ZFS(4) FreeBSD Kernel Interfaces Manual ZFS(4)
NAME
zfs - tuning of the ZFS kernel module
DESCRIPTION
The ZFS module supports these parameters:
dbuf_cache_max_bytes=ULONG_MAXB (ulong)
Maximum size in bytes of the dbuf cache. The target size is
determined by the MIN versus 1/2^dbuf_cache_shift (1/32nd) of the
target ARC size. The behavior of the dbuf cache and its
associated settings can be observed via the
/proc/spl/kstat/zfs/dbufstats kstat.
dbuf_metadata_cache_max_bytes=ULONG_MAXB (ulong)
Maximum size in bytes of the metadata dbuf cache. The target
size is determined by the MIN versus
1/2^dbuf_metadata_cache_shift (1/64th) of the target ARC size.
The behavior of the metadata dbuf cache and its associated
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 (int)
Set the size of the dbuf cache (dbuf_cache_max_bytes) to a log2
fraction of the target ARC size.
dbuf_metadata_cache_shift=6 (int)
Set the size of the dbuf metadata cache
(dbuf_metadata_cache_max_bytes) to a log2 fraction of the target
ARC size.
dmu_object_alloc_chunk_shift=7 (128) (int)
dnode slots allocated in a single operation as a power of 2. The
default value minimizes lock contention for the bulk operation
performed.
dmu_prefetch_max=134217728B (128MB) (int)
Limit the amount we can prefetch with one call to this amount in
bytes. This helps to limit the amount of memory that can be used
by prefetching.
ignore_hole_birth (int)
Alias for send_holes_without_birth_time.
l2arc_feed_again=1|0 (int)
Turbo L2ARC warm-up. When the L2ARC is cold the fill interval
will be set as fast as possible.
l2arc_feed_min_ms=200 (ulong)
Min feed interval in milliseconds. Requires l2arc_feed_again=1
and only applicable in related situations.
l2arc_feed_secs=1 (ulong)
Seconds between L2ARC writing.
l2arc_headroom=2 (ulong)
How far through the ARC lists to search for L2ARC cacheable
content, expressed as a multiplier of l2arc_write_max. ARC
persistence across reboots can be achieved with persistent L2ARC
by setting this parameter to 0, allowing the full length of ARC
lists to be searched for cacheable content.
l2arc_headroom_boost=200% (ulong)
Scales l2arc_headroom by this percentage when L2ARC contents are
being successfully compressed before writing. A value of 100
disables this feature.
l2arc_mfuonly=0|1 (int)
Controls whether only MFU metadata and data are cached from ARC
into L2ARC. This may be desired to avoid wasting space on L2ARC
when reading/writing large amounts of data that are not expected
to be accessed more than once.
The default is off, meaning both MRU and MFU data and metadata
are cached. When turning off this feature, some MRU buffers will
still be present in ARC and eventually cached on L2ARC. If
l2arc_noprefetch=0, some prefetched buffers will be cached to
L2ARC, and those might later transition to MRU, in which case the
l2arc_mru_asize arcstat will not be 0.
Regardless of l2arc_noprefetch, some MFU buffers might be evicted
from ARC, accessed later on as prefetches and 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% (int)
Percent of ARC size allowed for L2ARC-only headers. Since L2ARC
buffers are not evicted on memory pressure, too many headers on a
system with an irrationally large L2ARC can render it slow or
unusable. This parameter limits L2ARC writes and rebuilds to
achieve the target.
l2arc_trim_ahead=0% (ulong)
Trims ahead of the current write size (l2arc_write_max) on L2ARC
devices by this percentage of write size if we have filled the
device. If set to 100 we TRIM twice the space required to
accommodate upcoming writes. A minimum of 64MB will be trimmed.
It also enables TRIM of the whole L2ARC device upon creation or
addition to an existing pool or if the header of the device is
invalid upon importing a pool or onlining a cache device. A
value of 0 disables TRIM on L2ARC altogether and is the default
as it can put significant stress on the underlying storage
devices. This will vary depending of how well the specific
device handles these commands.
l2arc_noprefetch=1|0 (int)
Do not write buffers to L2ARC if they were prefetched but not
used by applications. In case there are prefetched buffers in
L2ARC and this option is later set, we do not read the prefetched
buffers from L2ARC. Unsetting this option is useful for caching
sequential reads from the disks to L2ARC and serve those reads
from L2ARC later on. This may be beneficial in case the L2ARC
device is significantly faster in sequential reads than the disks
of the pool.
Use 1 to disable and 0 to enable caching/reading prefetches
to/from L2ARC.
l2arc_norw=0|1 (int)
No reads during writes.
l2arc_write_boost=8388608B (8MB) (ulong)
Cold L2ARC devices will have l2arc_write_max increased by this
amount while they remain cold.
l2arc_write_max=8388608B (8MB) (ulong)
Max write bytes per interval.
l2arc_rebuild_enabled=1|0 (int)
Rebuild the L2ARC when importing a pool (persistent L2ARC). This
can be disabled if there are problems importing a pool or
attaching an L2ARC device (e.g. the L2ARC device is slow in
reading stored log metadata, or the metadata has become somehow
fragmented/unusable).
l2arc_rebuild_blocks_min_l2size=1073741824B (1GB) (ulong)
Mininum size of an L2ARC device required in order to write log
blocks in it. The log blocks are used upon importing the pool to
rebuild the persistent L2ARC.
For L2ARC devices less than 1GB, the amount of data l2arc_evict()
evicts is significant compared to the amount of restored L2ARC
data. In this case, do not write log blocks in L2ARC in order
not to waste space.
metaslab_aliquot=524288B (512kB) (ulong)
Metaslab granularity, in bytes. This is roughly similar to what
would be referred to as the "stripe size" in traditional RAID
arrays. In normal operation, ZFS will try to write this amount
of data to a top-level vdev before moving on to the next one.
metaslab_bias_enabled=1|0 (int)
Enable metaslab group biasing based on their vdevs' over- or
under-utilization relative to the pool.
metaslab_force_ganging=16777217BB (16MB + 1B) (ulong)
Make some blocks above a certain size be gang blocks. This
option is used by the test suite to facilitate testing.
zfs_history_output_max=1048576BB (1MB) (int)
When attempting to log an output nvlist of an ioctl in the on-
disk history, the output will not be stored if it is larger than
this size (in bytes). This must be less than DMU_MAX_ACCESS
(64MB). This applies primarily to zfs_ioc_channel_program() (cf.
zfs-program(8)).
zfs_keep_log_spacemaps_at_export=0|1 (int)
Prevent log spacemaps from being destroyed during pool exports
and destroys.
zfs_metaslab_segment_weight_enabled=1|0 (int)
Enable/disable segment-based metaslab selection.
zfs_metaslab_switch_threshold=2 (int)
When using segment-based metaslab selection, continue 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 (16MB) (int)
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 16MB, we typically see less than 500
iterations, even with very fragmented ashift=9 pools. The
maximum number of iterations possible is metaslab_df_max_search /
2^(ashift+1). With the default setting of 16MB this is 16*1024
(with ashift=9) or 2*1024 (with ashift=12).
metaslab_df_use_largest_segment=0|1 (int)
If not searching forward (due to metaslab_df_max_search,
metaslab_df_free_pct, or metaslab_df_alloc_threshold), this
tunable controls which segment is used. If set, we will use the
largest free segment. If unset, we will use a segment of at
least the requested size.
zfs_metaslab_max_size_cache_sec=3600s (1h) (ulong)
When we unload a metaslab, we cache the size of the largest free
chunk. We use that cached size to determine whether or not to
load a metaslab for a given allocation. As more frees accumulate
in that metaslab while it's unloaded, the cached max size becomes
less and less accurate. After a number of seconds controlled by
this tunable, we stop considering the cached max size and start
considering only the histogram instead.
zfs_metaslab_mem_limit=25% (int)
When we are loading a new metaslab, we check the amount of memory
being used to store metaslab range trees. If it is over a
threshold, we attempt to unload the least recently used metaslab
to prevent the system from clogging all of its memory with range
trees. This tunable sets the percentage of total system memory
that is the threshold.
zfs_metaslab_try_hard_before_gang=0|1 (int)
If unset, we will first try normal allocation.
If that fails then we will do a gang allocation.
If that fails then we will do a "try hard" gang allocation.
If that fails then we will have a multi-layer gang block.
