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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 deter-
mined 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 set-
tings 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 settings 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 target
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 memory.

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 operation per-
formed.

dmu_ddt_copies=3 (uint)
Controls the number of copies stored for DeDup Table (DDT) objects.
Reducing the number of copies to 1 from the previous default of 3
can reduce the write inflation caused by deduplication. This as-
sumes 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 con-
tent, expressed as a multiplier of l2arc_write_max. ARC persis-
tence across reboots can be achieved with persistent L2ARC by set-
ting 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 dis-
ables 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 transition 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 set-
ting 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 transition 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 onlining 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 toggling
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 unus-
able. 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 de-
vice. If set to 100 we TRIM twice the space required to accommo-
date upcoming writes. A minimum of 64 MiB will be trimmed. It
also enables TRIM of the whole L2ARC device upon creation or addi-
tion 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 underlying storage devices. This will
vary depending of how well the specific device handles these com-
mands.

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 sig-
nificantly 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/unus-
able).

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 operation, 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-uti-
lization relative to the metaslab class average. If disabled, each
metaslab group will receive allocations proportional to its capac-
ity.

metaslab_perf_bias=1|0|2 (int)
Controls metaslab groups biasing based on their write performance.
Setting to 0 makes all metaslab groups receive fixed amounts of al-
locations. 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 option
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 ex-
isting 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 ex-
isting 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 prop-
erty 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 un-
cached 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-pro-
gram(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 allocating
from the active metaslab until this option's worth of buckets 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. Without
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 re-
quested 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 consid-
ering only the histogram instead.

zfs_metaslab_mem_limit=25% (uint)
When we are loading a new metaslab, we check the amount of memory
being used to store metaslab range trees. If it is over a thresh-
old, we attempt to unload the least recently used metaslab to pre-
vent 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 satis-
fied (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 efficiency.

zfs_vdev_direct_write_verify=Linux 1 | FreeBSD 0 (uint)
If non-zero, then a Direct I/O write's checksum will be verified
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 de-
fault 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 recovering 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 un-
loading will occur.

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

reference_history=3 (uint)
Maximum reference holders being tracked when reference_tracking_en-
able 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_tracking_en-
able 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 fac-
tors involved may wish to specify a more realistic inflation fac-
tor, particularly if they operate close to quota or capacity lim-
its.

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

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

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 toggled once
the import has started to stop or start the traversal of non-meta-
data blocks.

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

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 instance.
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 performance
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 alloca-
tor 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 con-
verted to the new format when enabling the head_errlog feature.
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 normal
reads, instead its data will be reconstructed as needed from par-
ity. 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 seconds and a value of zero dis-
ables disk sit-outs in general, including slow disk outlier detec-
tion.

vdev_raidz_outlier_check_interval_ms=1000ms (1 sec) (ulong)
How often each RAID-Z and dRAID vdev will check for slow disk out-
liers. Increasing this interval will reduce the sensitivity of de-
tection (since all I/Os since the last check are included in the
statistics), but will slow the response to a disk developing a
problem. Defaults to once per second; setting extremely small val-
ues 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 be-
fore it counts as an event worth consdering. This is phrased as
"insensitivity" because larger values result in fewer detections.
Smaller values will result in more aggressive sitting out of disks
that may have problems, but may significantly increase 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 distribution under con-
sideration is probably an extreme-value distribution, 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 bandwidth 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 reg-
ular 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 en-
tire 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 feature is en-
abled.

