ZFS-MODULE-PARAMETERS(5)

NAME
zfs-module-parameters - ZFS module parameters

DESCRIPTION

Description of the different parameters to the ZFS module.

  Module parameters

ignore_hole_birth (int)
    When set, the hole_birth optimization will not be used, and all holes will always be sent on zfs send. Useful if you suspect your datasets are affected by a bug in hole_birth.

    Use 1 for on (default) and 0 for off.

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

    Use 1 for yes (default) and 0 to disable.

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

    Default value: 200.

l2arc_feed_secs (ulong)
    Seconds between L2ARC writing

    Default value: 1.

l2arc_headroom (ulong)
    How far through the ARC lists to search for L2ARC cacheable content, expressed as a multiplier of l2arc_write_max

    Default value: 2.

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

    Default value: 200.

l2arc_nocompress (int)
    Skip compressing L2ARC buffers

    Use 1 for yes and 0 for no (default).

l2arc_noprefetch (int)
    Do not write buffers to L2ARC if they were prefetched but not used by applications

    Use 1 for yes (default) and 0 to disable.

l2arc_norw (int)
    No reads during writes

    Use 1 for yes and 0 for no (default).

l2arc_write_boost (ulong)
    Cold L2ARC devices will have l2arc_write_max increased by this amount while they remain cold.

    Default value: 8,388,608.

l2arc_write_max (ulong)
    Max write bytes per interval

    Default value: 8,388,608.

metaslab_aliquot (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.

    Default value: 524,288.

metaslab_bias_enabled (int)
    Enable metaslab group biasing based on its vdev's over- or under-utilization relative to the pool.

    Use 1 for yes (default) and 0 for no.

zfs_metaslab_segment_weight_enabled (int)
    Enable/disable segment-based metaslab selection.

    Use 1 for yes (default) and 0 for no.

zfs_metaslab_switch_threshold (int)
    When using segment-based metaslab selection, continue allocating from the active metaslab until zfs_metaslab_switch_threshold worth of buckets have been exhausted.

    Default value: 2.

metaslab_debug_load (int)
    Load all metaslabs during pool import.

    Use 1 for yes and 0 for no (default).

metaslab_debug_unload (int)
    Prevent metaslabs from being unloaded.

    Use 1 for yes and 0 for no (default).

metaslab_fragmentation_factor_enabled (int)
    Enable use of the fragmentation metric in computing metaslab weights.

    Use 1 for yes (default) and 0 for no.

metaslabs_per_vdev (int)
    When a vdev is added, it will be divided into approximately (but no more than) this number of metaslabs.

    Default value: 200.

metaslab_preload_enabled (int)
    Enable metaslab group preloading.

    Use 1 for yes (default) and 0 for no.

metaslab_lba_weighting_enabled (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.

    Use 1 for yes (default) and 0 for no.

spa_config_path (charp)
    SPA config file

    Default value: /etc/zfs/zpool.cache.

spa_asize_inflation (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.

    Default value: 24.

spa_load_verify_data (int)
    Whether to traverse data blocks during an "extreme rewind" (-X) import. Use 0 to disable and 1 to enable. An extreme rewind import normally performs a full traversal of all blocks in the pool for verification. If this parameter is set to 0, 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.

    Default value: 1.

spa_load_verify_metadata (int)
    Whether to traverse blocks during an "extreme rewind" (-X) pool import. Use 0 to disable and 1 to enable. An extreme rewind import normally performs a full traversal of all blocks in the pool for verification. If this parameter is set to 0, the traversal is not performed. It can be toggled once the import has started to stop or start the traversal.

    Default value: 1.

spa_load_verify_maxinflight (int)
    Maximum concurrent I/Os during the traversal performed during an "extreme rewind" (-X) pool import.

    Default value: 10000.

spa_slop_shift (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.

    Default value: 5.

zfetch_array_rd_sz (ulong)
    If prefetching is enabled, disable prefetching for reads larger than this size.

    Default value: 1,048,576.

zfetch_max_distance (uint)
    Max bytes to prefetch per stream (default 8MB).

    Default value: 8,388,608.

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

    Default value: 8.

zfetch_min_sec_reap (uint)
    Min time before an active prefetch stream can be reclaimed

    Default value: 2.

zfs_arc_dnode_limit (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. See also 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.

