due to a
top-level vdev going offline), or
2. have localized, permanent errors (e.g. disk returns the
wrong data due to bit flip or firmware bug).
In the former case, 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 the latter, 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. It
is therefore 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.

zfs_free_min_time_ms=1000ms (1s) (uint)
During a zfs destroy operation using the async_destroy feature,
a minimum of this much time will be spent working on freeing
blocks per TXG.

zfs_obsolete_min_time_ms=500ms (uint)
Similar to zfs_free_min_time_ms, but for cleanup of old indi-
rection records for removed vdevs.

zfs_immediate_write_sz=32768B (32 KiB) (s64)
Largest write size to store the data directly into the ZIL if
logbias=latency. Larger writes may be written indirectly simi-
lar to logbias=throughput. In presence of SLOG this parameter
is ignored, as if it was set to infinity, storing all written
data into ZIL to not depend on regular vdev latency.

zil_special_is_slog=1|0 (int)
When enabled, and written blocks go to normal vdevs, treat
present special vdevs as SLOGs. Blocks that go to the special
vdevs are still written indirectly, as with logbias=throughput.
This parameter is ignored if an SLOG is present.

zfs_initialize_value=16045690984833335022 (0xDEADBEEFDEADBEEE) (u64)
Pattern written to vdev free space by zpool-initialize(8).

zfs_initialize_chunk_size=1048576B (1 MiB) (u64)
Size of writes used by zpool-initialize(8). This option is
used by the test suite.

zfs_livelist_max_entries=500000 (5*10^5) (u64)
The threshold size (in block pointers) at which we create a new
sub-livelist. Larger sublists are more costly from a memory
perspective but the fewer sublists there are, the lower the
cost of insertion.

zfs_livelist_min_percent_shared=75% (int)
If the amount of shared space between a snapshot and its clone
drops below this threshold, the clone turns off the livelist
and reverts to the old deletion method. This is in place be-
cause livelists no long give us a benefit once a clone has been
overwritten enough.

zfs_livelist_condense_new_alloc=0 (int)
Incremented each time an extra ALLOC blkptr is added to a
livelist entry while it is being condensed. This option is
used by the test suite to track race conditions.

zfs_livelist_condense_sync_cancel=0 (int)
Incremented each time livelist condensing is canceled while in
spa_livelist_condense_sync(). This option is used by the test
suite to track race conditions.

zfs_livelist_condense_sync_pause=0|1 (int)
When set, the livelist condense process pauses indefinitely be-
fore executing the synctask -- spa_livelist_condense_sync().
This option is used by the test suite to trigger race condi-
tions.

zfs_livelist_condense_zthr_cancel=0 (int)
Incremented each time livelist condensing is canceled while in
spa_livelist_condense_cb(). This option is used by the test
suite to track race conditions.

zfs_livelist_condense_zthr_pause=0|1 (int)
When set, the livelist condense process pauses indefinitely be-
fore executing the open context condensing work in
spa_livelist_condense_cb(). This option is used by the test
suite to trigger race conditions.

zfs_lua_max_instrlimit=100000000 (10^8) (u64)
The maximum execution time limit that can be set for a ZFS
channel program, specified as a number of Lua instructions.

zfs_lua_max_memlimit=104857600 (100 MiB) (u64)
The maximum memory limit that can be set for a ZFS channel pro-
gram, specified in bytes.

zfs_max_dataset_nesting=50 (int)
The maximum depth of nested datasets. This value can be tuned
temporarily to fix existing datasets that exceed the predefined
limit.

zfs_max_log_walking=5 (u64)
The number of past TXGs that the flushing algorithm of the log
spacemap feature uses to estimate incoming log blocks.

zfs_max_logsm_summary_length=10 (u64)
Maximum number of rows allowed in the summary of the spacemap
log.

zfs_max_recordsize=16777216 (16 MiB) (uint)
We currently support block sizes from 512 (512 B) to 16777216
(16 MiB). The benefits of larger blocks, and thus larger I/O,
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 allo-
cator. Therefore, we formerly forbade creating blocks larger
than 1M. Larger blocks could be created by changing it, and
pools with larger blocks can always be imported and used, re-
gardless of this setting.

Note that it is still limited by default to 1 MiB on x86_32,
because Linux's 3/1 memory split doesn't leave much room for
16M chunks.

zfs_allow_redacted_dataset_mount=0|1 (int)
Allow datasets received with redacted send/receive to be
mounted. Normally disabled because these datasets may be miss-
ing key data.

zfs_min_metaslabs_to_flush=1 (u64)
Minimum number of metaslabs to flush per dirty TXG.

zfs_metaslab_fragmentation_threshold=77% (uint)
Allow metaslabs to keep their active state as long as their
fragmentation percentage is no more than this value. An active
metaslab that exceeds this threshold will no longer keep its
active status allowing better metaslabs to be selected.

zfs_mg_fragmentation_threshold=95% (uint)
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.

zfs_mg_noalloc_threshold=0% (uint)
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 el-
igible for allocations.