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 completed in time to sat-
isfy demand request, i.e. prefetch depth didn't cover the read la-
tency 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 par-
allel 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 alloca-
tions to be linear in kernel memory. Disabling can improve perfor-
mance in some code paths at the expense of fragmented kernel mem-
ory.

zfs_abd_scatter_max_order=MAX_ORDER-1 (uint)
Maximum number of consecutive memory pages allocated in a single
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 footprint.

zfs_arc_eviction_pct=200% (uint)
When arc_is_overflowing(), arc_get_data_impl() waits for this per-
cent of the requested amount of data to be evicted. For example,
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 an-
other sub-list. This batch-style operation prevents entire sub-
lists from being evicted at once but comes at a cost of additional
unlocking and locking.

zfs_arc_evict_batches_limit=5 (uint)
Number of zfs_arc_evict_batch_limit batches to process per parallel
eviction task under heavy load to reduce number of context
switches.

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, un-
til 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 pres-
sure. 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 resume
growth after a memory pressure event.

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

zfs_arc_max=0B (u64)
Max size of ARC in bytes. If 0, then the max size of ARC is deter-
mined 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 can-
not be set back to 0 while running, and reducing it below the cur-
rent ARC size will not cause the ARC to shrink without memory pres-
sure 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(a!1s) (uint)
Minimum time prefetched blocks are locked in the ARC.

zfs_arc_min_prescient_prefetch_ms=0ms(a!6s) (uint)
Minimum time "prescient prefetched" blocks are locked in the ARC.
These blocks are meant to be prefetched fairly aggressively 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 ker-
nel to allocate an excessive amount of memory. When the limit is
exceeded, the ioctl fails with EINVAL and a description of the er-
ror 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 se-
ries of lists for both data and metadata objects. Locking is per-
formed at the level of these "sub-lists". This parameters controls
the number of sub-lists per ARC state, and also applies to other
uses of the multilist data structure.

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 cur-
rent ARC target size (arc_c) by thresholds determined by this para-
meter. Exceeding by (arc_c >> zfs_arc_overflow_shift) / 2 starts
ARC reclamation process. If that appears insufficient, exceeding
by (arc_c >> zfs_arc_overflow_shift) x 1.5 blocks new buffer allo-
cation until the reclaim thread catches up. Started reclamation
process continues till ARC size returns 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 allo-
cations 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 ker-
nel's LRU pagecache. It can guarantee that the ARC size won't col-
lapse 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 available
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 attempt-
ing to reclaim ARC memory to less than 100 ms per allocation at-
tempt, 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 subsystems. 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 transaction 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 at-
tempt to condense (see zfs_condense_indirect_vdevs_enable). In-
tended for use with the test suite to facilitate triggering con-
densing 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 obsolete
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 mappings.

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 number 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 op-
tion. The contents of the log can be accessed by reading
/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 un-
der /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 consid-
ered to be "hung". If zfs_deadman_enabled is set, then the deadman
behavior is invoked as described by zfs_deadman_failmode. By de-
fault, the deadman is enabled and set to wait which results in
"hung" I/O operations only being logged. The deadman is automati-
cally 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 de-
scribing that operation.
continue Attempt to recover from a "hung" operation by re-dis-
patching it to the I/O pipeline if possible.
panic Panic the system. This can be used to facilitate au-
tomatic 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 considered
to be "hung". Once this limit is exceeded the deadman will be in-
voked every zfs_deadman_checktime_ms milliseconds until 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 invoked every
zfs_deadman_checktime_ms milliseconds until the operation com-
pletes.

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 completes 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 esti-
mate, which prevents the backlog from completely draining if the
ingest rate falls. Raising it can force OpenZFS to flush more ag-
gressively, 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 aggressiveness
of the flushing to try to bring it back down to the soft cap. Set-
ting 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 increase
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 in-
crease the minimum number of log entries we flush per TXG. En-
abling it will reduce worst-case import times, at the cost of in-
creased 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), num-
ber 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 work-
loads, but will take longer for the flow rate to adjust to a sus-
tained 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 switching 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 instead.

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

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 infin-
ity. Larger values cause longer delays for a given amount of dirty
data.

For the smoothest delay, this value should be about 1 billion di-
vided by the maximum number of operations per second. This will
smoothly handle between ten times and a tenth of this number. 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 second.