    Default value: 0.

zfs_arc_dnode_limit_percent (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 set to nonzero value.

    Default value: 10.

zfs_arc_dnode_reduce_percent (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.

    Default value: 10% of the number of dnodes in the ARC.

zfs_arc_average_blocksize (int)
    The ARC's buffer hash table is sized based on the assumption of an average block size of zfs_arc_average_blocksize (default 8K). 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.

    Default value: 8192.

zfs_arc_evict_batch_limit (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.

    Default value: 10.

zfs_arc_grow_retry (int)
    After a memory pressure event the ARC will wait this many seconds before trying to resume growth

    Default value: 5.

zfs_arc_lotsfree_percent (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.

    Default value: 10.

zfs_arc_max (ulong)
    Max arc size of ARC in bytes. If set to 0 then it will consume 1/2 of system RAM. This value must be at least 67108864 (64 megabytes).

    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.

    Default value: 0.

zfs_arc_meta_limit (ulong)
    The maximum allowed size in bytes that meta data buffers are allowed to consume in the ARC. When this limit is reached meta data buffers will be reclaimed even if the overall arc_c_max has not been reached. This value defaults to 0 which indicates that a percent which is based on zfs_arc_meta_limit_percent of the ARC may be used for meta data.

    This value my be changed dynamically except that it cannot be set back to 0 for a specific percent of the ARC; it must be set to an explicit value.

    Default value: 0.

zfs_arc_meta_limit_percent (ulong)
    Percentage of ARC buffers that can be used for meta data. See also zfs_arc_meta_limit which serves a similar purpose but has a higher priority if set to nonzero value.

    Default value: 75.

zfs_arc_meta_min (ulong)
    The minimum allowed size in bytes that meta data buffers may consume in the ARC. This value defaults to 0 which disables a floor on the amount of the ARC devoted meta data.

    Default value: 0.

zfs_arc_meta_prune (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.

    Default value: 10,000.

zfs_arc_meta_adjust_restarts (ulong)
    The number of restart passes to make while scanning the ARC attempting the free buffers in order to stay below the zfs_arc_meta_limit. This value should not need to be tuned but is available to facilitate performance analysis.

    Default value: 4096.

zfs_arc_min (ulong)
    Min arc size

    Default value: 100.

zfs_arc_min_prefetch_lifespan (int)
    Minimum time prefetched blocks are locked in the ARC, specified in jiffies. A value of 0 will default to 1 second.

    Default value: 0.

zfs_multilist_num_sublists (int)
    To allow more fine-grained locking, each ARC state contains a series of lists for both data and meta data 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.

    Default value: 4 or the number of online CPUs, whichever is greater

zfs_arc_overflow_shift (int)
    The ARC size is considered to be overflowing if it exceeds the current ARC target size (arc_c) by a threshold determined by this parameter. The threshold is calculated as a fraction of arc_c using the formula "arc_c >> zfs_arc_overflow_shift". The default value of 8 causes the ARC to be considered to be overflowing if it exceeds the target size by 1/256th (0.3%) of the target size. When the ARC is overflowing, new buffer allocations are stalled until the reclaim thread catches up and the overflow condition no longer exists.

    Default value: 8.

zfs_arc_p_min_shift (int)
    arc_c shift to calc min/max arc_p

    Default value: 4.

zfs_arc_p_aggressive_disable (int)
    Disable aggressive arc_p growth

    Use 1 for yes (default) and 0 to disable.

zfs_arc_p_dampener_disable (int)
    Disable arc_p adapt dampener

    Use 1 for yes (default) and 0 to disable.

zfs_arc_shrink_shift (int)
    log2(fraction of arc to reclaim)

    Default value: 5.

zfs_arc_pc_percent (uint)
    Percent of pagecache to reclaim arc to This tunable allows 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 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.

    Default value: 0 (disabled).

zfs_arc_sys_free (ulong)
    The target number of bytes the ARC should leave as free memory on the system. Defaults to the larger of 1/64 of physical memory or 512K. Setting this option to a non-zero value will override the default.

    Default value: 0.

zfs_autoimport_disable (int)
    Disable pool import at module load by ignoring the cache file (typically /etc/zfs/zpool.cache).