This parameter allows one to deal with pools having heavily im-
balanced 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 un-
der the old zfs_mg_alloc_failures facility.

zfs_ddt_data_is_special=1|0 (int)
If enabled, ZFS will place DDT data into the special allocation
class.

zfs_user_indirect_is_special=1|0 (int)
If enabled, ZFS will place user data indirect blocks into the
special allocation class.

zfs_multihost_history=0 (uint)
Historical statistics for this many latest multihost updates
will be available in /proc/spl/kstat/zfs/<pool>/multihost.

zfs_multihost_interval=1000ms (1 s) (u64)
Used to control the frequency of multihost writes which are
performed when the multihost pool property is on. This is one
of the factors used to determine the length of the activity
check during import.

The multihost write period is zfs_multihost_interval /
leaf-vdevs. On average a multihost write will be issued for
each leaf vdev every zfs_multihost_interval 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.

zfs_multihost_import_intervals=20 (uint)
Used to control the duration of the activity test on import.
Smaller values of zfs_multihost_import_intervals 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.

On import the activity check waits a minimum amount of time de-
termined by zfs_multihost_interval x
zfs_multihost_import_intervals, or the same product computed on
the host which last had the pool imported, whichever is
greater. 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
zfs_multihost_interval. A minimum of 100 ms is enforced.

0 is equivalent to 1.

zfs_multihost_fail_intervals=10 (uint)
Controls the behavior of the pool when multihost write failures
or delays are detected.

When 0, multihost write failures or delays 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.

Otherwise, the pool will be suspended if
zfs_multihost_fail_intervals x zfs_multihost_interval millisec-
onds pass without a successful MMP write. This guarantees the
activity test will see MMP writes if the pool is imported. 1
is equivalent to 2; this is necessary to prevent the pool from
being suspended due to normal, small I/O latency variations.

zfs_no_scrub_io=0|1 (int)
Set to disable scrub I/O. This results in scrubs not actually
scrubbing data and simply doing a metadata crawl of the pool
instead.

zfs_no_scrub_prefetch=0|1 (int)
Set to disable block prefetching for scrubs.

zfs_nocacheflush=0|1 (int)
Disable cache flush operations on disks when writing. Setting
this will cause pool corruption on power loss if a volatile
out-of-order write cache is enabled.

zfs_nopwrite_enabled=1|0 (int)
Allow no-operation writes. The occurrence of nopwrites will
further depend on other pool properties (i.a. the checksumming
and compression algorithms).

zfs_dmu_offset_next_sync=1|0 (int)
Enable forcing TXG sync to find holes. When enabled forces ZFS
to sync data when SEEK_HOLE or SEEK_DATA flags are used allow-
ing holes in a file to be accurately reported. When disabled
holes will not be reported in recently dirtied files.

zfs_pd_bytes_max=52428800B (50 MiB) (int)
The number of bytes which should be prefetched during a pool
traversal, like zfs send or other data crawling operations.

zfs_traverse_indirect_prefetch_limit=32 (uint)
The number of blocks pointed by indirect (non-L0) block which
should be prefetched during a pool traversal, like zfs send or
other data crawling operations.

zfs_per_txg_dirty_frees_percent=30% (u64)
Control percentage of dirtied indirect blocks from frees al-
lowed into one TXG. After this threshold is crossed, addi-
tional frees will wait until the next TXG. 0 disables this
throttle.

zfs_prefetch_disable=0|1 (int)
Disable predictive prefetch. Note that it leaves "prescient"
prefetch (for, e.g., zfs send) intact. Unlike predictive
prefetch, prescient prefetch never issues I/O that ends up not
being needed, so it can't hurt performance.

zfs_qat_checksum_disable=0|1 (int)
Disable QAT hardware acceleration for SHA256 checksums. May be
unset after the ZFS modules have been loaded to initialize the
QAT hardware as long as support is compiled in and the QAT dri-
ver is present.

zfs_qat_compress_disable=0|1 (int)
Disable QAT hardware acceleration for gzip compression. May be
unset after the ZFS modules have been loaded to initialize the
QAT hardware as long as support is compiled in and the QAT dri-
ver is present.

zfs_qat_encrypt_disable=0|1 (int)
Disable QAT hardware acceleration for AES-GCM encryption. May
be unset after the ZFS modules have been loaded to initialize
the QAT hardware as long as support is compiled in and the QAT
driver is present.

zfs_vnops_read_chunk_size=33554432B (32 MiB) (u64)
Bytes to read per chunk.

zfs_read_history=0 (uint)
Historical statistics for this many latest reads will be avail-
able in /proc/spl/kstat/zfs/<pool>/reads.

zfs_read_history_hits=0|1 (int)
Include cache hits in read history

zfs_rebuild_max_segment=1048576B (1 MiB) (u64)
Maximum read segment size to issue when sequentially resilver-
ing a top-level vdev.

zfs_rebuild_scrub_enabled=1|0 (int)
Automatically start a pool scrub when the last active sequen-
tial resilver completes in order to verify the checksums of all
blocks which have been resilvered. This is enabled by default
and strongly recommended.