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 versions 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 max-
imum.

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 waiting 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 un-
flushed 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 between
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 ap-
proximately 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 un-
flushed. 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 dele-
tion. Files containing more than zfs_delete_blocks will be deleted
asynchronously, while smaller files are deleted synchronously. De-
creasing this value will reduce the time spent in an unlink(2) sys-
tem 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 ex-
ceeded, new writes are halted until space frees up. This parameter
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 ig-
nored 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 per-
centage 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 can-
not be preallocated for a file in order to guarantee that later
writes will not run out of space. Instead, fallocate(2) space pre-
allocation only checks that sufficient space is currently available
in the pool or the user's project quota allocation, and then cre-
ates a sparse file of the requested size. The requested space is
multiplied by zfs_fallocate_reserve_percent to allow additional
space for indirect 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 runtime. If multi-
ple 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 results in vector instructions from
the respective CPU instruction 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 modi-
fied 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 in-
struction set extensions to be available, and will only appear if
ZFS detects that they are present at runtime. If multiple imple-
mentations of BLAKE3 are available, the fastest will be chosen
using a micro benchmark. You can see the benchmark results by read-
ing 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=250000 (u64)
Maximum number of dedup, clone or gang blocks freed in a single
TXG. These frees may require additional I/O, making them more ex-
pensive.

zfs_async_free_zio_wait_interval=2000 (u64)
After freeing this many dedup, clone or gang blocks wait for all
pending I/Os to complete before continuing.

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 active 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 active 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 device.
See "ZFS I/O SCHEDULER".

zfs_vdev_async_write_min_active=2 (uint)
Minimum asynchronous write I/O operations active to each device.
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 improve resilver
performance further at a cost of further increasing 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. Ide-
ally, 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 device.
See "ZFS I/O SCHEDULER".

zfs_vdev_rebuild_min_active=1 (uint)
Minimum sequential resilver I/O operations active to each device.
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 lim-
ited to zfs_*_min_active, unless the vdev is "idle". When there
are no interactive I/O operations active (synchronous or other-
wise), and zfs_vdev_nia_delay operations have completed since the
last interactive operation, then the vdev is considered to be
"idle", and the number of concurrently-active non-interactive oper-
ations 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 con-
current random I/O latency reaches several seconds. On some HDDs
this happens even if sequential I/O operations are submitted one at
a time, and so setting zfs_*_max_active= 1 does not help. To pre-
vent non-interactive I/O, like scrub, from monopolizing 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 reason-
able 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 kernel
itself, it will be clamped. Setting it to zero will cause the ker-
nel's ideal size to be used. This parameter only applies 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 re-
naming 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 trig-
gered 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 bit-
wise-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 cumulative:
+---------------------------------------------------------------+
| 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. indirect
blocks), space referenced by the missing metadata can not be freed.
Normally this causes the background destroy to become "stalled", as
it is unable to make forward progress. While in this stalled
state, all remaining space to free from the error-encountering
filesystem is "temporarily leaked". Set this flag to cause it to
ignore the EIO, permanently leak the space from indirect blocks
that can not be read, and continue to free everything else that it
can.

The default "stalling" behavior is useful if the storage partially
fails (i.e. some but not all I/O operations fail), and then later
recovers. In this case, we will be able to continue pool opera-
tions while it is partially failed, and when it recovers, 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 for-
ward progress regardless. In the latter, because the error is per-
manent, the best we can do is leak the minimum amount of space,
which is what setting this flag will do. It is therefore reason-
able for this flag to normally be set, but we chose the more con-
servative approach of not setting it, so that there is no possibil-
ity of leaking space in the "partial temporary" failure case.

zfs_free_min_time_ms=500ms (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 indirection
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 similar
to logbias=throughput. In presence of SLOG this parameter is ig-
nored, 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 parame-
ter is ignored if an SLOG is present.

zfs_import_defer_txgs=5 (uint)
Number of transaction groups to wait after pool import before
starting background work such as asynchronous block freeing (from
snapshots, clones, and deduplication) and scrub or resilver opera-
tions. This allows the pool import and filesystem mounting to com-
plete more quickly without interference from background activities.
The default value of 5 transaction groups typically provides suffi-
cient time for import and mount operations to complete on most sys-
tems.