If set, we will first try normal allocation.
If that fails then we will do a "try hard" allocation.
If that fails we will do a gang allocation.
If that fails we will do a "try hard" gang allocation.
If that fails then we will have a multi-layer gang block.
zfs_metaslab_find_max_tries=100 (int)
When not trying hard, we only consider this number of the best
metaslabs. This improves performance, especially when there are
many metaslabs per vdev and the allocation can't actually be
satisfied (so we would otherwise iterate all metaslabs).
zfs_vdev_default_ms_count=200 (int)
When a vdev is added, target this number of metaslabs per top-
level vdev.
zfs_vdev_default_ms_shift=29 (512MB) (int)
Default limit for metaslab size.
zfs_vdev_max_auto_ashift=ASHIFT_MAX (16) (ulong)
Maximum ashift used when optimizing for logical -> physical
sector size on new top-level vdevs.
zfs_vdev_min_auto_ashift=ASHIFT_MIN (9) (ulong)
Minimum ashift used when creating new top-level vdevs.
zfs_vdev_min_ms_count=16 (int)
Minimum number of metaslabs to create in a top-level vdev.
vdev_validate_skip=0|1 (int)
Skip label validation steps during pool import. Changing is not
recommended unless you know what you're doing and are recovering
a damaged label.
zfs_vdev_ms_count_limit=131072 (128k) (int)
Practical upper limit of total metaslabs per top-level vdev.
metaslab_preload_enabled=1|0 (int)
Enable metaslab group preloading.
metaslab_lba_weighting_enabled=1|0 (int)
Give more weight to metaslabs with lower LBAs, assuming they have
greater bandwidth, as is typically the case on a modern constant
angular velocity disk drive.
metaslab_unload_delay=32 (int)
After a metaslab is used, we keep it loaded for this many TXGs,
to attempt to reduce unnecessary reloading. Note that both this
many TXGs and metaslab_unload_delay_ms milliseconds must pass
before unloading will occur.
metaslab_unload_delay_ms=600000ms (10min) (int)
After a metaslab is used, we keep it loaded for this many
milliseconds, to attempt to reduce unnecessary reloading. Note,
that both this many milliseconds and metaslab_unload_delay TXGs
must pass before unloading will occur.
reference_history=3 (int)
Maximum reference holders being tracked when
reference_tracking_enable is active.
reference_tracking_enable=0|1 (int)
Track reference holders to refcount_t objects (debug builds
only).
send_holes_without_birth_time=1|0 (int)
When set, the hole_birth optimization will not be used, and all
holes will always be sent during a zfs send. This is useful if
you suspect your datasets are affected by a bug in hole_birth.
spa_config_path=/etc/zfs/zpool.cache (charp)
SPA config file.
spa_asize_inflation=24 (int)
Multiplication factor used to estimate actual disk consumption
from the size of data being written. The default value is a
worst case estimate, but lower values may be valid for a given
pool depending on its configuration. Pool administrators who
understand the factors involved may wish to specify a more
realistic inflation factor, particularly if they operate close to
quota or capacity limits.
spa_load_print_vdev_tree=0|1 (int)
Whether to print the vdev tree in the debugging message buffer
during pool import.
spa_load_verify_data=1|0 (int)
Whether to traverse data blocks during an "extreme rewind" (-X)
import.
An extreme rewind import normally performs a full traversal of
all blocks in the pool for verification. If this parameter is
unset, the traversal skips non-metadata blocks. It can be
toggled once the import has started to stop or start the
traversal of non-metadata blocks.
spa_load_verify_metadata=1|0 (int)
Whether to traverse blocks during an "extreme rewind" (-X) pool
import.
An extreme rewind import normally performs a full traversal of
all blocks in the pool for verification. If this parameter is
unset, the traversal is not performed. It can be toggled once
the import has started to stop or start the traversal.
spa_load_verify_shift=4 (1/16th) (int)
Sets the maximum number of bytes to consume during pool import to
the log2 fraction of the target ARC size.
spa_slop_shift=5 (1/32nd) (int)
Normally, we don't allow the last 3.2% (1/2^spa_slop_shift) of
space in the pool to be consumed. This ensures that we don't run
the pool completely out of space, due to unaccounted changes
(e.g. to the MOS). It also limits the worst-case time to
allocate space. If we have less than this amount of free space,
most ZPL operations (e.g. write, create) will return ENOSPC.
vdev_removal_max_span=32768B (32kB) (int)
During top-level vdev removal, chunks of data are copied from the
vdev which may include free space in order to trade 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
regular reads (but there's no reason it has to be the same).
vdev_file_logical_ashift=9 (512B) (ulong)
Logical ashift for file-based devices.
vdev_file_physical_ashift=9 (512B) (ulong)
Physical ashift for file-based devices.
zap_iterate_prefetch=1|0 (int)
If set, when we start iterating over a ZAP object, prefetch the
entire object (all leaf blocks). However, this is limited by
dmu_prefetch_max.
zfetch_array_rd_sz=1048576B (1MB) (ulong)
If prefetching is enabled, disable prefetching for reads larger
than this size.
zfetch_max_distance=8388608B (8MB) (uint)
Max bytes to prefetch per stream.
zfetch_max_idistance=67108864B (64MB) (uint)
Max bytes to prefetch indirects for per stream.
zfetch_max_streams=8 (uint)
Max number of streams per zfetch (prefetch streams per file).
zfetch_min_sec_reap=2 (uint)
Min time before an active prefetch stream can be reclaimed
zfs_abd_scatter_enabled=1|0 (int)
Enables ARC from using scatter/gather lists and forces all
allocations to be linear in kernel memory. Disabling can improve
performance in some code paths at the expense of fragmented
kernel memory.
zfs_abd_scatter_max_order=MAX_ORDER-1 (uint)
Maximum number of consecutive memory pages allocated in a single
block for scatter/gather lists.
The value of MAX_ORDER depends on kernel configuration.
zfs_abd_scatter_min_size=1536B (1.5kB) (uint)
This is the minimum allocation size that will use scatter (page-
based) ABDs. Smaller allocations will use linear ABDs.
zfs_arc_dnode_limit=0B (ulong)
When the number of bytes consumed by dnodes in the ARC exceeds
this number of bytes, try to unpin some of it in response to
demand for non-metadata. This value acts as a ceiling to the
amount of dnode metadata, and defaults to 0, which indicates that
a percent which is based on zfs_arc_dnode_limit_percent of the
ARC meta buffers that may be used for dnodes.
Also see zfs_arc_meta_prune which serves a similar purpose but is
used when the amount of metadata in the ARC exceeds
zfs_arc_meta_limit rather than in response to overall demand for
non-metadata.
zfs_arc_dnode_limit_percent=10% (ulong)
Percentage that can be consumed by dnodes of ARC meta buffers.
See also zfs_arc_dnode_limit, which serves a similar purpose but
has a higher priority if nonzero.
zfs_arc_dnode_reduce_percent=10% (ulong)
Percentage of ARC dnodes to try to scan in response to demand for
non-metadata when the number of bytes consumed by dnodes exceeds
zfs_arc_dnode_limit.
zfs_arc_average_blocksize=8192B (8kB) (int)
The ARC's buffer hash table is sized based on the assumption of
an average block size of this value. This works out to roughly
1MB of hash table per 1GB of physical memory with 8-byte
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% (int)
When arc_is_overflowing(), arc_get_data_impl() waits for this
percent of the requested amount of data to be evicted. For
example, by default, for every 2kB that's evicted, 1kB of it may
be "reused" by a new allocation. Since this is above 100%, it
ensures that progress is made towards getting arc_size under
arc_c. Since this is finite, it ensures that allocations can
still happen, even during the potentially long time that arc_size
is more than arc_c.
zfs_arc_evict_batch_limit=10 (int)
Number ARC headers to evict per sub-list before proceeding to
another sub-list. This batch-style operation prevents entire
sub-lists from being evicted at once but comes at a cost of
additional unlocking and locking.
zfs_arc_grow_retry=0s (int)
If set to a non zero value, it will replace the arc_grow_retry
value with this value. The arc_grow_retry value (default 5s) is
the number of seconds the ARC will wait before trying to 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 (ulong)
Max size of ARC in bytes. If 0, then the max size of ARC is
determined by the amount of system memory installed. Under
Linux, half of system memory will be used as the limit. Under
FreeBSD, the larger of all_system_memory - 1GB and 5/8 *
all_system_memory will be used as the limit. This value must be
at least 67108864B (64MB).