    Use 1 for yes (default) and 0 for no.

zfs_dbgmsg_enable (int)
    Internally ZFS keeps a small log to facilitate debugging. By default the log is disabled, to enable it set this option to 1. The contents of the log can be accessed by reading the /proc/spl/kstat/zfs/dbgmsg file. Writing 0 to this proc file clears the log.

    Default value: 0.

zfs_dbgmsg_maxsize (int)
    The maximum size in bytes of the internal ZFS debug log.

    Default value: 4M.

zfs_dbuf_state_index (int)
    This feature is currently unused. It is normally used for controlling what reporting is available under /proc/spl/kstat/zfs.

    Default value: 0.

zfs_deadman_enabled (int)
    When a pool sync operation takes longer than zfs_deadman_synctime_ms milliseconds, a "slow spa_sync" message is logged to the debug log (see zfs_dbgmsg_enable). If zfs_deadman_enabled is set, all pending IO operations are also checked and if any haven't completed within zfs_deadman_synctime_ms milliseconds, a "SLOW IO" message is logged to the debug log and a "delay" system event with the details of the hung IO is posted.

    Use 1 (default) to enable the slow IO check and 0 to disable.

zfs_deadman_checktime_ms (int)
    Once a pool sync operation has taken longer than zfs_deadman_synctime_ms milliseconds, continue to check for slow operations every zfs_deadman_checktime_ms milliseconds.

    Default value: 5,000.

zfs_deadman_synctime_ms (ulong)
    Interval in milliseconds after which the deadman is triggered and also the interval after which an IO operation is considered to be "hung" if zfs_deadman_enabled is set. See zfs_deadman_enabled.

    Default value: 1,000,000.

zfs_dedup_prefetch (int)
    Enable prefetching dedup-ed blks

    Use 1 for yes and 0 to disable (default).

zfs_delay_min_dirty_percent (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 >= zfs_vdev_async_write_active_max_dirty_percent. See the section "ZFS TRANSACTION DELAY".

    Default value: 60.

zfs_delay_scale (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 10x and 1/10th this number.

    See the section "ZFS TRANSACTION DELAY".

    Note: zfs_delay_scale * zfs_dirty_data_max must be < 2^64.

    Default value: 500,000.

zfs_delete_blocks (ulong)
    This is the used to define a large file for the purposes of delete. 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.

    Default value: 20,480.

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 the section "ZFS TRANSACTION DELAY".

    Default value: 10 percent of all memory, 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 the section "ZFS TRANSACTION DELAY".

    Default value: 25% of physical RAM.

zfs_dirty_data_max_max_percent (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 the section "ZFS TRANSACTION DELAY".

    Default value: 25.

zfs_dirty_data_max_percent (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 the section "ZFS TRANSACTION DELAY".

    Default value: 10%, subject to zfs_dirty_data_max_max.

zfs_dirty_data_sync (int)
    Start syncing out a transaction group if there is at least this much dirty data.

    Default value: 67,108,864.

zfs_fletcher_4_impl (string)
    Select a fletcher 4 implementation.

    Supported selectors are: fastest, scalar, sse2, ssse3, avx2, avx512f, and aarch64_neon. All of the selectors 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 and scalar results in vector instructions from the respective CPU instruction set being used.

    Default value: fastest.

zfs_free_bpobj_enabled (int)
    Enable/disable the processing of the free_bpobj object.

    Default value: 1.

zfs_free_max_blocks (ulong)
    Maximum number of blocks freed in a single txg.

    Default value: 100,000.

zfs_vdev_async_read_max_active (int)
    Maximum asynchronous read I/Os active to each device. See the section "ZFS I/O SCHEDULER".

    Default value: 3.

zfs_vdev_async_read_min_active (int)
    Minimum asynchronous read I/Os active to each device. See the section "ZFS I/O SCHEDULER".

    Default value: 1.

zfs_vdev_async_write_active_max_dirty_percent (int)
    When the pool has more than zfs_vdev_async_write_active_max_dirty_percent dirty data, use zfs_vdev_async_write_max_active to limit active async writes. If the dirty data is between min and max, the active I/O limit is linearly interpolated. See the section "ZFS I/O SCHEDULER".