zfs_rebuild_vdev_limit=67108864B (64 MiB) (u64)
Maximum amount of I/O that can be concurrently issued for a se-
quential resilver per leaf device, given in bytes.

zfs_reconstruct_indirect_combinations_max=4096 (int)
If an indirect split block contains more than this many possi-
ble unique combinations when being reconstructed, consider it
too computationally expensive to check them all. Instead, try
at most this many randomly selected combinations each time the
block is accessed. This allows all segment copies to partici-
pate fairly in the reconstruction when all combinations cannot
be checked and prevents repeated use of one bad copy.

zfs_recover=0|1 (int)
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.

zfs_removal_ignore_errors=0|1 (int)
Ignore hard I/O errors during device removal. When set, if a
device encounters a hard I/O error during the removal process
the removal will not be canceled. This can result in a nor-
mally recoverable block becoming permanently damaged and is
hence not recommended. This should only be used as a last re-
sort when the pool cannot be returned to a healthy state prior
to removing the device.

zfs_removal_suspend_progress=0|1 (uint)
This is used by the test suite so that it can ensure that cer-
tain actions happen while in the middle of a removal.

zfs_remove_max_segment=16777216B (16 MiB) (uint)
The largest contiguous segment that we will attempt to allocate
when removing a device. If there is a performance problem with
attempting to allocate large blocks, consider decreasing this.
The default value is also the maximum.

zfs_resilver_disable_defer=0|1 (int)
Ignore the resilver_defer feature, causing an operation that
would start a resilver to immediately restart the one in
progress.

zfs_resilver_defer_percent=10% (uint)
If the ongoing resilver progress is below this threshold, a new
resilver will restart from scratch instead of being deferred
after the current one finishes, even if the resilver_defer fea-
ture is enabled.

zfs_resilver_min_time_ms=3000ms (3 s) (uint)
Resilvers are processed by the sync thread. While resilvering,
it will spend at least this much time working on a resilver be-
tween TXG flushes.

zfs_scan_ignore_errors=0|1 (int)
If set, remove the DTL (dirty time list) upon completion of a
pool scan (scrub), even if there were unrepairable errors. In-
tended to be used during pool repair or recovery to stop resil-
vering when the pool is next imported.

zfs_scrub_after_expand=1|0 (int)
Automatically start a pool scrub after a RAIDZ expansion com-
pletes in order to verify the checksums of all blocks which
have been copied during the expansion. This is enabled by de-
fault and strongly recommended.

zfs_scrub_min_time_ms=1000ms (1 s) (uint)
Scrubs are processed by the sync thread. While scrubbing, it
will spend at least this much time working on a scrub between
TXG flushes.

zfs_scrub_error_blocks_per_txg=4096 (uint)
Error blocks to be scrubbed in one txg.

zfs_scan_checkpoint_intval=7200s (2 hour) (uint)
To preserve progress across reboots, the sequential scan algo-
rithm periodically needs to stop metadata scanning and issue
all the verification I/O to disk. The frequency of this flush-
ing is determined by this tunable.

zfs_scan_fill_weight=3 (uint)
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 in-
sertion. Changing the value afterwards will have no effect on
scrub or resilver performance.

zfs_scan_issue_strategy=0 (uint)
Determines the order that data will be verified while scrubbing
or resilvering:
1 Data will be verified as sequentially as possible, given
the amount of memory reserved for scrubbing (see
zfs_scan_mem_lim_fact). This may improve scrub perfor-
mance if the pool's data is very fragmented.
2 The largest mostly-contiguous chunk of found data will
be verified first. By deferring scrubbing of small seg-
ments, we may later find adjacent data to coalesce and
increase the segment size.
0 Use strategy 1 during normal verification and strategy 2
while taking a checkpoint.

zfs_scan_legacy=0|1 (int)
If unset, indicates that scrubs and resilvers will gather meta-
data in memory before issuing sequential I/O. Otherwise indi-
cates that the legacy algorithm will be used, where I/O is ini-
tiated as soon as it is discovered. Unsetting will not affect
scrubs or resilvers that are already in progress.

zfs_scan_max_ext_gap=2097152B (2 MiB) (int)
Sets the largest gap in bytes between scrub/resilver I/O opera-
tions that will still be considered sequential for sorting pur-
poses. Changing this value will not affect scrubs or resilvers
that are already in progress.

zfs_scan_mem_lim_fact=20^-1 (uint)
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.

zfs_scan_mem_lim_soft_fact=20^-1 (uint)
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 below no action is taken. When we
cross this limit from above it is because we are issuing veri-
fication 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.

zfs_scan_report_txgs=0|1 (uint)
When reporting resilver throughput and estimated completion
time use the performance observed over roughly the last
zfs_scan_report_txgs TXGs. When set to zero performance is
calculated over the time between checkpoints.

zfs_scan_strict_mem_lim=0|1 (int)
Enforce tight memory limits on pool scans when a sequential
scan is in progress. When disabled, the memory limit may be
exceeded by fast disks.