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 per-
spective 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 because
livelists no long give us a benefit once a clone has been overwrit-
ten 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 before
executing the synctask -- spa_livelist_condense_sync(). This op-
tion is used by the test suite to trigger race conditions.

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 before
executing the open context condensing work in spa_livelist_con-
dense_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 program,
specified in bytes.

zfs_max_dataset_nesting=50 (int)
The maximum depth of nested datasets. This value can be tuned tem-
porarily 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 allocator. 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, regardless of this setting.

Note that it is still limited by default to 1 MiB on x86_32, be-
cause 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 missing 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 frag-
mentation 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 allo-
cations. The default value of 0 disables the feature and causes
all metaslab groups to be eligible for allocations.

This parameter allows one to deal with pools having heavily imbal-
anced vdevs such as would be the case when a new vdev has been
added. Setting the threshold to a non-zero percentage will stop
allocations from being made to vdevs that aren't filled to the
specified percentage and allow lesser filled vdevs to acquire more
allocations than they otherwise would under the old
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 spe-
cial 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 per-
formed 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 ob-
served 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 deter-
mined 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 in-
tervals than zfs_multihost_interval. A minimum of 100 ms is en-
forced.

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 fail-
ures will still be reported to the ZED which depending on its con-
figuration 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 milliseconds
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 in-
stead.

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 fur-
ther 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 allowing 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 tra-
versal, 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 allowed
into one TXG. After this threshold is crossed, additional 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 un-
set 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_qat_compress_disable=0|1 (int)
Disable QAT hardware acceleration for gzip compression. May be un-
set 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_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 available
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 resilvering a
top-level vdev.

zfs_rebuild_scrub_enabled=1|0 (int)
Automatically start a pool scrub when the last active sequential
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 sequen-
tial resilver per leaf device, given in bytes.

zfs_reconstruct_indirect_combinations_max=4096 (int)
If an indirect split block contains more than this many possible
unique combinations when being reconstructed, consider it too com-
putationally expensive to check them all. Instead, try at most
this many randomly selected combinations each time the block is ac-
cessed. This allows all segment copies to participate fairly in
the reconstruction when all combinations cannot be checked and pre-
vents 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 de-
vice encounters a hard I/O error during the removal process the re-
moval will not be canceled. This can result in a normally recover-
able block becoming permanently damaged and is hence not recom-
mended. This should only be used as a last resort 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 certain
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 at-
tempting 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 re-
silver will restart from scratch instead of being deferred after
the current one finishes, even if the resilver_defer feature is en-
abled.

zfs_resilver_min_time_ms=1500ms (uint)
Resilvers are processed by the sync thread. While resilvering, it
will spend at least this much time working on a resilver between
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. Intended to
be used during pool repair or recovery to stop resilvering when the
pool is next imported.

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

zfs_scrub_min_time_ms=750ms (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 algorithm
periodically needs to stop metadata scanning and issue all the ver-
ification I/O to disk. The frequency of this flushing is deter-
mined by this tunable.

zfs_scan_fill_weight=3 (uint)
This tunable affects how scrub and resilver I/O segments are or-
dered. A higher number indicates that we care more about how
filled in a segment is, while a lower number indicates we care more
about the size of the extent without considering the gaps within a
segment. This value is only tunable upon module insertion. Chang-
ing 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 performance
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 segments,
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 metadata
in memory before issuing sequential I/O. Otherwise indicates that
the legacy algorithm will be used, where I/O is initiated 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 operations
that will still be considered sequential for sorting purposes.
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 al-
gorithm. 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 un-
til 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 verification I/O. In this
case (unless the metadata scan is done) we stop issuing verifica-
tion 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 calcu-
lated 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 certain
circumstances, previous versions of ZFS could incorrectly remove
the spill block from an existing object. Including unmodified
copies of the spill blocks creates a backwards-compatible stream
which will recreate a spill block if it was incorrectly removed.