    Default value: 60.

zfs_vdev_async_write_active_min_dirty_percent (int)
    When the pool has less than zfs_vdev_async_write_active_min_dirty_percent dirty data, use zfs_vdev_async_write_min_active to limit active async writes. If the dirty data is between min and max, the active I/O limit is linearly interpolated. See the section "ZFS I/O SCHEDULER".

    Default value: 30.

zfs_vdev_async_write_max_active (int)
    Maximum asynchronous write I/Os active to each device. See the section "ZFS I/O SCHEDULER".

    Default value: 10.

zfs_vdev_async_write_min_active (int)
    Minimum asynchronous write I/Os active to each device. See the section "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.

    Default value: 2.

zfs_vdev_max_active (int)
    The maximum number of I/Os active to each device. Ideally, this will be >= the sum of each queue's max_active. It must be at least the sum of each queue's min_active. See the section "ZFS I/O SCHEDULER".

    Default value: 1,000.

zfs_vdev_scrub_max_active (int)
    Maximum scrub I/Os active to each device. See the section "ZFS I/O SCHEDULER".

    Default value: 2.

zfs_vdev_scrub_min_active (int)
    Minimum scrub I/Os active to each device. See the section "ZFS I/O SCHEDULER".

    Default value: 1.

zfs_vdev_sync_read_max_active (int)
    Maximum synchronous read I/Os active to each device. See the section "ZFS I/O SCHEDULER".

    Default value: 10.

zfs_vdev_sync_read_min_active (int)
    Minimum synchronous read I/Os active to each device. See the section "ZFS I/O SCHEDULER".

    Default value: 10.

zfs_vdev_sync_write_max_active (int)
    Maximum synchronous write I/Os active to each device. See the section "ZFS I/O SCHEDULER".

    Default value: 10.

zfs_vdev_sync_write_min_active (int)
    Minimum synchronous write I/Os active to each device. See the section "ZFS I/O SCHEDULER".

    Default value: 10.

zfs_vdev_queue_depth_pct (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. It allows for dynamic allocation distribution when devices are imbalanced as fuller devices will tend to be slower than empty devices. See also zio_dva_throttle_enabled.

    Default value: 1000.

zfs_disable_dup_eviction (int)
    Disable duplicate buffer eviction

    Use 1 for yes and 0 for no (default).

zfs_expire_snapshot (int)
    Seconds to expire .zfs/snapshot

    Default value: 300.

zfs_admin_snapshot (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. This functionality is disabled by default.

    Use 1 for yes and 0 for no (default).

zfs_flags (int)
    Set additional debugging flags. The following flags may be bitwise-or'd 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.
    32 | ZFS_DEBUG_SPA
     | Enable spa_dbgmsg entries in the debug log.
    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-core range_trees.
    512 | ZFS_DEBUG_SET_ERROR
     | Enable SET_ERROR and dprintf entries in the debug log.

    * Requires debug build.

    Default value: 0.

zfs_free_leak_on_eio (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/os 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. However, note that this case is actually fairly rare. Typically pools either (a) fail completely (but perhaps temporarily, e.g. a top-level vdev going offline), or (b) have localized, permanent errors (e.g. disk returns the wrong data due to bit flip or firmware bug). In case (a), 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 case (b), 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. Therefore, it is 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.

    Default value: 0.

zfs_free_min_time_ms (int)
    During a zfs destroy operation using feature@async_destroy a minimum of this much time will be spent working on freeing blocks per txg.

    Default value: 1,000.

zfs_immediate_write_sz (long)
    Largest data block to write to zil. Larger blocks will be treated as if the dataset being written to had the property setting logbias=throughput.

    Default value: 32,768.

zfs_max_recordsize (int)
    We currently support block sizes from 512 bytes to 16MB. The benefits of larger blocks, and thus larger IO, 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 zfs_max_recordsize (default 1MB). Larger blocks can be created by changing this tunable, and pools with larger blocks can always be imported and used, regardless of this setting.

    Default value: 1,048,576.

zfs_mdcomp_disable (int)
    Disable meta data compression

    Use 1 for yes and 0 for no (default).

zfs_metaslab_fragmentation_threshold (int)
    Allow metaslabs to keep their active state as long as their fragmentation percentage is less than or equal to this value. An active metaslab that exceeds this threshold will no longer keep its active status allowing better metaslabs to be selected.

    Default value: 70.

zfs_mg_fragmentation_threshold (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.

