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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=ULONG_MAXB (ulong)
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=ULONG_MAXB (ulong)
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 (int)
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 (int)
Set the size of the dbuf metadata cache
(dbuf_metadata_cache_max_bytes) to a log2 fraction of the tar-
get ARC size.

dmu_object_alloc_chunk_shift=7 (128) (int)
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_prefetch_max=134217728B (128MB) (int)
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.

ignore_hole_birth (int)
Alias for send_holes_without_birth_time.

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

l2arc_feed_secs=1 (ulong)
Seconds between L2ARC writing.

l2arc_headroom=2 (ulong)
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% (ulong)
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 eligibile
for caching into L2ARC. If set to 1, exclude dbufs on special
vdevs from being cached to L2ARC.

l2arc_mfuonly=0|1 (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 off, meaning both MRU and MFU data and metadata
are cached. When turning off this feature, 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 transition to MRU, in which
case the l2arc_mru_asize arcstat will not be 0.

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% (int)
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% (ulong)
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 64MB will
be trimmed. It also enables TRIM of the whole L2ARC device
upon creation or addition to an existing pool or if the header
of the device is invalid upon importing a pool or onlining a
cache device. A value of 0 disables TRIM on L2ARC altogether
and is the default as it can put significant stress on the un-
derlying 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=8388608B (8MB) (ulong)
Cold L2ARC devices will have l2arc_write_max increased by this
amount while they remain cold.

l2arc_write_max=8388608B (8MB) (ulong)
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 (1GB) (ulong)
Mininum 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 1GB, 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=1048576B (1MB) (ulong)
Metaslab granularity, in bytes. This is roughly similar to
what would be referred to as the "stripe size" in traditional
RAID arrays. In normal operation, ZFS will try to write this
amount of data to each disk before moving on to the next top-
level vdev.

metaslab_bias_enabled=1|0 (int)
Enable metaslab group biasing based on their vdevs' over- or
under-utilization relative to the pool.

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

zfs_history_output_max=1048576BB (1MB) (int)
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 (64MB). 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 (16MB) (int)
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 16MB, 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 16MB 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 (1h) (ulong)
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% (int)
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 (int)
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 (int)
When a vdev is added, target this number of metaslabs per top-
level vdev.

zfs_vdev_default_ms_shift=29 (512MB) (int)
Default limit for metaslab size.

zfs_vdev_max_auto_ashift=14 (ulong)
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_min_auto_ashift=ASHIFT_MIN (9) (ulong)
Minimum ashift used when creating new top-level vdevs.

zfs_vdev_min_ms_count=16 (int)
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) (int)
Practical upper limit of total metaslabs per top-level vdev.

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

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 (int)
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 (10min) (int)
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 (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 (int)
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) (int)
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.

vdev_removal_max_span=32768B (32kB) (int)
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 (512B) (ulong)
Logical ashift for file-based devices.

vdev_file_physical_ashift=9 (512B) (ulong)
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.

zfetch_array_rd_sz=1048576B (1MB) (ulong)
If prefetching is enabled, disable prefetching for reads larger
than this size.

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 (64MB) (uint)
Max bytes to prefetch indirects for per stream.

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 (1.5kB) (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 (ulong)
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.

Also see zfs_arc_meta_prune which serves a similar purpose but
is used when the amount of metadata in the ARC exceeds
zfs_arc_meta_limit rather than in response to overall demand
for non-metadata.

zfs_arc_dnode_limit_percent=10% (ulong)
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% (ulong)
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 (8kB) (int)
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
1MB of hash table per 1GB of physical memory with 8-byte point-
ers. 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% (int)
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 2kB that's evicted, 1kB 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 (int)
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_grow_retry=0s (int)
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 (ulong)
Max size of ARC in bytes. If 0, then the max size of ARC is
determined by the amount of system memory installed. Under
Linux, half of system memory will be used as the limit. Under
FreeBSD, the larger of all_system_memory - 1GB and 5/8 *
all_system_memory will be used as the limit. This value must
be at least 67108864B (64MB).