zfs_scan_suspend_progress=0|1 (int)
Freezes a scrub/resilver in progress without actually pausing
it. Intended for testing/debugging.

zfs_scan_vdev_limit=16777216B (16 MiB) (int)
Maximum amount of data that can be concurrently issued at once
for scrubs and resilvers per leaf device, given in bytes.

zfs_send_corrupt_data=0|1 (int)
Allow sending of corrupt data (ignore read/checksum errors when
sending).

zfs_send_unmodified_spill_blocks=1|0 (int)
Include unmodified spill blocks in the send stream. Under cer-
tain circumstances, previous versions of ZFS could incorrectly
remove the spill block from an existing object. Including un-
modified copies of the spill blocks creates a backwards-compat-
ible stream which will recreate a spill block if it was incor-
rectly removed.

zfs_send_no_prefetch_queue_ff=20^-1 (uint)
The fill fraction of the zfs send internal queues. The fill
fraction controls the timing with which internal threads are
woken up.

zfs_send_no_prefetch_queue_length=1048576B (1 MiB) (uint)
The maximum number of bytes allowed in zfs send's internal
queues.

zfs_send_queue_ff=20^-1 (uint)
The fill fraction of the zfs send prefetch queue. The fill
fraction controls the timing with which internal threads are
woken up.

zfs_send_queue_length=16777216B (16 MiB) (uint)
The maximum number of bytes allowed that will be prefetched by
zfs send. This value must be at least twice the maximum block
size in use.

zfs_recv_queue_ff=20^-1 (uint)
The fill fraction of the zfs receive queue. The fill fraction
controls the timing with which internal threads are woken up.

zfs_recv_queue_length=16777216B (16 MiB) (uint)
The maximum number of bytes allowed in the zfs receive queue.
This value must be at least twice the maximum block size in
use.

zfs_recv_write_batch_size=1048576B (1 MiB) (uint)
The maximum amount of data, in bytes, that zfs receive will
write in one DMU transaction. This is the uncompressed size,
even when receiving a compressed send stream. This setting
will not reduce the write size below a single block. Capped at
a maximum of 32 MiB.

zfs_recv_best_effort_corrective=0 (int)
When this variable is set to non-zero a corrective receive:
1. Does not enforce the restriction of source & destination
snapshot GUIDs matching.
2. If there is an error during healing, the healing receive
is not terminated instead it moves on to the next record.

zfs_override_estimate_recordsize=0|1 (uint)
Setting this variable overrides the default logic for estimat-
ing block sizes when doing a zfs send. The default heuristic
is that the average block size will be the current recordsize.
Override this value if most data in your dataset is not of that
size and you require accurate zfs send size estimates.

zfs_sync_pass_deferred_free=2 (uint)
Flushing of data to disk is done in passes. Defer frees start-
ing in this pass.

zfs_spa_discard_memory_limit=16777216B (16 MiB) (int)
Maximum memory used for prefetching a checkpoint's space map on
each vdev while discarding the checkpoint.

zfs_spa_note_txg_time=600 (uint)
This parameter defines, in seconds, how often the TXG time
database will record a new TXG if it has changed. After the
specified time interval has passed, and if the TXG number has
changed, the new value is recorded in the database. These
timestamps can later be used for more granular operations, such
as scrubbing.

zfs_spa_flush_txg_time=600 (uint)
This parameter defines, in seconds, how often the ZFS will
flush the TXG time database to disk. It ensures that the data
is actually written to persistent storage, which helps preserve
the database in case of unexpected shutdown. The database is
also automatically flushed during the export sequence.

zfs_special_class_metadata_reserve_pct=25% (uint)
Only allow small data blocks to be allocated on the special and
dedup vdev types when the available free space percentage on
these vdevs exceeds this value. This ensures reserved space is
available for pool metadata as the special vdevs approach ca-
pacity.

zfs_sync_pass_dont_compress=8 (uint)
Starting in this sync pass, disable compression (including of
metadata). With the default setting, in practice, we don't
have this many sync passes, so this has no effect.

The original intent was that disabling compression would help
the sync passes to converge. However, in practice, disabling
compression increases the average number of sync passes; be-
cause when we turn compression off, many blocks' size will
change, and thus we have to re-allocate (not overwrite) them.
It also increases the number of 128 KiB allocations (e.g. for
indirect blocks and spacemaps) because these will not be com-
pressed. The 128 KiB allocations are especially detrimental to
performance on highly fragmented systems, which may have very
few free segments of this size, and may need to load new
metaslabs to satisfy these allocations.

zfs_sync_pass_rewrite=2 (uint)
Rewrite new block pointers starting in this pass.

zfs_trim_extent_bytes_max=134217728B (128 MiB) (uint)
Maximum size of TRIM command. Larger ranges will be split into
chunks no larger than this value before issuing.

zfs_trim_extent_bytes_min=32768B (32 KiB) (uint)
Minimum size of TRIM commands. TRIM ranges smaller than this
will be skipped, unless they're part of a larger range which
was chunked. This is done because it's common for these small
TRIMs to negatively impact overall performance.