    Default value: 85.

zfs_mg_noalloc_threshold (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.

    Default value: 0.

zfs_multihost_history (int)
    Historical statistics for the last N multihost updates will be available in /proc/spl/kstat/zfs/<pool>/multihost

    Default value: 0.

zfs_multihost_interval (ulong)
    Used to control the frequency of multihost writes which are performed when the multihost pool property is on. This is one factor used to determine the length of the activity check during import.

    The multihost write period is zfs_multihost_interval / leaf-vdevs milliseconds. This means that 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.

    On import the activity check waits a minimum amount of time determined by zfs_multihost_interval * zfs_multihost_import_intervals. 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 value of 100ms is enforced.

    Default value: 1000.

zfs_multihost_import_intervals (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. A value of 0 is ignored and treated as if it was set to 1

    Default value: 10.

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

    When zfs_multihost_fail_intervals = 0 then multihost write failures 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.

    When zfs_multihost_fail_intervals > 0 then sequential multihost write failures will cause the pool to be suspended. This occurs when zfs_multihost_fail_intervals * zfs_multihost_interval milliseconds have passed since the last successful multihost write. This guarantees the activity test will see multihost writes if the pool is imported.

    Default value: 5.

zfs_no_scrub_io (int)
    Set for no scrub I/O. This results in scrubs not actually scrubbing data and simply doing a metadata crawl of the pool instead.

    Use 1 for yes and 0 for no (default).

zfs_no_scrub_prefetch (int)
    Set to disable block prefetching for scrubs.

    Use 1 for yes and 0 for no (default).

zfs_nocacheflush (int)
    Disable cache flush operations on disks when writing. Beware, this may cause corruption if disks re-order writes.

    Use 1 for yes and 0 for no (default).

zfs_nopwrite_enabled (int)
    Enable NOP writes

    Use 1 for yes (default) and 0 to disable.

zfs_dmu_offset_next_sync (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 txg's to be synced so that this data can be found.

    Use 1 for yes and 0 to disable (default).

zfs_pd_bytes_max (int)
    The number of bytes which should be prefetched during a pool traversal (eg: zfs send or other data crawling operations)

    Default value: 52,428,800.

zfs_per_txg_dirty_frees_percent (ulong)
    Tunable to control percentage of dirtied blocks from frees in one TXG. After this threshold is crossed, additional dirty blocks from frees wait until the next TXG. A value of zero will disable this throttle.

    Default value: 30 and 0 to disable.

zfs_prefetch_disable (int)
    This tunable disables predictive prefetch. Note that it leaves "prescient" prefetch (e.g. prefetch for zfs send) intact. Unlike predictive prefetch, prescient prefetch never issues i/os that end up not being needed, so it can't hurt performance.

    Use 1 for yes and 0 for no (default).

zfs_read_chunk_size (long)
    Bytes to read per chunk

    Default value: 1,048,576.

zfs_read_history (int)
    Historical statistics for the last N reads will be available in /proc/spl/kstat/zfs/<pool>/reads

    Default value: 0 (no data is kept).

zfs_read_history_hits (int)
    Include cache hits in read history

    Use 1 for yes and 0 for no (default).

zfs_recover (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.

    Use 1 for yes and 0 for no (default).

zfs_resilver_delay (int)
    Number of ticks to delay prior to issuing a resilver I/O operation when a non-resilver or non-scrub I/O operation has occurred within the past zfs_scan_idle ticks.

    Default value: 2.

zfs_resilver_min_time_ms (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.

    Default value: 3,000.

zfs_scan_idle (int)
    Idle window in clock ticks. During a scrub or a resilver, if a non-scrub or non-resilver I/O operation has occurred during this window, the next scrub or resilver operation is delayed by, respectively zfs_scrub_delay or zfs_resilver_delay ticks.

    Default value: 50.

zfs_scan_min_time_ms (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.

    Default value: 1,000.

zfs_scrub_delay (int)
    Number of ticks to delay prior to issuing a scrub I/O operation when a non-scrub or non-resilver I/O operation has occurred within the past zfs_scan_idle ticks.