zfs_send_no_prefetch_queue_ff=20^-1 (uint)
The fill fraction of the zfs send internal queues. The fill frac-
tion 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 frac-
tion 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 con-
trols 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 estimating
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 starting
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 actu-
ally written to persistent storage, which helps preserve the data-
base in case of unexpected shutdown. The database is also automat-
ically 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 capacity.

zfs_sync_pass_dont_compress=8 (uint)
Starting in this sync pass, disable compression (including of meta-
data). 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 compres-
sion increases the average number of sync passes; because 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 compressed. The 128 KiB allo-
cations are especially detrimental to performance on highly frag-
mented 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 chun-
ked. 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 option
is useful for pools constructed from large thinly-provisioned de-
vices where TRIM operations are slow. As a pool ages, an increas-
ing fraction of the pool's metaslabs will be initialized, progres-
sively degrading the usefulness of this option. This setting is
stored when starting a manual TRIM and will persist for the dura-
tion of the requested TRIM.

zfs_trim_queue_limit=10 (uint)
Maximum number of queued TRIMs outstanding per leaf vdev. The num-
ber of concurrent TRIM commands issued to the device is controlled
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 effi-
cient TRIM operations and the delay before the recently 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 poten-
tially increased memory usage. Decreasing this value will have the
opposite effect. The default of 32 was determined to be a reason-
able compromise.

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

zfs_txg_timeout=5s (uint)
Flush dirty data to disk at least every this many seconds (maximum
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 cal-
culation for the purpose of selecting the least busy mirror member
when an I/O operation immediately follows its predecessor on rota-
tional 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 cal-
culation 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 incre-
mented 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 locality. See
"ZFS I/O SCHEDULER".

zfs_vdev_mirror_non_rotating_inc=0 (int)
A number by which the balancing algorithm increments the load cal-
culation for the purpose of selecting the least busy mirror member
on non-rotational vdevs when I/O operations do not immediately fol-
low one another.

zfs_vdev_mirror_non_rotating_seek_inc=1 (int)
A number by which the balancing algorithm increments the load cal-
culation 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 incre-
mented 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 selected
on module load, as they are supported on all systems. The remain-
ing 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 avail-
able 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 frag-
mented pools, lowering this (typically to 36 KiB) can improve per-
formance.

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 prior-
ity. Any writes above that will be executed with lower (asynchro-
nous) priority to limit potential SLOG device abuse by single ac-
tive 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 enabled 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 conditions in which a
thread in the zio pipeline is looping indefinitely.

zio_slow_io_ms=30000ms (30 s) (int)
When an I/O operation takes more than this much time to complete,
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 performance.

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 prefixed, to be backwards com-
patible 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 structures
in memory, which can result in large amounts of "unusable" memory
on systems with lots of infrequently-accessed files, until the ker-
nel's memory pressure mechanism asks OpenZFS 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 in-
odes and their associated memory back to the dbuf cache or the ARC
for reuse, but may reduce performance if inodes 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. Intended for
testing/debugging. Since a dentry structure holds an inode refer-
ence, a cached dentry can "pin" an inode in memory 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 refer-
enced. They will be reclaimed as part of the kernel's normal cache
management processes. Setting it to 1 will instruct 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 compression, en-
cryption, checksum and parity 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 perfor-
mance, especially when slower compression and/or checksumming is
enabled. Set value only applies to pools imported/created after
that.