This value can be changed dynamically, with some caveats. It
cannot be set back to 0 while running, and reducing it below the
current ARC size will not cause the ARC to shrink without memory
pressure to induce shrinking.
zfs_arc_meta_adjust_restarts=4096 (ulong)
The number of restart passes to make while scanning the ARC
attempting the free buffers in order to stay below the
fs_arc_meta_limit. This value should not need to be tuned but is
available to facilitate performance analysis.
zfs_arc_meta_limit=0B (ulong)
The maximum allowed size in bytes that metadata buffers are
allowed to consume in the ARC. When this limit is reached,
metadata buffers will be reclaimed, even if the overall arc_c_max
has not been reached. It defaults to 0, which indicates that a
percentage based on zfs_arc_meta_limit_percent of the ARC may be
used for metadata.
This value my be changed dynamically, except that must be set to
an explicit value (cannot be set back to 0).
zfs_arc_meta_limit_percent=75% (ulong)
Percentage of ARC buffers that can be used for metadata.
See also zfs_arc_meta_limit, which serves a similar purpose but
has a higher priority if nonzero.
zfs_arc_meta_min=0B (ulong)
The minimum allowed size in bytes that metadata buffers may
consume in the ARC.
zfs_arc_meta_prune=10000 (int)
The number of dentries and inodes to be scanned looking for
entries which can be dropped. This may be required when the ARC
reaches the zfs_arc_meta_limit because dentries and inodes can
pin buffers in the ARC. Increasing this value will cause to
dentry and inode caches to be pruned more aggressively. Setting
this value to 0 will disable pruning the inode and dentry caches.
zfs_arc_meta_strategy=1|0 (int)
Define the strategy for ARC metadata buffer eviction (meta
reclaim strategy):
0 (META_ONLY) evict only the ARC metadata buffers
1 (BALANCED) additional data buffers may be evicted if
required to evict the required number of
metadata buffers.
zfs_arc_min=0B (ulong)
Min size of ARC in bytes. If set to 0, arc_c_min will default to
consuming the larger of 32MB or all_system_memory/32.
zfs_arc_min_prefetch_ms=0ms(==1s) (int)
Minimum time prefetched blocks are locked in the ARC.
zfs_arc_min_prescient_prefetch_ms=0ms(==6s) (int)
Minimum time "prescient prefetched" blocks are locked in the ARC.
These blocks are meant to be prefetched fairly 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 (ulong)
Maximum size in bytes allowed to be passed as zc_nvlist_src_size
for ioctls on /dev/zfs. This prevents a user from causing the
kernel to allocate an excessive amount of memory. When the limit
is exceeded, the ioctl fails with EINVAL and a description of the
error is sent to the zfs-dbgmsg log. This parameter should not
need to be touched under normal circumstances. If 0, equivalent
to a quarter of the user-wired memory limit under FreeBSD and to
134217728B (128MB) under Linux.
zfs_multilist_num_sublists=0 (int)
To allow more fine-grained locking, each ARC state contains a
series of lists for both data and metadata objects. Locking is
performed at the level of these "sub-lists". This parameters
controls the number of sub-lists per ARC state, and also 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
current ARC target size (arc_c) by thresholds determined by this
parameter. Exceeding by (arc_c >> zfs_arc_overflow_shift) * 0.5
starts ARC reclamation process. If that appears insufficient,
exceeding by (arc_c >> zfs_arc_overflow_shift) * 1.5 blocks new
buffer allocation until the reclaim thread catches up. Started
reclamation process continues till ARC size 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
allocations by 0.6%.
zfs_arc_p_min_shift=0 (int)
If nonzero, this will update arc_p_min_shift (default 4) with the
new value. arc_p_min_shift is used as a shift of arc_c when
calculating the minumum arc_p size.
zfs_arc_p_dampener_disable=1|0 (int)
Disable arc_p adapt dampener, which reduces the maximum single
adjustment to arc_p.
zfs_arc_shrink_shift=0 (int)
If nonzero, this will update arc_shrink_shift (default 7) with
the new value.
zfs_arc_pc_percent=0% (off) (uint)
Percent of pagecache to reclaim ARC to.
This tunable allows the ZFS ARC to play more nicely with the
kernel's LRU pagecache. It can guarantee that the ARC size won't
collapse under scanning pressure on the pagecache, yet still
allows the ARC to be reclaimed down to zfs_arc_min if necessary.
This value is specified as percent of pagecache size (as measured
by NR_FILE_PAGES), where that percent may exceed 100. This only
operates during memory pressure/reclaim.
zfs_arc_shrinker_limit=10000 (int)
This is a limit on how many pages the ARC shrinker makes
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.
The default limit of 10000 (in practice, 160MB per allocation
attempt with 4kB pages) limits the amount of time spent
attempting to reclaim ARC memory to less than 100ms per
allocation attempt, even with a small average compressed block
size of ~8kB.
The parameter can be set to 0 (zero) to disable the limit, and
only applies on Linux.
zfs_arc_sys_free=0B (ulong)
The target number of bytes the ARC should leave as free memory on
the system. If zero, equivalent to the bigger of 512kB and
all_system_memory/64.
zfs_autoimport_disable=1|0 (int)
Disable pool import at module load by ignoring the cache file
(spa_config_path).
zfs_checksum_events_per_second=20/s (uint)
Rate limit checksum events to this many per second. Note that
this should not be set below the ZED thresholds (currently 10
checksums over 10 seconds) or else the daemon may not trigger any
action.
zfs_commit_timeout_pct=5% (int)
This controls the amount of time that a ZIL block (lwb) will
remain "open" when it isn't "full", and it has a thread waiting
for it to be committed to stable storage. The timeout is scaled
based on a percentage of the last lwb latency to avoid
significantly impacting the latency of each individual
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% (int)
Minimum percent of obsolete bytes in vdev mapping required to
attempt to condense (see zfs_condense_indirect_vdevs_enable).
Intended for use with the test suite to facilitate triggering
condensing as needed.
zfs_condense_indirect_vdevs_enable=1|0 (int)
Enable condensing indirect vdev mappings. When set, attempt to
condense indirect vdev mappings if the mapping uses more than
zfs_condense_min_mapping_bytes bytes of memory and if the
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 (1GB) (ulong)
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 (128kB) (ulong)
Minimum size vdev mapping to attempt to condense (see
zfs_condense_indirect_vdevs_enable).
zfs_dbgmsg_enable=1|0 (int)
Internally ZFS keeps a small log to facilitate debugging. The
log is enabled by default, and can be disabled by unsetting this
option. The contents of the log can be accessed by 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 (4MB) (int)
Maximum size of the internal ZFS debug log.
zfs_dbuf_state_index=0 (int)
Historically used for controlling what reporting was available
under /proc/spl/kstat/zfs. No effect.
zfs_deadman_enabled=1|0 (int)
When a pool sync operation takes longer than
zfs_deadman_synctime_ms, or when an individual I/O operation
takes longer than zfs_deadman_ziotime_ms, then the operation is
considered to be "hung". If zfs_deadman_enabled is set, then the
deadman behavior is invoked as described by zfs_deadman_failmode.
By default, the deadman is enabled and set to wait which results
in "hung" I/Os only being logged. The deadman is automatically
disabled when a pool gets suspended.
zfs_deadman_failmode=wait (charp)
Controls the failure behavior when the deadman detects a "hung"
I/O operation. Valid values are:
wait Wait for a "hung" operation to complete. For each
"hung" operation a "deadman" event will be posted
describing that operation.
continue Attempt to recover from a "hung" operation by re-
dispatching it to the I/O pipeline if possible.
panic Panic the system. This can be used to facilitate
automatic fail-over to a properly configured fail-
over partner.
zfs_deadman_checktime_ms=60000ms (1min) (int)
Check time in milliseconds. This defines the frequency at which
we check for hung I/O requests and potentially invoke the
zfs_deadman_failmode behavior.
zfs_deadman_synctime_ms=600000ms (10min) (ulong)
Interval in milliseconds after which the deadman is triggered and
also the interval after which a pool sync operation is considered
to be "hung". Once this limit is exceeded the deadman will be
invoked every zfs_deadman_checktime_ms milliseconds until the
pool sync completes.
zfs_deadman_ziotime_ms=300000ms (5min) (ulong)
Interval in milliseconds after which the deadman is triggered and
an individual I/O operation is considered to be "hung". As long
as the operation remains "hung", the deadman will be invoked
every zfs_deadman_checktime_ms milliseconds until the operation
completes.
zfs_dedup_prefetch=0|1 (int)
Enable prefetching dedup-ed blocks which are going to be freed.
zfs_delay_min_dirty_percent=60% (int)
Start to delay each transaction once there is this amount of
dirty data, expressed as a percentage of zfs_dirty_data_max.