zfs_trim_metaslab_skip=0|1 (uint)
Skip uninitialized metaslabs during the TRIM process. This op-
tion is useful for pools constructed from large thinly-provi-
sioned devices where TRIM operations are slow. As a pool ages,
an increasing fraction of the pool's metaslabs will be initial-
ized, progressively degrading the usefulness of this option.
This setting is stored when starting a manual TRIM and will
persist for the duration of the requested TRIM.

zfs_trim_queue_limit=10 (uint)
Maximum number of queued TRIMs outstanding per leaf vdev. The
number of concurrent TRIM commands issued to the device is con-
trolled by zfs_vdev_trim_min_active and
zfs_vdev_trim_max_active.

zfs_trim_txg_batch=32 (uint)
The number of transaction groups' worth of frees which should
be aggregated before TRIM operations are issued to the device.
This setting represents a trade-off between issuing larger,
more efficient TRIM operations and the delay before the re-
cently trimmed space is available for use by the device.

Increasing this value will allow frees to be aggregated for a
longer time. This will result is larger TRIM operations and
potentially increased memory usage. Decreasing this value will
have the opposite effect. The default of 32 was determined to
be a reasonable compromise.

zfs_txg_history=100 (uint)
Historical statistics for this many latest TXGs will be avail-
able in /proc/spl/kstat/zfs/<pool>/TXGs.

zfs_txg_timeout=5s (uint)
Flush dirty data to disk at least every this many seconds (max-
imum TXG duration).

zfs_vdev_aggregation_limit=1048576B (1 MiB) (uint)
Max vdev I/O aggregation size.

zfs_vdev_aggregation_limit_non_rotating=131072B (128 KiB) (uint)
Max vdev I/O aggregation size for non-rotating media.

zfs_vdev_mirror_rotating_inc=0 (int)
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 operation immediately follows its predeces-
sor on rotational vdevs for the purpose of making decisions
based on load.

zfs_vdev_mirror_rotating_seek_inc=5 (int)
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 operation lacks locality as defined by
zfs_vdev_mirror_rotating_seek_offset. Operations within this
that are not immediately following the previous operation are
incremented by half.

zfs_vdev_mirror_rotating_seek_offset=1048576B (1 MiB) (int)
The maximum distance for the last queued I/O operation in which
the balancing algorithm considers an operation to have local-
ity. See "ZFS I/O SCHEDULER".

zfs_vdev_mirror_non_rotating_inc=0 (int)
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/O operations do not imme-
diately follow one another.

zfs_vdev_mirror_non_rotating_seek_inc=1 (int)
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 operation lacks locality as defined by the
zfs_vdev_mirror_rotating_seek_offset. Operations within this
that are not immediately following the previous operation are
incremented by half.

zfs_vdev_read_gap_limit=32768B (32 KiB) (uint)
Aggregate read I/O operations if the on-disk gap between them
is within this threshold.

zfs_vdev_write_gap_limit=4096B (4 KiB) (uint)
Aggregate write I/O operations if the on-disk gap between them
is within this threshold.

zfs_vdev_raidz_impl=fastest (string)
Select the raidz parity implementation to use.

Variants that don't depend on CPU-specific features may be se-
lected 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,
/sys/module/zfs/parameters/zfs_vdev_raidz_impl will show the
available options, with the currently selected one enclosed in
square brackets.

fastest selected by built-in benchmark
original original implementation
scalar scalar implementation
sse2 SSE2 instruction set 64-bit x86
ssse3 SSSE3 instruction set 64-bit x86
avx2 AVX2 instruction set 64-bit x86
avx512f AVX512F instruction set 64-bit x86
avx512bw AVX512F & AVX512BW instruction sets 64-bit x86
aarch64_neon NEON Aarch64/64-bit ARMv8
aarch64_neonx2 NEON with more unrolling Aarch64/64-bit ARMv8
powerpc_altivec Altivec PowerPC

zfs_zevent_len_max=512 (uint)
Max event queue length. Events in the queue can be viewed with
zpool-events(8).

zfs_zevent_retain_max=2000 (int)
Maximum recent zevent records to retain for duplicate checking.
Setting this to 0 disables duplicate detection.

zfs_zevent_retain_expire_secs=900s (15 min) (int)
Lifespan for a recent ereport that was retained for duplicate
checking.

zfs_zil_clean_taskq_maxalloc=1048576 (int)
The maximum number of taskq entries that are allowed to be
cached. When this limit is exceeded transaction records (itxs)
will be cleaned synchronously.

zfs_zil_clean_taskq_minalloc=1024 (int)
The number of taskq entries that are pre-populated when the
taskq is first created and are immediately available for use.

zfs_zil_clean_taskq_nthr_pct=100% (int)
This controls the number of threads used by dp_zil_clean_taskq.
The default value of 100% will create a maximum of one thread
per CPU.