zio_taskq_batch_tpq=0 (uint)
Number of worker threads per taskq. Higher values improve I/O or-
dering and CPU utilization, while lower reduce lock contention.
Set value only applies to pools imported/created after 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 unless
you understand what it does. Each of the four values corresponds
to the issue, issue high-priority, interrupt, and interrupt high-
priority queues. Valid values are fixed,N,M (M queues with N
threads each), scale[,MIN] (scale with CPUs, minimum MIN total
threads), sync, and null. Set values only apply to pools im-
ported/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 unless
you understand what it does. Each of the four values corresponds
to the issue, issue high-priority, interrupt, and interrupt high-
priority queues. Valid values are fixed,N,M (M queues with N
threads each), scale[,MIN] (scale with CPUs, minimum MIN total
threads), sync, and null. Set values only apply to pools im-
ported/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 unless
you understand what it does. Each of the four values corresponds
to the issue, issue high-priority, interrupt, and interrupt high-
priority queues. Valid values are fixed,N,M (M queues with N
threads each), scale[,MIN] (scale with CPUs, minimum 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 vol-
ume is desirable, because they are likely to be accessed immedi-
ately by blkid(8) or the kernel partitioner.

zvol_request_sync=0|1 (uint)
When processing I/O requests for a zvol, submit them synchronously.
This effectively limits the queue depth to 1 for each I/O submit-
ter. 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 run-
ning on a kernel that supports block multiqueue (blk-mq).

zvol_num_taskqs=0 (uint)
Number of zvol taskqs. If 0 (the default) then scaling is done in-
ternally 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 default)
to use the legacy zvol APIs. This setting can give better or worse
zvol performance depending on the workload. This parameter will
only appear if your kernel supports blk-mq and is only read and as-
signed 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 parame-
ter will only appear if your kernel supports blk-mq and is only ap-
plied 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 enforcement
may have a performance impact.

ZFS I/O SCHEDULER
ZFS issues I/O operations to leaf vdevs to satisfy and complete I/O opera-
tions. The scheduler determines when and in what order those operations
are issued. The scheduler divides operations into five I/O classes, prior-
itized in the following order: sync read, sync write, async read, async
write, and scrub/resilver. Each queue defines the minimum and maximum num-
ber of concurrent operations that may be issued to the device. In addi-
tion, 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 min-
ima have been met.

For many physical devices, throughput increases with the number of concur-
rent operations, but latency typically suffers. Furthermore, physical de-
vices 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 satisfied and
the aggregate maximum has not been hit, the scheduler looks for classes
whose maximum has not been satisfied. Iteration through the I/O classes is
done in the order specified above. No further operations are issued if the
aggregate maximum number of concurrent operations has been hit, or if there
are no operations queued for an I/O class that has not hit its maximum.
Every time an I/O operation is queued or an operation completes, the sched-
uler looks for new operations to issue.

In general, smaller max_actives will lead to lower latency of synchronous
operations. Larger max_actives may lead to higher overall throughput, de-
pending on underlying storage.

The ratio of the queues' max_actives determines the balance of performance
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, ex-
cept for the async write class. Asynchronous writes represent the data
that is committed to stable storage during the syncing stage for transac-
tion groups. Transaction groups enter the syncing state periodically, so
the number of queued async writes will quickly burst up and then bleed down
to zero. Rather than servicing them as quickly as possible, the I/O sched-
uler 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 phys-
ical devices, reducing the burstiness in the number of simultaneous opera-
tions also stabilizes the response time of operations from other queues, in
particular synchronous ones. In broad strokes, the I/O scheduler will is-
sue 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 con-
current operations to the minimum. As that threshold is crossed, the num-
ber 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 in-
dependent 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 al-
ready 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 de-
lay after writing at full speed has failed to keep up with the incoming
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 repre-
sented 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 en-
suring that the appropriate limits are set for the I/O scheduler 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 ports 15.1 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+15.1-RELEASE+and+Ports>

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