This value should be at least
zfs_vdev_async_write_active_max_dirty_percent. See ZFS
TRANSACTION DELAY.
zfs_delay_scale=500000 (int)
This controls how quickly the transaction delay approaches
infinity. Larger values cause longer delays for a given amount
of dirty data.
For the smoothest delay, this value should be about 1 billion
divided by the maximum number of operations per second. This
will smoothly handle between ten times and a tenth of this
number. See ZFS TRANSACTION DELAY.
zfs_delay_scale * zfs_dirty_data_max must be smaller than 2^64.
zfs_disable_ivset_guid_check=0|1 (int)
Disables requirement for IVset GUIDs to be present and match when
doing a raw receive of encrypted datasets. Intended for users
whose pools were created with OpenZFS pre-release 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
maximum.
zfs_object_mutex_size=64 (uint)
Size of the znode hashtable used for holds.
Due to the need to hold locks on objects that may not exist yet,
kernel mutexes are not created per-object and instead a hashtable
is used where collisions will result in objects waiting when
there is not actually contention on the same object.
zfs_slow_io_events_per_second=20/s (int)
Rate limit delay and deadman zevents (which report slow I/Os) to
this many per second.
zfs_unflushed_max_mem_amt=1073741824B (1GB) (ulong)
Upper-bound limit for unflushed metadata changes to be held by
the log spacemap in memory, in bytes.
zfs_unflushed_max_mem_ppm=1000ppm (0.1%) (ulong)
Part of overall system memory that ZFS allows to be used for
unflushed metadata changes by the log spacemap, in millionths.
zfs_unflushed_log_block_max=262144 (256k) (ulong)
Describes the maximum number of log spacemap blocks allowed for
each pool. The default value means that the space in all the log
spacemaps can add up to no more than 262144 blocks (which means
32GB of logical space before compression and ditto blocks,
assuming that blocksize is 128kB).
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/Os for spacemap
updates per TXG. At the same time though, that means that in the
event of an unclean export, there will be more log spacemap
blocks for us to read, inducing overhead in the import time of
the pool. The lower the number, the amount of flushing
increases, destroying log blocks quicker as they become obsolete
faster, which leaves less blocks to be read during import time
after a crash.
Each log spacemap block existing during pool import leads to
approximately one extra logical I/O issued. This is the reason
why this tunable is exposed in terms of blocks rather than space
used.
zfs_unflushed_log_block_min=1000 (ulong)
If the number of metaslabs is small and our incoming rate is
high, we could get into a situation that we are flushing all our
metaslabs every TXG. Thus we always allow at least this many log
blocks.
zfs_unflushed_log_block_pct=400% (ulong)
Tunable used to determine the number of blocks that can be used
for the spacemap log, expressed as a percentage of the total
number of metaslabs in the pool.
zfs_unlink_suspend_progress=0|1 (uint)
When enabled, files will not be asynchronously removed from the
list of pending unlinks and the space they consume will be
leaked. Once this option has been disabled and the dataset is
remounted, the pending unlinks will be processed and the freed
space returned to the pool. This option is used by the test
suite.
zfs_delete_blocks=20480 (ulong)
This is the used to define a large file for the purposes of
deletion. Files containing more than zfs_delete_blocks will be
deleted asynchronously, while smaller files are deleted
synchronously. Decreasing this value will reduce the time spent
in an unlink(2) system call, at the expense of a longer delay
before the freed space is available.
zfs_dirty_data_max= (int)
Determines the dirty space limit in bytes. Once this limit is
exceeded, new writes are halted until space frees up. This
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 ignored if zfs_dirty_data_max is later changed. This
parameter takes precedence over zfs_dirty_data_max_max_percent.
See ZFS TRANSACTION DELAY.
Defaults to physical_ram/4,
zfs_dirty_data_max_max_percent=25% (int)
Maximum allowable value of zfs_dirty_data_max, expressed as a
percentage of physical RAM. This limit is only enforced at
module load time, and will be ignored if zfs_dirty_data_max is
later changed. The parameter zfs_dirty_data_max_max takes
precedence over this one. See ZFS TRANSACTION DELAY.
zfs_dirty_data_max_percent=10% (int)
Determines the dirty space limit, expressed as a percentage of
all memory. Once this limit is exceeded, new writes are halted
until space frees up. The parameter zfs_dirty_data_max takes
precedence over this one. See ZFS TRANSACTION DELAY.
Subject to zfs_dirty_data_max_max.
zfs_dirty_data_sync_percent=20% (int)
Start syncing out a transaction group if there's at least this
much dirty data (as a percentage of zfs_dirty_data_max). This
should be less than
zfs_vdev_async_write_active_min_dirty_percent.
zfs_fallocate_reserve_percent=110% (uint)
Since ZFS is a copy-on-write filesystem with snapshots, blocks
cannot be preallocated for a file in order to guarantee that
later writes will not run out of space. Instead, fallocate(2)
space preallocation only checks that sufficient space is
currently available in the pool or the user's project quota
allocation, and then creates a sparse file of the requested size.
The requested space is multiplied by
zfs_fallocate_reserve_percent to allow additional space for
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 multiple implementations of fletcher 4 are available, the
fastest will be chosen using a micro benchmark. Selecting scalar
results in the original CPU-based calculation being used.
Selecting any option other than fastest or scalar results in
vector instructions from the respective CPU instruction set being
used.
zfs_free_bpobj_enabled=1|0 (int)
Enable/disable the processing of the free_bpobj object.
zfs_async_block_max_blocks=ULONG_MAX (unlimited) (ulong)
Maximum number of blocks freed in a single TXG.
zfs_max_async_dedup_frees=100000 (10^5) (ulong)
Maximum number of dedup blocks freed in a single TXG.
zfs_override_estimate_recordsize=0 (ulong)
If nonzer, override record size calculation for zfs send
estimates.
zfs_vdev_async_read_max_active=3 (int)
Maximum asynchronous read I/O operations active to each device.
See ZFS I/O SCHEDULER.
zfs_vdev_async_read_min_active=1 (int)
Minimum asynchronous read I/O operation active to each device.
See ZFS I/O SCHEDULER.
zfs_vdev_async_write_active_max_dirty_percent=60% (int)
When the pool has more than this much dirty data, use
zfs_vdev_async_write_max_active to limit active async writes. If
the dirty data is between the minimum and maximum, the active I/O
limit is linearly interpolated. See ZFS I/O SCHEDULER.
zfs_vdev_async_write_active_min_dirty_percent=30% (int)
When the pool has less than this much dirty data, use
zfs_vdev_async_write_min_active to limit active async writes. If
the dirty data is between the minimum and maximum, the active I/O
limit is linearly interpolated. See ZFS I/O SCHEDULER.
zfs_vdev_async_write_max_active=30 (int)
Maximum asynchronous write I/O operations active to each device.
See ZFS I/O SCHEDULER.
zfs_vdev_async_write_min_active=2 (int)
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 (int)
Maximum initializing I/O operations active to each device. See
ZFS I/O SCHEDULER.
zfs_vdev_initializing_min_active=1 (int)
Minimum initializing I/O operations active to each device. See
ZFS I/O SCHEDULER.
zfs_vdev_max_active=1000 (int)
The maximum number of I/O operations active to each device.
Ideally, this will be at least the sum of each queue's
max_active. See ZFS I/O SCHEDULER.
zfs_vdev_rebuild_max_active=3 (int)
Maximum sequential resilver I/O operations active to each device.
See ZFS I/O SCHEDULER.
zfs_vdev_rebuild_min_active=1 (int)
Minimum sequential resilver I/O operations active to each device.