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_adjust_restarts=4096 (ulong)
The number of restart passes to make while scanning the ARC at-
tempting the free buffers in order to stay below the
fs_arc_meta_limit. This value should not need to be tuned but
is available to facilitate performance analysis.

zfs_arc_meta_limit=0B (ulong)
The maximum allowed size in bytes that metadata buffers are al-
lowed to consume in the ARC. When this limit is reached, meta-
data buffers will be reclaimed, even if the overall arc_c_max
has not been reached. It defaults to 0, which indicates that a
percentage based on zfs_arc_meta_limit_percent of the ARC may
be used for metadata.

This value my be changed dynamically, except that must be set
to an explicit value (cannot be set back to 0).

zfs_arc_meta_limit_percent=75% (ulong)
Percentage of ARC buffers that can be used for metadata.

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

zfs_arc_meta_min=0B (ulong)
The minimum allowed size in bytes that metadata buffers may
consume in the ARC.

zfs_arc_meta_prune=10000 (int)
The number of dentries and inodes to be scanned looking for en-
tries which can be dropped. This may be required when the ARC
reaches the zfs_arc_meta_limit because dentries and inodes can
pin buffers in the ARC. Increasing this value will cause to
dentry and inode caches to be pruned more aggressively. Set-
ting this value to 0 will disable pruning the inode and dentry
caches.

zfs_arc_meta_strategy=1|0 (int)
Define the strategy for ARC metadata buffer eviction (meta re-
claim strategy):
0 (META_ONLY) evict only the ARC metadata buffers
1 (BALANCED) additional data buffers may be evicted if
required to evict the required number of
metadata buffers.

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

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

zfs_arc_min_prescient_prefetch_ms=0ms(6s) (int)
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 (ulong)
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 (128MB) under
Linux.

zfs_multilist_num_sublists=0 (int)
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)
* 0.5 starts ARC reclamation process. If that appears insuffi-
cient, exceeding by (arc_c >> zfs_arc_overflow_shift) * 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_p_min_shift=0 (int)
If nonzero, this will update arc_p_min_shift (default 4) with
the new value. arc_p_min_shift is used as a shift of arc_c
when calculating the minumum arc_p size.

zfs_arc_p_dampener_disable=1|0 (int)
Disable arc_p adapt dampener, which reduces the maximum single
adjustment to arc_p.

zfs_arc_shrink_shift=0 (int)
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_FILE_PAGES), where that percent may ex-
ceed 100. This only operates during memory pressure/reclaim.

zfs_arc_shrinker_limit=10000 (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.

The default limit of 10000 (in practice, 160MB per allocation
attempt with 4kB pages) limits the amount of time spent at-
tempting to reclaim ARC memory to less than 100ms per alloca-
tion attempt, even with a small average compressed block size
of ~8kB.

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

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

zfs_autoimport_disable=1|0 (int)
Disable pool import at module load by ignoring the cache file
(spa_config_path).

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=5% (int)
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% (int)
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 (1GB) (ulong)
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 (128kB) (ulong)
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 (4MB) (int)
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_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/Os only being logged.
The deadman is automatically disabled when a pool gets sus-
pended.

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_checktime_ms=60000ms (1min) (int)
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_synctime_ms=600000ms (10min) (ulong)
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 (5min) (ulong)
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_delay_min_dirty_percent=60% (int)
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 * zfs_dirty_data_max must be smaller than 2^64.

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 and deadman zevents (which report slow I/Os)
to this many per second.

zfs_unflushed_max_mem_amt=1073741824B (1GB) (ulong)
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%) (ulong)
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) (ulong)
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 16GB of logical space before compression and ditto
blocks, assuming that blocksize is 128kB).

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/Os
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 (ulong)
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% (ulong)
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 (ulong)
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.

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 physical_ram/4,

zfs_dirty_data_max_max_percent=25% (int)
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% (int)
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% (int)
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_free_bpobj_enabled=1|0 (int)
Enable/disable the processing of the free_bpobj object.

zfs_async_block_max_blocks=ULONG_MAX (unlimited) (ulong)
Maximum number of blocks freed in a single TXG.