zil_maxblocksize=131072B (128 KiB) (uint)
This sets the maximum block size used by the ZIL. On very
fragmented pools, lowering this (typically to 36 KiB) can im-
prove performance.

zil_maxcopied=7680B (7.5 KiB) (uint)
This sets the maximum number of write bytes logged via
WR_COPIED. It tunes a tradeoff between additional memory copy
and possibly worse log space efficiency vs additional range
lock/unlock.

zil_nocacheflush=0|1 (int)
Disable the cache flush commands that are normally sent to disk
by the ZIL after an LWB write has completed. Setting this will
cause ZIL corruption on power loss if a volatile out-of-order
write cache is enabled.

zil_replay_disable=0|1 (int)
Disable intent logging replay. Can be disabled for recovery
from corrupted ZIL.

zil_slog_bulk=67108864B (64 MiB) (u64)
Limit SLOG write size per commit executed with synchronous pri-
ority. Any writes above that will be executed with lower
(asynchronous) priority to limit potential SLOG device abuse by
single active ZIL writer.

zfs_zil_saxattr=1|0 (int)
Setting this tunable to zero disables ZIL logging of new
xattr=sa records if the org.openzfs:zilsaxattr feature is en-
abled on the pool. This would only be necessary to work around
bugs in the ZIL logging or replay code for this record type.
The tunable has no effect if the feature is disabled.

zfs_embedded_slog_min_ms=64 (uint)
Usually, one metaslab from each normal and special class vdev
is dedicated for use by the ZIL to log synchronous writes.
However, if there are fewer than zfs_embedded_slog_min_ms
metaslabs in the vdev, this functionality is disabled. This
ensures that we don't set aside an unreasonable amount of space
for the ZIL.

zstd_earlyabort_pass=1 (uint)
Whether heuristic for detection of incompressible data with
zstd levels >= 3 using LZ4 and zstd-1 passes is enabled.

zstd_abort_size=131072 (uint)
Minimal uncompressed size (inclusive) of a record before the
early abort heuristic will be attempted.

zio_deadman_log_all=0|1 (int)
If non-zero, the zio deadman will produce debugging messages
(see zfs_dbgmsg_enable) for all zios, rather than only for leaf
zios possessing a vdev. This is meant to be used by developers
to gain diagnostic information for hang conditions which don't
involve a mutex or other locking primitive: typically condi-
tions in which a thread in the zio pipeline is looping indefi-
nitely.

zio_slow_io_ms=30000ms (30 s) (int)
When an I/O operation takes more than this much time to com-
plete, it's marked as slow. Each slow operation causes a delay
zevent. Slow I/O counters can be seen with zpool status -s.

zio_dva_throttle_enabled=1|0 (int)
Throttle block allocations in the I/O pipeline. This allows
for dynamic allocation distribution based on device perfor-
mance.

zfs_xattr_compat=0|1 (int)
Control the naming scheme used when setting new xattrs in the
user namespace. If 0 (the default on Linux), user namespace
xattr names are prefixed with the namespace, to be backwards
compatible with previous versions of ZFS on Linux. If 1 (the
default on FreeBSD), user namespace xattr names are not pre-
fixed, to be backwards compatible with previous versions of ZFS
on illumos and FreeBSD.

Either naming scheme can be read on this and future versions of
ZFS, regardless of this tunable, but legacy ZFS on illumos or
FreeBSD are unable to read user namespace xattrs written in the
Linux format, and legacy versions of ZFS on Linux are unable to
read user namespace xattrs written in the legacy ZFS format.

An existing xattr with the alternate naming scheme is removed
when overwriting the xattr so as to not accumulate duplicates.

zio_requeue_io_start_cut_in_line=0|1 (int)
Prioritize requeued I/O.

zfs_delete_inode=0|1 (int)
Sets whether the kernel should free an inode structure when the
last reference is released, or cache it in memory. Intended
for testing/debugging.

A live inode structure "pins" versious internal OpenZFS struc-
tures in memory, which can result in large amounts of "unus-
able" memory on systems with lots of infrequently-accessed
files, until the kernel's memory pressure mechanism asks Open-
ZFS to release them.

The default value of 0 always caches inodes that appear to
still exist on disk. Setting it to 1 will immediately release
unused inodes and their associated memory back to the dbuf
cache or the ARC for reuse, but may reduce performance if in-
odes are frequently evicted and reloaded.

This parameter is only available on Linux.

zfs_delete_dentry=0|1 (int)
Sets whether the kernel should free a dentry structure when it
is no longer required, or hold it in the dentry cache. In-
tended for testing/debugging. Since a dentry structure holds
an inode reference, a cached dentry can "pin" an inode in mem-
ory indefinitely, along with associated OpenZFS structures (See
zfs_delete_inode).

The default value of 0 instructs the kernel to cache entries
and their associated inodes when they are no longer directly
referenced. They will be reclaimed as part of the kernel's
normal cache management processes. Setting it to 1 will in-
struct the kernel to release directory entries and their inodes
as soon as they are no longer referenced by the filesystem.