See ZFS I/O SCHEDULER.
zfs_vdev_removal_max_active=2 (int)
Maximum removal I/O operations active to each device. See ZFS
I/O SCHEDULER.
zfs_vdev_removal_min_active=1 (int)
Minimum removal I/O operations active to each device. See ZFS
I/O SCHEDULER.
zfs_vdev_scrub_max_active=2 (int)
Maximum scrub I/O operations active to each device. See ZFS I/O
SCHEDULER.
zfs_vdev_scrub_min_active=1 (int)
Minimum scrub I/O operations active to each device. See ZFS I/O
SCHEDULER.
zfs_vdev_sync_read_max_active=10 (int)
Maximum synchronous read I/O operations active to each device.
See ZFS I/O SCHEDULER.
zfs_vdev_sync_read_min_active=10 (int)
Minimum synchronous read I/O operations active to each device.
See ZFS I/O SCHEDULER.
zfs_vdev_sync_write_max_active=10 (int)
Maximum synchronous write I/O operations active to each device.
See ZFS I/O SCHEDULER.
zfs_vdev_sync_write_min_active=10 (int)
Minimum synchronous write I/O operations active to each device.
See ZFS I/O SCHEDULER.
zfs_vdev_trim_max_active=2 (int)
Maximum trim/discard I/O operations active to each device. See
ZFS I/O SCHEDULER.
zfs_vdev_trim_min_active=1 (int)
Minimum trim/discard I/O operations active to each device. See
ZFS I/O SCHEDULER.
zfs_vdev_nia_delay=5 (int)
For non-interactive I/O (scrub, resilver, removal, initialize and
rebuild), the number of concurrently-active I/O operations is
limited to zfs_*_min_active, unless the vdev is "idle". When
there are no interactive I/O operatinons active (synchronous or
otherwise), 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 operations is increased to zfs_*_max_active. See ZFS
I/O SCHEDULER.
zfs_vdev_nia_credit=5 (int)
Some HDDs tend to prioritize sequential I/O so strongly, that
concurrent random I/O latency reaches several seconds. On some
HDDs this happens even if sequential I/O operations are submitted
one at a time, and so setting zfs_*_max_active= 1 does not help.
To prevent 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 reasonable amount of time. See ZFS I/O SCHEDULER.
zfs_vdev_queue_depth_pct=1000% (int)
Maximum number of queued allocations per top-level vdev expressed
as a percentage of zfs_vdev_async_write_max_active, which allows
the system to detect devices that are more capable of handling
allocations and to allocate more blocks to those devices. This
allows for dynamic allocation distribution when devices are
imbalanced, as fuller devices will tend to be slower than empty
devices.
Also see zio_dva_throttle_enabled.
zfs_expire_snapshot=300s (int)
Time before expiring .zfs/snapshot.
zfs_admin_snapshot=0|1 (int)
Allow the creation, removal, or renaming of entries in the
.zfs/snapshot directory to cause the creation, destruction, or
renaming of snapshots. When enabled, this functionality works
both locally and over NFS exports which have the no_root_squash
option set.
zfs_flags=0 (int)
Set additional debugging flags. The following flags may be
bitwise-ored together:
+---------------------------------------------------------------------------------------------------------+
| Value Symbolic Name Description |
+---------------------------------------------------------------------------------------------------------+
| 1 ZFS_DEBUG_DPRINTF Enable dprintf entries in the debug log. |
|* 2 ZFS_DEBUG_DBUF_VERIFY Enable extra dbuf verifications. |
|* 4 ZFS_DEBUG_DNODE_VERIFY Enable extra dnode verifications. |
| 8 ZFS_DEBUG_SNAPNAMES Enable snapshot name verification. |
| 16 ZFS_DEBUG_MODIFY Check for illegally modified ARC buffers. |
| 64 ZFS_DEBUG_ZIO_FREE Enable verification of block frees. |
| 128 ZFS_DEBUG_HISTOGRAM_VERIFY Enable extra spacemap histogram verifications. |
| 256 ZFS_DEBUG_METASLAB_VERIFY Verify space accounting on disk matches in-memory range_trees. |
| 512 ZFS_DEBUG_SET_ERROR Enable SET_ERROR and dprintf entries in the debug log. |
| 1024 ZFS_DEBUG_INDIRECT_REMAP Verify split blocks created by device removal. |
| 2048 ZFS_DEBUG_TRIM Verify TRIM ranges are always within the allocatable range tree. |
| 4096 ZFS_DEBUG_LOG_SPACEMAP Verify that the log summary is consistent with the spacemap log |
| and enable zfs_dbgmsgs for metaslab loading and flushing. |
+---------------------------------------------------------------------------------------------------------+
* Requires debug build.
zfs_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 operations 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
forward progress regardless. In the latter, because the error is
permanent, the best we can do is leak the minimum amount of
space, which is what setting this flag will do. It is therefore
reasonable for this flag to normally be set, but we chose the
more conservative approach of not setting it, so that there is no
possibility of leaking space in the "partial temporary" failure
case.
zfs_free_min_time_ms=1000ms (1s) (int)
During a zfs destroy operation using the async_destroy feature, a
minimum of this much time will be spent working on freeing blocks
per TXG.
zfs_obsolete_min_time_ms=500ms (int)
Similar to zfs_free_min_time_ms, but for cleanup of old
indirection records for removed vdevs.
zfs_immediate_write_sz=32768B (32kB) (long)
Largest data block to write to the ZIL. Larger blocks will be
treated as if the dataset being written to had the
logbias=throughput property set.
zfs_initialize_value=16045690984833335022 (0xDEADBEEFDEADBEEE) (ulong)
Pattern written to vdev free space by zpool-initialize(8).
zfs_initialize_chunk_size=1048576B (1MB) (ulong)
Size of writes used by zpool-initialize(8). This option is used
by the test suite.
zfs_livelist_max_entries=500000 (5*10^5) (ulong)
The threshold size (in block pointers) at which we create a new
sub-livelist. Larger sublists are more costly from a memory
perspective but the fewer sublists there are, the lower the cost
of insertion.
zfs_livelist_min_percent_shared=75% (int)
If the amount of shared space between a snapshot and its clone
drops below this threshold, the clone turns off the livelist and
reverts to the old deletion method. This is in place because
livelists no long give us a benefit once a clone has been
overwritten enough.
zfs_livelist_condense_new_alloc=0 (int)
Incremented each time an extra ALLOC blkptr is added to a
livelist entry while it is being condensed. This option is used
by the test suite to track race conditions.
zfs_livelist_condense_sync_cancel=0 (int)
Incremented each time livelist condensing is canceled while in
spa_livelist_condense_sync(). This option is used by the test
suite to track race conditions.
zfs_livelist_condense_sync_pause=0|1 (int)
When set, the livelist condense process pauses indefinitely
before executing the synctask - spa_livelist_condense_sync().
This option 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_condense_cb(). This option is used by the test
suite to trigger race conditions.
zfs_lua_max_instrlimit=100000000 (10^8) (ulong)
The maximum execution time limit that can be set for a ZFS
channel program, specified as a number of Lua instructions.
zfs_lua_max_memlimit=104857600 (100MB) (ulong)
The maximum memory limit that can be set for a ZFS channel
program, specified in bytes.
zfs_max_dataset_nesting=50 (int)
The maximum depth of nested datasets. This value can be tuned
temporarily to fix existing datasets that exceed the predefined
limit.
zfs_max_log_walking=5 (ulong)
The number of past TXGs that the flushing algorithm of the log
spacemap feature uses to estimate incoming log blocks.
zfs_max_logsm_summary_length=10 (ulong)
Maximum number of rows allowed in the summary of the spacemap
log.
zfs_max_recordsize=1048576 (1MB) (int)
We currently support block sizes from 512B to 16MB. The 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 do not allow the recordsize to be set larger than
this tunable. Larger blocks can be created by changing it, and
pools with larger blocks can always be imported and used,
regardless of this setting.
zfs_allow_redacted_dataset_mount=0|1 (int)
Allow datasets received with redacted send/receive to be mounted.
Normally disabled because these datasets may be missing key data.
zfs_min_metaslabs_to_flush=1 (ulong)
Minimum number of metaslabs to flush per dirty TXG.
zfs_metaslab_fragmentation_threshold=70% (int)
Allow metaslabs to keep their active state as long as their
fragmentation percentage is no more than this value. An active
metaslab that exceeds this threshold will no longer keep its
active status allowing better metaslabs to be selected.
zfs_mg_fragmentation_threshold=95% (int)
Metaslab groups are considered eligible for allocations if their
fragmentation metric (measured as a percentage) is less than or
equal to this value. If a metaslab group exceeds this threshold
then it will be skipped unless all metaslab groups within the
metaslab class have also crossed this threshold.
zfs_mg_noalloc_threshold=0% (int)
Defines a threshold at which metaslab groups should be eligible
for allocations. The value is expressed as a percentage of free
space beyond which a metaslab group is always eligible for
allocations. If a metaslab group's free space is less than or
equal to the threshold, the allocator will avoid allocating to
that group unless all groups in the pool have reached the
threshold. Once all groups have reached the threshold, all
groups are allowed to accept allocations. The default value of 0
disables the feature and causes all metaslab groups to be
eligible for allocations.