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

zfs_override_estimate_recordsize=0 (ulong)
If nonzer, override record size calculation for zfs send esti-
mates.

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

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

zfs_vdev_async_write_active_max_dirty_percent=60% (int)
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% (int)
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=30 (int)
Maximum asynchronous write I/O operations active to each de-
vice. See "ZFS I/O SCHEDULER".

zfs_vdev_async_write_min_active=2 (int)
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 (int)
Maximum initializing I/O operations active to each device. See
"ZFS I/O SCHEDULER".

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

zfs_vdev_max_active=1000 (int)
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 (int)
Maximum sequential resilver I/O operations active to each de-
vice. See "ZFS I/O SCHEDULER".

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

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

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

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

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

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

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

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

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

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

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

zfs_vdev_nia_delay=5 (int)
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 operatinons 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 (int)
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_queue_depth_pct=1000% (int)
Maximum number of queued allocations per top-level vdev ex-
pressed as a percentage of zfs_vdev_async_write_max_active,
which allows the system to detect devices that are more capable
of handling allocations and to allocate more blocks to those
devices. This allows for dynamic allocation distribution when
devices are imbalanced, as fuller devices will tend to be
slower than empty devices.

Also see zio_dva_throttle_enabled.

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_flags=0 (int)
Set additional debugging flags. The following flags may be
bitwise-ored together:
+-----------------------------------------------------------------------------------------------------------+
| Value Symbolic Name Description |
+-----------------------------------------------------------------------------------------------------------+
| 1 ZFS_DEBUG_DPRINTF Enable dprintf entries in the debug log. |
| * 2 ZFS_DEBUG_DBUF_VERIFY Enable extra dbuf verifications. |
| * 4 ZFS_DEBUG_DNODE_VERIFY Enable extra dnode verifications. |
| 8 ZFS_DEBUG_SNAPNAMES Enable snapshot name verification. |
| 16 ZFS_DEBUG_MODIFY Check for illegally modified ARC buffers. |
| 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. |
+-----------------------------------------------------------------------------------------------------------+
* Requires debug build.

zfs_btree_verify_intensity=0 (uint)
Enables btree verification. The following settings are culmi-
native:
+---------------------------------------------------------------+
| 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) (int)
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 (int)
Similar to zfs_free_min_time_ms, but for cleanup of old indi-
rection records for removed vdevs.

zfs_immediate_write_sz=32768B (32kB) (long)
Largest data block to write to the ZIL. Larger blocks will be
treated as if the dataset being written to had the
logbias=throughput property set.

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

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

zfs_livelist_max_entries=500000 (5*10^5) (ulong)
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) (ulong)
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 (100MB) (ulong)
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 (ulong)
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 (ulong)
Maximum number of rows allowed in the summary of the spacemap
log.

zfs_max_recordsize=1048576 (1MB) (int)
We currently support block sizes from 512B to 16MB. The bene-
fits 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 la-
tency, and also potentially on the memory allocator. There-
fore, we do not allow the recordsize to be set larger than this
tunable. Larger blocks can be created by changing it, and
pools with larger blocks can always be imported and used, re-
gardless of this setting.

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 (ulong)
Minimum number of metaslabs to flush per dirty TXG.

zfs_metaslab_fragmentation_threshold=70% (int)
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% (int)
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% (int)
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 (int)
Historical statistics for this many latest multihost updates
will be available in /proc/spl/kstat/zfs/<pool>/multihost.

zfs_multihost_interval=1000ms (1s) (ulong)
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 *
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 100ms 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 * 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 (50MB) (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 (int)
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% (ulong)
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=1048576B (1MB) (long)
Bytes to read per chunk.