    Default value: 4.

zfs_send_corrupt_data (int)
    Allow sending of corrupt data (ignore read/checksum errors when sending data)

    Use 1 for yes and 0 for no (default).

zfs_sync_pass_deferred_free (int)
    Flushing of data to disk is done in passes. Defer frees starting in this pass

    Default value: 2.

zfs_sync_pass_dont_compress (int)
    Don't compress starting in this pass

    Default value: 5.

zfs_sync_pass_rewrite (int)
    Rewrite new block pointers starting in this pass

    Default value: 2.

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

    Default value: 75.

zfs_top_maxinflight (int)
    Max concurrent I/Os per top-level vdev (mirrors or raidz arrays) allowed during scrub or resilver operations.

    Default value: 32.

zfs_txg_history (int)
    Historical statistics for the last N txgs will be available in /proc/spl/kstat/zfs/<pool>/txgs

    Default value: 100.

zfs_txg_timeout (int)
    Flush dirty data to disk at least every N seconds (maximum txg duration)

    Default value: 5.

zfs_vdev_aggregation_limit (int)
    Max vdev I/O aggregation size

    Default value: 131,072.

zfs_vdev_cache_bshift (int)
    Shift size to inflate reads too

    Default value: 16 (effectively 65536).

zfs_vdev_cache_max (int)
    Inflate reads small than this value to meet the zfs_vdev_cache_bshift size.

    Default value: 16384.

zfs_vdev_cache_size (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.

    Default value: 0.

zfs_vdev_mirror_rotating_inc (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 immediately follows its predecessor on rotational vdevs for the purpose of making decisions based on load.

    Default value: 0.

zfs_vdev_mirror_rotating_seek_inc (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 lacks locality as defined by the zfs_vdev_mirror_rotating_seek_offset. I/Os within this that are not immediately following the previous I/O are incremented by half.

    Default value: 5.

zfs_vdev_mirror_rotating_seek_offset (int)
    The maximum distance for the last queued I/O in which the balancing algorithm considers an I/O to have locality. See the section "ZFS I/O SCHEDULER".

    Default value: 1048576.

zfs_vdev_mirror_non_rotating_inc (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/Os do not immediately follow one another.

    Default value: 0.

zfs_vdev_mirror_non_rotating_seek_inc (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 lacks locality as defined by the zfs_vdev_mirror_rotating_seek_offset. I/Os within this that are not immediately following the previous I/O are incremented by half.

    Default value: 1.

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

    Default value: 32,768.

zfs_vdev_scheduler (charp)
    Set the Linux I/O scheduler on whole disk vdevs to this scheduler

    Default value: noop.

zfs_vdev_write_gap_limit (int)
    Aggregate write I/O over gap

    Default value: 4,096.

zfs_vdev_raidz_impl (string)
    Parameter for selecting raidz parity implementation to use. Options marked (always) below 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, the content of /sys/module/zfs/parameters/zfs_vdev_raidz_impl will show available options with the currently selected one enclosed in []. Possible options are:
      fastest - (always) implementation selected using built-in benchmark
      original - (always) original raidz implementation
      scalar - (always) scalar raidz implementation
      sse2 - implementation using SSE2 instruction set (64bit x86 only)
      ssse3 - implementation using SSSE3 instruction set (64bit x86 only)
      avx2 - implementation using AVX2 instruction set (64bit x86 only)
      avx512f - implementation using AVX512F instruction set (64bit x86 only)
      avx512bw - implementation using AVX512F & AVX512BW instruction sets (64bit x86 only)
      aarch64_neon - implementation using NEON (Aarch64/64 bit ARMv8 only)
      aarch64_neonx2 - implementation using NEON with more unrolling (Aarch64/64 bit ARMv8 only)

    Default value: fastest.

zfs_zevent_cols (int)
    When zevents are logged to the console use this as the word wrap width.

    Default value: 80.

zfs_zevent_console (int)
    Log events to the console

    Use 1 for yes and 0 for no (default).

zfs_zevent_len_max (int)
    Max event queue length. A value of 0 will result in a calculated value which increases with the number of CPUs in the system (minimum 64 events). Events in the queue can be viewed with the zpool events command.

    Default value: 0.

zfs_zil_clean_taskq_maxalloc (int)
    The maximum number of taskq entries that are allowed to be cached. When this limit is exceeded itx's will be cleaned synchronously.

    Default value: 1048576.

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

    Default value: 1024.

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

    Default value: 100.