This parameter allows one to deal with pools having heavily
imbalanced vdevs such as would be the case when a new vdev has
been added. Setting the threshold to a non-zero percentage will
stop allocations from being made to vdevs that aren't filled to
the specified percentage and allow lesser filled vdevs to acquire
more allocations than they otherwise would under the old
zfs_mg_alloc_failures facility.
zfs_ddt_data_is_special=1|0 (int)
If enabled, ZFS will place DDT data into the special allocation
class.
zfs_user_indirect_is_special=1|0 (int)
If enabled, ZFS will place user data indirect blocks into the
special allocation class.
zfs_multihost_history=0 (int)
Historical statistics for this many latest multihost updates will
be available in /proc/spl/kstat/zfs/<pool>/multihost.
zfs_multihost_interval=1000ms (1s) (ulong)
Used to control the frequency of multihost writes which are
performed when the multihost pool property is on. This is one of
the factors used to determine the length of the activity check
during import.
The multihost write period is zfs_multihost_interval /
leaf-vdevs. On average a multihost write will be issued for each
leaf vdev every zfs_multihost_interval milliseconds. In
practice, the observed period can vary with the I/O load and this
observed value is the delay which is stored in the uberblock.
zfs_multihost_import_intervals=20 (uint)
Used to control the duration of the activity test on import.
Smaller values of zfs_multihost_import_intervals will reduce the
import time but increase the risk of failing to detect an active
pool. The total activity check time is never allowed to drop
below one second.
On import the activity check waits a minimum amount of time
determined by zfs_multihost_interval *
zfs_multihost_import_intervals, or the same product computed on
the host which last had the pool imported, whichever is greater.
The activity check time may be further extended if the value of
MMP delay found in the best uberblock indicates actual multihost
updates happened at longer intervals than zfs_multihost_interval.
A minimum of 100ms is enforced.
0 is equivalent to 1.
zfs_multihost_fail_intervals=10 (uint)
Controls the behavior of the pool when multihost write failures
or delays are detected.
When 0, multihost write failures or delays are ignored. The
failures will still be reported to the ZED which depending on its
configuration may take action such as suspending the pool or
offlining a device.
Otherwise, the pool will be suspended if
zfs_multihost_fail_intervals * zfs_multihost_interval
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
instead.
zfs_no_scrub_prefetch=0|1 (int)
Set to disable block prefetching for scrubs.
zfs_nocacheflush=0|1 (int)
Disable cache flush operations on disks when writing. Setting
this will cause pool corruption on power loss if a volatile out-
of-order write cache is enabled.
zfs_nopwrite_enabled=1|0 (int)
Allow no-operation writes. The occurrence of nopwrites will
further depend on other pool properties (i.a. the checksumming
and compression algorithms).
zfs_dmu_offset_next_sync=0|1 (int)
Enable forcing TXG sync to find holes. When enabled forces ZFS
to act like prior versions when SEEK_HOLE or SEEK_DATA flags are
used, which, when a dnode is dirty, causes TXGs to be synced so
that this data can be found.
zfs_pd_bytes_max=52428800B (50MB) (int)
The number of bytes which should be prefetched during a pool
traversal, like zfs send or other data crawling operations.
zfs_traverse_indirect_prefetch_limit=32 (int)
The number of blocks pointed by indirect (non-L0) block which
should be prefetched during a pool traversal, like zfs send or
other data crawling operations.
zfs_per_txg_dirty_frees_percent=5% (ulong)
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
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_qat_compress_disable=0|1 (int)
Disable QAT hardware acceleration for gzip compression. May be
unset after the ZFS modules have been loaded to initialize the
QAT hardware as long as support is compiled in and the QAT 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=1048576B (1MB) (long)
Bytes to read per chunk.
zfs_read_history=0 (int)
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 (1MB) (ulong)
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=33554432B (32MB) (ulong)
Maximum amount of I/O that can be concurrently issued for a
sequential 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
computationally expensive to check them all. Instead, try at
most this many randomly selected combinations each time the block
is accessed. This allows all segment copies to participate
fairly in the reconstruction when all combinations cannot be
checked and prevents repeated use of one bad copy.
zfs_recover=0|1 (int)
Set to attempt to recover from fatal errors. This should only be
used as a last resort, as it typically results in leaked space,
or worse.
zfs_removal_ignore_errors=0|1 (int)
Ignore hard IO errors during device removal. When set, if a
device encounters a hard IO error during the removal process the
removal will not be cancelled. This can result in a normally
recoverable block becoming permanently damaged and is hence not
recommended. 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 (int)
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 (16MB) (int)
The largest contiguous segment that we will attempt to allocate
when removing a device. If there is a performance problem with
attempting to allocate large blocks, consider decreasing this.
The default value is also the maximum.
zfs_resilver_disable_defer=0|1 (int)
Ignore the resilver_defer feature, causing an operation that
would start a resilver to immediately restart the one in
progress.
zfs_resilver_min_time_ms=3000ms (3s) (int)
Resilvers are processed by the sync thread. While resilvering,
it will spend at least this much time working on a resilver
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_min_time_ms=1000ms (1s) (int)
Scrubs are processed by the sync thread. While scrubbing, it
will spend at least this much time working on a scrub between TXG
flushes.
zfs_scan_checkpoint_intval=7200s (2h) (int)
To preserve progress across reboots, the sequential scan
algorithm periodically needs to stop metadata scanning and issue
all the verification I/O to disk. The frequency of this flushing
is determined by this tunable.
zfs_scan_fill_weight=3 (int)
This tunable affects how scrub and resilver I/O segments are
ordered. A higher number indicates that we care more about how
filled in a segment is, while a lower number indicates we care
more about the size of the extent without considering the gaps
within a segment. This value is only tunable upon module
insertion. Changing the value afterwards will have no affect on
scrub or resilver performance.
zfs_scan_issue_strategy=0 (int)
Determines the order that data will be verified while scrubbing
or resilvering:
1 Data will be verified as sequentially as possible, given
the amount of memory reserved for scrubbing (see
zfs_scan_mem_lim_fact). This may improve scrub
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 (2MB) (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 (int)
Maximum fraction of RAM used for I/O sorting by sequential scan
algorithm. This tunable determines the hard limit for I/O
sorting memory usage. When the hard limit is reached we stop
scanning metadata and start issuing data verification I/O. This
is done until we get below the soft limit.
zfs_scan_mem_lim_soft_fact=20^-1 (int)
The fraction of the hard limit used to determined the soft limit
for I/O sorting by the sequential scan algorithm. When we cross
this limit from below no action is taken. When we cross this
limit from above it is because we are issuing verification I/O.
In this case (unless the metadata scan is done) we stop issuing
verification I/O and start scanning metadata again until we get
to the hard limit.
zfs_scan_strict_mem_lim=0|1 (int)
Enforce tight memory limits on pool scans when a sequential scan
is in progress. When disabled, the memory limit may be exceeded
by fast disks.
zfs_scan_suspend_progress=0|1 (int)
Freezes a scrub/resilver in progress without actually pausing it.
Intended for testing/debugging.
zfs_scan_vdev_limit=4194304B (4MB) (int)
Maximum amount of data that can be concurrently issued at once
for scrubs and resilvers per leaf device, given in bytes.
zfs_send_corrupt_data=0|1 (int)
Allow sending of corrupt data (ignore read/checksum errors when
sending).
zfs_send_unmodified_spill_blocks=1|0 (int)
Include unmodified spill blocks in the send stream. Under
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 (int)
The fill fraction of the zfs send internal queues. The fill
fraction controls the timing with which internal threads are
woken up.
zfs_send_no_prefetch_queue_length=1048576B (1MB) (int)
The maximum number of bytes allowed in zfs send's internal
queues.
zfs_send_queue_ff=20^-1 (int)
The fill fraction of the zfs send prefetch queue. The fill
fraction controls the timing with which internal threads are
woken up.
zfs_send_queue_length=16777216B (16MB) (int)
The maximum number of bytes allowed that will be prefetched by
zfs send. This value must be at least twice the maximum block
size in use.
zfs_recv_queue_ff=20^-1 (int)
The fill fraction of the zfs receive queue. The fill fraction
controls the timing with which internal threads are woken up.
zfs_recv_queue_length=16777216B (16MB) (int)
The maximum number of bytes allowed in the zfs receive queue.