This parameter is only available on Linux.

zio_taskq_batch_pct=80% (uint)
Percentage of online CPUs which will run a worker thread for
I/O. These workers are responsible for I/O work such as com-
pression, encryption, checksum and parity calculations. Frac-
tional number of CPUs will be rounded down.

The default value of 80% was chosen to avoid using all CPUs
which can result in latency issues and inconsistent application
performance, especially when slower compression and/or check-
summing is enabled. Set value only applies to pools im-
ported/created after that.

zio_taskq_batch_tpq=0 (uint)
Number of worker threads per taskq. Higher values improve I/O
ordering and CPU utilization, while lower reduce lock con-
tention. Set value only applies to pools imported/created af-
ter that.

If 0, generate a system-dependent value close to 6 threads per
taskq. Set value only applies to pools imported/created after
that.

zio_taskq_write_tpq=16 (uint)
Determines the minimum number of threads per write issue taskq.
Higher values improve CPU utilization on high throughput, while
lower reduce taskq locks contention on high IOPS. Set value
only applies to pools imported/created after that.

zio_taskq_read=fixed,1,8 null scale null (charp)
Set the queue and thread configuration for the IO read queues.
This is an advanced debugging parameter. Don't change this un-
less you understand what it does. Each of the four values cor-
responds to the issue, issue high-priority, interrupt, and in-
terrupt high-priority queues. Valid values are fixed,N,M (M
queues with N threads each), scale[,MIN] (scale with CPUs, min-
imum MIN total threads), sync, and null. Set values only apply
to pools imported/created after that.

zio_taskq_write=sync null scale null (charp)
Set the queue and thread configuration for the IO write queues.
This is an advanced debugging parameter. Don't change this un-
less you understand what it does. Each of the four values cor-
responds to the issue, issue high-priority, interrupt, and in-
terrupt high-priority queues. Valid values are fixed,N,M (M
queues with N threads each), scale[,MIN] (scale with CPUs, min-
imum MIN total threads), sync, and null. Set values only apply
to pools imported/created after that.

zio_taskq_free=scale,32 null null null (charp)
Set the queue and thread configuration for the IO free queues.
This is an advanced debugging parameter. Don't change this un-
less you understand what it does. Each of the four values cor-
responds to the issue, issue high-priority, interrupt, and in-
terrupt high-priority queues. Valid values are fixed,N,M (M
queues with N threads each), scale[,MIN] (scale with CPUs, min-
imum MIN total threads), sync, and null. The default uses a
minimum of 32 threads to improve parallelism for DDT and BRT
metadata operations during frees. Set values only apply to
pools imported/created after that.

zvol_inhibit_dev=0|1 (uint)
Do not create zvol device nodes. This may slightly improve
startup time on systems with a very large number of zvols.

zvol_major=230 (uint)
Major number for zvol block devices.

zvol_max_discard_blocks=16384 (long)
Discard (TRIM) operations done on zvols will be done in batches
of this many blocks, where block size is determined by the
volblocksize property of a zvol.

zvol_prefetch_bytes=131072B (128 KiB) (uint)
When adding a zvol to the system, prefetch this many bytes 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 blkid(8) or the kernel partitioner.

zvol_request_sync=0|1 (uint)
When processing I/O requests for a zvol, submit them synchro-
nously. This effectively limits the queue depth to 1 for each
I/O submitter. When unset, requests are handled asynchronously
by a thread pool. The number of requests which can be handled
concurrently is controlled by zvol_threads. zvol_request_sync
is ignored when running on a kernel that supports block multi-
queue (blk-mq).

zvol_num_taskqs=0 (uint)
Number of zvol taskqs. If 0 (the default) then scaling is done
internally to prefer 6 threads per taskq. This only applies on
Linux.

zvol_threads=0 (uint)
The number of system wide threads to use for processing zvol
block IOs. If 0 (the default) then internally set zvol_threads
to the number of CPUs present or 32 (whichever is greater).

zvol_blk_mq_threads=0 (uint)
The number of threads per zvol to use for queuing IO requests.
This parameter will only appear if your kernel supports blk-mq
and is only read and assigned to a zvol at zvol load time. If
0 (the default) then internally set zvol_blk_mq_threads to the
number of CPUs present.

zvol_use_blk_mq=0|1 (uint)
Set to 1 to use the blk-mq API for zvols. Set to 0 (the de-
fault) to use the legacy zvol APIs. This setting can give bet-
ter or worse zvol performance depending on the workload. This
parameter will only appear if your kernel supports blk-mq and
is only read and assigned to a zvol at zvol load time.

zvol_blk_mq_blocks_per_thread=8 (uint)
If zvol_use_blk_mq is enabled, then process this number of
volblocksize-sized blocks per zvol thread. This tunable can be
use to favor better performance for zvol reads (lower values)
or writes (higher values). If set to 0, then the zvol layer
will process the maximum number of blocks per thread that it
can. This parameter will only appear if your kernel supports
blk-mq and is only applied at each zvol's load time.

zvol_blk_mq_queue_depth=0 (uint)
The queue_depth value for the zvol blk-mq interface. This pa-
rameter will only appear if your kernel supports blk-mq and is
only applied at each zvol's load time. If 0 (the default) then
use the kernel's default queue depth. Values are clamped to
the kernel's BLKDEV_MIN_RQ and BLKDEV_MAX_RQ/BLKDEV_DEFAULT_RQ
limits.

zvol_volmode=1 (uint)
Defines zvol block devices behavior when volmode=default:
1 equivalent to full
2 equivalent to dev
3 equivalent to none

zvol_enforce_quotas=0|1 (uint)
Enable strict ZVOL quota enforcement. The strict quota en-
forcement may have a performance impact.