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

    Use 1 for yes and 0 for no (default).

zil_slog_bulk (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.

    Default value: 786,432.

zio_delay_max (int)
    A zevent will be logged if a ZIO operation takes more than N milliseconds to complete. Note that this is only a logging facility, not a timeout on operations.

    Default value: 30,000.

zio_dva_throttle_enabled (int)
    Throttle block allocations in the ZIO 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.

    Default value: 1.

zio_requeue_io_start_cut_in_line (int)
    Prioritize requeued I/O

    Default value: 0.

zio_taskq_batch_pct (uint)
    Percentage of online CPUs (or CPU cores, etc) which will run a worker thread for IO. These workers are responsible for IO work such as compression and checksum calculations. Fractional number of CPUs will be rounded down.

    The default value of 75 was chosen to avoid using all CPUs which can result in latency issues and inconsistent application performance, especially when high compression is enabled.

    Default value: 75.

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

    Use 1 for yes and 0 for no (default).

zvol_major (uint)
    Major number for zvol block devices

    Default value: 230.

zvol_max_discard_blocks (ulong)
    Discard (aka 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.

    Default value: 16,384.

zvol_prefetch_bytes (uint)
    When adding a zvol to the system prefetch zvol_prefetch_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 by the kernel scanning for a partition table.

    Default value: 131,072.

zvol_request_sync (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 set to 0 requests are handled asynchronously by a thread pool. The number of requests which can be handled concurrently is controller by zvol_threads.

    Default value: 0.

zvol_threads (uint)
    Max number of threads which can handle zvol I/O requests concurrently.

    Default value: 32.

zvol_volmode (uint)
    Defines zvol block devices behaviour when volmode is set to default. Valid values are 1 (full), 2 (dev) and 3 (none).

    Default value: 1.

zfs_qat_disable (int)
    This tunable disables qat hardware acceleration for gzip compression. It is available only if qat acceleration is compiled in and qat driver is present.

    Use 1 for yes and 0 for no (default).

ZFS I/O SCHEDULER
ZFS issues I/O operations to leaf vdevs to satisfy and complete I/Os. The I/O scheduler determines when and in what order those operations are issued. The I/O scheduler divides operations into five I/O classes prioritized in the following order: sync read, sync write, async read, async write, and scrub/resilver. Each queue defines the minimum and maximum number of concurrent operations that may be issued to the device. In addition, the device has an aggregate maximum, zfs_vdev_max_active. Note that the sum of the per-queue minimums must not exceed the aggregate maximum. If the sum of the per-queue maximums exceeds the aggregate maximum, then the number of active I/Os may reach zfs_vdev_max_active, in which case no further I/Os will be issued regardless of whether all per-queue minimums have been met.

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

The scheduler selects the next operation to issue by first looking for an I/O class whose minimum has not been satisfied. Once all are satisfied and the aggregate maximum has not been hit, the scheduler looks for classes whose maximum has not been satisfied. Iteration through the I/O classes is done in the order specified above. No further operations are issued if the aggregate maximum number of concurrent operations has been hit or if there are no operations queued for an I/O class that has not hit its maximum. Every time an I/O is queued or an operation completes, the I/O scheduler looks for new operations to issue.

In general, smaller max_active's will lead to lower latency of synchronous operations. Larger max_active's may lead to higher overall throughput, depending on underlying storage.

The ratio of the queues' max_actives determines the balance of performance between reads, writes, and scrubs. E.g., 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 I/Os according to the amount of dirty data in the pool. Since both throughput and latency typically increase with the number of concurrent operations issued to physical devices, reducing the burstiness in the number of concurrent operations also stabilizes the response time of operations from other -- and in particular synchronous -- queues. In broad strokes, the I/O scheduler will issue more concurrent operations from the async write queue as there's more dirty data in the pool.

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 = zfs_delay_scale * (dirty - min) / (max - dirty)
    min_time is then capped at 100 milliseconds.

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, the delay is effectively the inverse of IOPS. Here the midpoint of 500us translates to 2000 IOPS. The shape of the curve was chosen such that small changes in the amount of accumulated dirty data in the first 3/4 of the curve yield relatively small differences in the amount of delay.

The effects can be easier to understand when the amount of delay is represented on a log 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 backend storage, and then by changing the value of zfs_delay_scale to increase the steepness of the curve.