This value must be at least twice the maximum block size in use.
zfs_recv_write_batch_size=1048576B (1MB) (int)
The maximum amount of data, in bytes, that zfs receive will write
in one DMU transaction. This is the uncompressed size, even when
receiving a compressed send stream. This setting will not reduce
the write size below a single block. Capped at a maximum of
32MB.
zfs_override_estimate_recordsize=0|1 (ulong)
Setting this variable overrides the default logic for 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 (int)
Flushing of data to disk is done in passes. Defer frees starting
in this pass.
zfs_spa_discard_memory_limit=16777216B (16MB) (int)
Maximum memory used for prefetching a checkpoint's space map on
each vdev while discarding the checkpoint.
zfs_special_class_metadata_reserve_pct=25% (int)
Only allow small data blocks to be allocated on the special and
dedup vdev types when the available free space percentage on
these vdevs exceeds this value. This ensures reserved space is
available for pool metadata as the special vdevs approach
capacity.
zfs_sync_pass_dont_compress=8 (int)
Starting in this sync pass, disable compression (including of
metadata). With the default setting, in practice, we don't have
this many sync passes, so this has no effect.
The original intent was that disabling compression would help the
sync passes to converge. However, in practice, disabling
compression increases the average number of sync passes; 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 128kB allocations (e.g. for indirect
blocks and spacemaps) because these will not be compressed. The
128kB allocations are especially detrimental to performance on
highly fragmented systems, which may have very few free segments
of this size, and may need to load new metaslabs to satisfy these
allocations.
zfs_sync_pass_rewrite=2 (int)
Rewrite new block pointers starting in this pass.
zfs_sync_taskq_batch_pct=75% (int)
This controls the number of threads used by dp_sync_taskq. The
default value of 75% will create a maximum of one thread per CPU.
zfs_trim_extent_bytes_max=134217728B (128MB) (uint)
Maximum size of TRIM command. Larger ranges will be split into
chunks no larger than this value before issuing.
zfs_trim_extent_bytes_min=32768B (32kB) (uint)
Minimum size of TRIM commands. TRIM ranges smaller than this
will be skipped, unless they're part of a larger range which was
chunked. This is done because it's common for these small TRIMs
to negatively impact overall performance.
zfs_trim_metaslab_skip=0|1 (uint)
Skip uninitialized metaslabs during the TRIM process. This
option is useful for pools constructed from large thinly-
provisioned devices where TRIM operations are slow. As a pool
ages, an increasing fraction of the pool's metaslabs will be
initialized, progressively degrading the usefulness of this
option. This setting is stored when starting a manual TRIM and
will persist for the duration of the requested TRIM.
zfs_trim_queue_limit=10 (uint)
Maximum number of queued TRIMs outstanding per leaf vdev. The
number of concurrent TRIM commands issued to the device is
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
efficient 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
potentially increased memory usage. Decreasing this value will
have the opposite effect. The default of 32 was determined to be
a reasonable compromise.
zfs_txg_history=0 (int)
Historical statistics for this many latest TXGs will be available
in /proc/spl/kstat/zfs/<pool>/TXGs.
zfs_txg_timeout=5s (int)
Flush dirty data to disk at least every this many seconds
(maximum TXG duration).
zfs_vdev_aggregate_trim=0|1 (int)
Allow TRIM I/Os to be aggregated. This is normally not helpful
because the extents to be trimmed will have been already been
aggregated by the metaslab. This option is provided for
debugging and performance analysis.
zfs_vdev_aggregation_limit=1048576B (1MB) (int)
Max vdev I/O aggregation size.
zfs_vdev_aggregation_limit_non_rotating=131072B (128kB) (int)
Max vdev I/O aggregation size for non-rotating media.
zfs_vdev_cache_bshift=16 (64kB) (int)
Shift size to inflate reads to.
zfs_vdev_cache_max=16384B (16kB) (int)
Inflate reads smaller than this value to meet the
zfs_vdev_cache_bshift size (default 64kB).
zfs_vdev_cache_size=0 (int)
Total size of the per-disk cache in bytes.
Currently this feature is disabled, as it has been found to not
be helpful for performance and in some cases harmful.
zfs_vdev_mirror_rotating_inc=0 (int)
A number by which the balancing algorithm increments the load
calculation for the purpose of selecting the least busy mirror
member when an I/O operation immediately follows its predecessor
on rotational vdevs for the purpose of making decisions based on
load.
zfs_vdev_mirror_rotating_seek_inc=5 (int)
A number by which the balancing algorithm increments the load
calculation for the purpose of selecting the least busy mirror
member when an I/O operation lacks locality as defined by
zfs_vdev_mirror_rotating_seek_offset. Operations within this
that are not immediately following the previous operation are
incremented by half.
zfs_vdev_mirror_rotating_seek_offset=1048576B (1MB) (int)
The maximum distance for the last queued I/O operation in which
the balancing algorithm considers an operation to have locality.
See ZFS I/O SCHEDULER.
zfs_vdev_mirror_non_rotating_inc=0 (int)
A number by which the balancing algorithm increments the load
calculation for the purpose of selecting the least busy mirror
member on non-rotational vdevs when I/O operations do not
immediately follow one another.
zfs_vdev_mirror_non_rotating_seek_inc=1 (int)
A number by which the balancing algorithm increments the load
calculation for the purpose of selecting the least busy mirror
member when an I/O operation lacks locality as defined by the
zfs_vdev_mirror_rotating_seek_offset. Operations within this
that are not immediately following the previous operation are
incremented by half.
zfs_vdev_read_gap_limit=32768B (32kB) (int)
Aggregate read I/O operations if the on-disk gap between them is
within this threshold.
zfs_vdev_write_gap_limit=4096B (4kB) (int)
Aggregate write I/O operations if the on-disk gap between them is
within this threshold.
zfs_vdev_raidz_impl=fastest (string)
Select the raidz parity implementation to use.
Variants that don't depend on CPU-specific features may be
selected on module load, as they are supported on all systems.
The remaining options may only be set after the module is loaded,
as they are available only if the implementations are compiled in
and supported on the running system.
Once the module is loaded,
/sys/module/zfs/parameters/zfs_vdev_raidz_impl will show the
available options, with the currently selected one enclosed in
square brackets.
fastest selected by built-in benchmark
original original implementation
scalar scalar implementation
sse2 SSE2 instruction set 64-bit x86
ssse3 SSSE3 instruction set 64-bit x86
avx2 AVX2 instruction set 64-bit x86
avx512f AVX512F instruction set 64-bit x86
avx512bw AVX512F & AVX512BW instruction sets 64-bit x86
aarch64_neon NEON Aarch64/64-bit ARMv8
aarch64_neonx2 NEON with more unrolling Aarch64/64-bit ARMv8
powerpc_altivec Altivec PowerPC
zfs_vdev_scheduler (charp)
DEPRECATED. Prints warning to kernel log for compatibility.
zfs_zevent_len_max=512 (int)
Max event queue length. Events in the queue can be viewed with
zpool-events(8).
zfs_zevent_retain_max=2000 (int)
Maximum recent zevent records to retain for duplicate checking.
Setting this to 0 disables duplicate detection.
zfs_zevent_retain_expire_secs=900s (15min) (int)
Lifespan for a recent ereport that was retained for duplicate
checking.
zfs_zil_clean_taskq_maxalloc=1048576 (int)
The maximum number of taskq entries that are allowed to be
cached. When this limit is exceeded transaction records (itxs)
will be cleaned synchronously.
zfs_zil_clean_taskq_minalloc=1024 (int)
The number of taskq entries that are pre-populated when the taskq
is first created and are immediately available for use.
zfs_zil_clean_taskq_nthr_pct=100% (int)
This controls the number of threads used by dp_zil_clean_taskq.