ZFS I/O SCHEDULER
ZFS issues I/O operations to leaf vdevs to satisfy and complete I/O op-
erations. The scheduler determines when and in what order those opera-
tions are issued. The 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,
zfs_vdev_max_active. Note that the sum of the per-queue minima must
not exceed the aggregate maximum. If the sum of the per-queue maxima
exceeds the aggregate maximum, then the number of active operations may
reach zfs_vdev_max_active, in which case no further operations will be
issued, regardless of whether all per-queue minima have been met.

For many physical devices, throughput increases with the number of con-
current operations, but latency typically suffers. Furthermore, physi-
cal devices typically have a limit at which more concurrent operations
have no effect on throughput or can actually cause it to decrease.

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 satis-
fied 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 oper-
ations are issued if the aggregate maximum number of concurrent opera-
tions 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 operation is
queued or an operation completes, the scheduler looks for new opera-
tions to issue.

In general, smaller max_actives will lead to lower latency of synchro-
nous operations. Larger max_actives may lead to higher overall
throughput, depending on underlying storage.

The ratio of the queues' max_actives determines the balance of perfor-
mance between reads, writes, and scrubs. For example, increasing
zfs_vdev_scrub_max_active will cause the scrub or resilver to complete
more quickly, but reads and writes to have higher latency and lower
throughput.

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 period-
ically, so the number of queued async writes will quickly burst up and
then bleed down to zero. Rather than servicing them as quickly as pos-
sible, the I/O scheduler changes the maximum number of active async
write operations 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 bursti-
ness in the number of simultaneous operations also stabilizes the re-
sponse time of operations from other queues, in particular synchronous
ones. In broad strokes, the I/O scheduler will issue more concurrent
operations from the async write queue as there is more dirty data in
the pool.

Async Writes
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:

| 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

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.

Ideally, the amount of dirty data on a busy pool will stay in the
sloped part of the function between
zfs_vdev_async_write_active_min_dirty_percent and
zfs_vdev_async_write_active_max_dirty_percent. If it exceeds the maxi-
mum 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.

ZFS TRANSACTION DELAY
We delay transactions when we've determined that the backend storage
isn't able to accommodate the rate of incoming writes.

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.

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.

The minimum time for a transaction to take is calculated as
min_time = min(zfs_delay_scale x (dirty - min) / (max - dirty),
100ms)

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
zfs_delay_min_dirty_percent. This should typically be at or above
zfs_vdev_async_write_active_max_dirty_percent, so that we only start to
delay after writing at full speed has failed to keep up with the incom-
ing write rate. The scale of the curve is defined by zfs_delay_scale.
Roughly speaking, this variable determines the amount of delay at the
midpoint of the curve.

delay
10ms +-------------------------------------------------------------*+
| *|
9ms + *+
| *|
8ms + *+
| * |
7ms + * +
| * |
6ms + * +
| * |
5ms + * +
| * |
4ms + * +
| * |
3ms + * +
| * |
2ms + (midpoint) * +
| | ** |
1ms + v *** +
| zfs_delay_scale ----------> ******** |
0 +-------------------------------------*********----------------+
0% <- zfs_dirty_data_max -> 100%

Note, that since the delay is added to the outstanding time remaining
on the most recent transaction it's effectively the inverse of IOPS.
Here, the midpoint of 500 us 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 three quarters of the curve yield relatively
small differences in the amount of delay.

The effects can be easier to understand when the amount of delay is
represented on a logarithmic scale:

delay
100ms +-------------------------------------------------------------++
+ +
| |
+ *+
10ms + *+
+ ** +
| (midpoint) ** |
+ | ** +
1ms + v **** +
+ zfs_delay_scale ----------> ***** +
| **** |
+ **** +
100us + ** +
+ * +
| * |
+ * +
10us + * +
+ +
| |
+ +
+--------------------------------------------------------------+
0% <- zfs_dirty_data_max -> 100%

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 sched-
uler to reach optimal throughput on the back-end storage, and then by
changing the value of zfs_delay_scale to increase the steepness of the
curve.

FreeBSD September 15, 2025 ZFS(4)

NAME | DESCRIPTION | ZFS I/O SCHEDULER | ZFS TRANSACTION DELAY

Want to link to this manual page? Use this URL:
<https://man.freebsd.org/cgi/man.cgi?query=zfs&sektion=4&manpath=FreeBSD+15.0-RELEASE>

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