zfs_read_history=0 (int)
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 (1MB) (ulong)
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=33554432B (32MB) (ulong)
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 IO errors during device removal. When set, if a
device encounters a hard IO error during the removal process
the removal will not be cancelled. 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 (int)
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 (16MB) (int)
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_min_time_ms=3000ms (3s) (int)
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_min_time_ms=1000ms (1s) (int)
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_scan_checkpoint_intval=7200s (2h) (int)
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 (int)
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 affect on
scrub or resilver performance.

zfs_scan_issue_strategy=0 (int)
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 (2MB) (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 (int)
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 (int)
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_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=4194304B (4MB) (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 (int)
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 (1MB) (int)
The maximum number of bytes allowed in zfs send's internal
queues.

zfs_send_queue_ff=20^-1 (int)
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 (16MB) (int)
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 (int)
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 (16MB) (int)
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 (1MB) (int)
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 32MB.

zfs_override_estimate_recordsize=0|1 (ulong)
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 (int)
Flushing of data to disk is done in passes. Defer frees start-
ing in this pass.

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

zfs_special_class_metadata_reserve_pct=25% (int)
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 (int)
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 128kB allocations (e.g. for in-
direct blocks and spacemaps) because these will not be com-
pressed. The 128kB 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 (int)
Rewrite new block pointers starting in this pass.

zfs_sync_taskq_batch_pct=75% (int)
This controls the number of threads used by dp_sync_taskq. The
default value of 75% will create a maximum of one thread per
CPU.

zfs_trim_extent_bytes_max=134217728B (128MB) (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 (32kB) (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=0 (int)
Historical statistics for this many latest TXGs will be avail-
able in /proc/spl/kstat/zfs/<pool>/TXGs.

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

zfs_vdev_aggregate_trim=0|1 (int)
Allow TRIM I/Os to be aggregated. This is normally not helpful
because the extents to be trimmed will have been already been
aggregated by the metaslab. This option is provided for debug-
ging and performance analysis.

zfs_vdev_aggregation_limit=1048576B (1MB) (int)
Max vdev I/O aggregation size.

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

zfs_vdev_cache_bshift=16 (64kB) (int)
Shift size to inflate reads to.

zfs_vdev_cache_max=16384B (16kB) (int)
Inflate reads smaller than this value to meet the
zfs_vdev_cache_bshift size (default 64kB).

zfs_vdev_cache_size=0 (int)
Total size of the per-disk cache in bytes.

Currently this feature is disabled, as it has been found to not
be helpful for performance and in some cases harmful.

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 (1MB) (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 (32kB) (int)
Aggregate read I/O operations if the on-disk gap between them
is within this threshold.

zfs_vdev_write_gap_limit=4096B (4kB) (int)
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_vdev_scheduler (charp)
DEPRECATED. Prints warning to kernel log for compatibility.

zfs_zevent_len_max=512 (int)
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 (15min) (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 (128kB) (int)
This sets the maximum block size used by the ZIL. On very
fragmented pools, lowering this (typically to 36kB) can improve
performance.

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=786432B (768kB) (ulong)
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_embedded_slog_min_ms=64 (int)
Usually, one metaslab from each normal-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.

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 (30s) (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 when devices are imbal-
anced. When enabled, the maximum number of pending allocations
per top-level vdev is limited by zfs_vdev_queue_depth_pct.

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

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 and checksum calculations. Fractional 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.

zio_taskq_batch_tpq=0 (uint)
Number of worker threads per taskq. Lower values improve I/O
ordering and CPU utilization, while higher reduces lock con-
tention.

If 0, generate a system-dependent value close to 6 threads per
taskq.

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 (ulong)
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 (128kB) (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_threads=32 (uint)
Max number of threads which can handle zvol I/O requests con-
currently.

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

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 concurrent operations also stabilizes the re-
sponse time of operations from other and in particular synchronous
queues. In broad strokes, the I/O scheduler will issue more concurrent
operations from the async write queue as there's more dirty data in the
pool.

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 * (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 500us translates to 2000 IOPS. The shape of the
curve was chosen such that small changes in the amount of accumulated
dirty data in the first 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 13.2 June 1, 2021 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+13.2-RELEASE>

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