The default value of 100% will create a maximum of one thread per
cpu.
zil_maxblocksize=131072B (128kB) (int)
This sets the maximum block size used by the ZIL. On very
fragmented pools, lowering this (typically to 36kB) can improve
performance.
zil_nocacheflush=0|1 (int)
Disable the cache flush commands that are normally sent to disk
by the ZIL after an LWB write has completed. Setting this will
cause ZIL corruption on power loss if a volatile out-of-order
write cache is enabled.
zil_replay_disable=0|1 (int)
Disable intent logging replay. Can be disabled for recovery from
corrupted ZIL.
zil_slog_bulk=786432B (768kB) (ulong)
Limit SLOG write size per commit executed with synchronous
priority. Any writes above that will be executed with lower
(asynchronous) priority to limit potential SLOG device abuse by
single active ZIL writer.
zfs_embedded_slog_min_ms=64 (int)
Usually, one metaslab from each normal-class vdev is dedicated
for use by the ZIL to log synchronous writes. However, if there
are fewer than zfs_embedded_slog_min_ms metaslabs in the vdev,
this functionality is disabled. This ensures that we don't set
aside an unreasonable amount of space for the ZIL.
zio_deadman_log_all=0|1 (int)
If non-zero, the zio deadman will produce debugging messages (see
zfs_dbgmsg_enable) for all zios, rather than only for leaf zios
possessing a vdev. This is meant to be used by developers to
gain diagnostic information for hang conditions which don't
involve a mutex or other locking primitive: typically conditions
in which a thread in the zio pipeline is looping indefinitely.
zio_slow_io_ms=30000ms (30s) (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 when devices are imbalanced.
When enabled, the maximum number of pending allocations per top-
level vdev is limited by zfs_vdev_queue_depth_pct.
zio_requeue_io_start_cut_in_line=0|1 (int)
Prioritize requeued I/O.
zio_taskq_batch_pct=80% (uint)
Percentage of online CPUs which will run a worker thread for I/O.
These workers are responsible for I/O work such as compression
and checksum calculations. Fractional number of CPUs will be
rounded down.
The default value of 80% was chosen to avoid using all CPUs which
can result in latency issues and inconsistent application
performance, especially when slower compression and/or
checksumming is enabled.
zio_taskq_batch_tpq=0 (uint)
Number of worker threads per taskq. Lower values improve I/O
ordering and CPU utilization, while higher reduces lock
contention.
If 0, generate a system-dependent value close to 6 threads per
taskq.
zvol_inhibit_dev=0|1 (uint)
Do not create zvol device nodes. This may slightly improve
startup time on systems with a very large number of zvols.
zvol_major=230 (uint)
Major number for zvol block devices.
zvol_max_discard_blocks=16384 (ulong)
Discard (TRIM) operations done on zvols will be done in batches
of this many blocks, where block size is determined by the
volblocksize property of a zvol.
zvol_prefetch_bytes=131072B (128kB) (uint)
When adding a zvol to the system, prefetch this many bytes from
the start and end of the volume. Prefetching these regions of
the volume is desirable, because they are likely to be accessed
immediately by blkid(8) or the kernel partitioner.
zvol_request_sync=0|1 (uint)
When processing I/O requests for a zvol, submit them
synchronously. This effectively limits the queue depth to 1 for
each I/O submitter. When unset, requests are handled
asynchronously by a thread pool. The number of requests which
can be handled concurrently is controlled by zvol_threads.
zvol_threads=32 (uint)
Max number of threads which can handle zvol I/O requests
concurrently.
zvol_volmode=1 (uint)
Defines zvol block devices behaviour when volmode=default:
1 equivalent to full
2 equivalent to dev
3 equivalent to none
ZFS I/O SCHEDULER
ZFS issues I/O operations to leaf vdevs to satisfy and complete I/O
operations. The scheduler determines when and in what order those
operations are issued. The scheduler divides operations into five I/O
classes, prioritized in the following order: sync read, sync write, async
read, async write, and scrub/resilver. Each queue defines the minimum
and maximum number of concurrent operations that may be issued to the
device. In addition, the device has an aggregate maximum,
zfs_vdev_max_active. Note that the sum of the per-queue minima must not
exceed the aggregate maximum. If the sum of the per-queue maxima exceeds
the aggregate maximum, then the number of active operations may reach
zfs_vdev_max_active, in which case no further operations will be issued,
regardless of whether all per-queue minima have been met.
For many physical devices, throughput increases with the number of
concurrent operations, but latency typically suffers. Furthermore,
physical devices typically have a limit at which more concurrent
operations have no effect on throughput or can actually cause it to
decrease.
The scheduler selects the next operation to issue by first looking for an
I/O class whose minimum has not been satisfied. Once all are 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 scheduler 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,
depending 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,
except for the async write class. Asynchronous writes represent the data
that is committed to stable storage during the syncing stage for
transaction groups. Transaction groups enter the syncing state
periodically, so the number of queued async writes will quickly burst up
and then bleed down to zero. Rather than servicing them as quickly as
possible, the I/O scheduler changes the maximum number of active async
write operations according to the amount of dirty data in the pool.
Since both throughput and latency typically increase with the number of
concurrent operations issued to physical devices, reducing the burstiness
in the number of concurrent operations also stabilizes the response time
of operations from other - and in particular synchronous - queues. In
broad strokes, the I/O scheduler will issue more concurrent operations
from the async write queue as there's more dirty data in the pool.
Async Writes
The number of concurrent operations issued for the async write I/O class
follows a piece-wise linear function defined by a few adjustable points:
| o---------| <-- zfs_vdev_async_write_max_active
^ | /^ |
| | / | |
active | / | |
I/O | / | |
count | / | |
| / | |
|-------o | | <-- zfs_vdev_async_write_min_active
0|_______^______|_________|
0% | | 100% of zfs_dirty_data_max
| |
| `-- zfs_vdev_async_write_active_max_dirty_percent
`--------- zfs_vdev_async_write_active_min_dirty_percent
Until the amount of dirty data exceeds a minimum percentage of the dirty
data allowed in the pool, the I/O scheduler will limit the number of
concurrent operations to the minimum. As that threshold is crossed, the
number of concurrent operations issued increases linearly to the maximum
at the specified maximum percentage of the dirty data allowed in the
pool.
Ideally, the amount of dirty data on a busy pool will stay in the sloped
part of the function between
zfs_vdev_async_write_active_min_dirty_percent and
zfs_vdev_async_write_active_max_dirty_percent. If it exceeds the maximum
percentage, this indicates that the rate of incoming data is greater than
the rate that the backend storage can handle. In this case, we must
further throttle incoming writes, as described in the next section.
ZFS TRANSACTION DELAY
We delay transactions when we've determined that the backend storage
isn't able to accommodate the rate of incoming writes.
If there is already a transaction waiting, we delay relative to when that
transaction will finish waiting. This way the calculated delay time is
independent of the number of threads concurrently executing transactions.
If we are the only waiter, wait relative to when the transaction started,
rather than the current time. This credits the transaction for "time
already served", e.g. reading indirect blocks.
The minimum time for a transaction to take is calculated as
min_time = min(zfs_delay_scale * (dirty - min) / (max - dirty),
100ms)
The delay has two degrees of freedom that can be adjusted via tunables.
The percentage of dirty data at which we start to delay is defined by
zfs_delay_min_dirty_percent. This should typically be at or above
zfs_vdev_async_write_active_max_dirty_percent, so that we only start to
delay after writing at full speed has failed to keep up with the 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 500us translates to 2000 IOPS. The shape of the curve
was chosen such that small changes in the amount of accumulated dirty
data in the first three quarters of the curve yield relatively small
differences in the amount of delay.
The effects can be easier to understand when the amount of delay is
represented on a logarithmic scale:
delay
100ms +-------------------------------------------------------------++
+ +
| |
+ *+
10ms + *+
+ ** +
| (midpoint) ** |
+ | ** +
1ms + v **** +
+ zfs_delay_scale ----------> ***** +
| **** |
+ **** +
100us + ** +
+ * +
| * |
+ * +
10us + * +
+ +
| |
+ +
+--------------------------------------------------------------+
0% <- zfs_dirty_data_max -> 100%
Note here that only as the amount of dirty data approaches its limit does
the delay start to increase rapidly. The goal of a properly tuned system
should be to keep the amount of dirty data out of that range by first
ensuring that the appropriate limits are set for the I/O 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 13.1-RELEASE-p6 June 1, 2021 FreeBSD 13.1-RELEASE-p6
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