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15 .TH ZFS-MODULE-PARAMETERS 5 "Nov 16, 2013"
17 zfs\-module\-parameters \- ZFS module parameters
21 Description of the different parameters to the ZFS module.
23 .SS "Module parameters"
30 \fBignore_hole_birth\fR (int)
33 When set, the hole_birth optimization will not be used, and all holes will
34 always be sent on zfs send. Useful if you suspect your datasets are affected
35 by a bug in hole_birth.
37 Use \fB1\fR for on (default) and \fB0\fR for off.
43 \fBl2arc_feed_again\fR (int)
46 Turbo L2ARC warm-up. When the L2ARC is cold the fill interval will be set as
49 Use \fB1\fR for yes (default) and \fB0\fR to disable.
55 \fBl2arc_feed_min_ms\fR (ulong)
58 Min feed interval in milliseconds. Requires \fBl2arc_feed_again=1\fR and only
59 applicable in related situations.
61 Default value: \fB200\fR.
67 \fBl2arc_feed_secs\fR (ulong)
70 Seconds between L2ARC writing
72 Default value: \fB1\fR.
78 \fBl2arc_headroom\fR (ulong)
81 How far through the ARC lists to search for L2ARC cacheable content, expressed
82 as a multiplier of \fBl2arc_write_max\fR
84 Default value: \fB2\fR.
90 \fBl2arc_headroom_boost\fR (ulong)
93 Scales \fBl2arc_headroom\fR by this percentage when L2ARC contents are being
94 successfully compressed before writing. A value of 100 disables this feature.
96 Default value: \fB200\fR.
102 \fBl2arc_nocompress\fR (int)
105 Skip compressing L2ARC buffers
107 Use \fB1\fR for yes and \fB0\fR for no (default).
113 \fBl2arc_noprefetch\fR (int)
116 Do not write buffers to L2ARC if they were prefetched but not used by
119 Use \fB1\fR for yes (default) and \fB0\fR to disable.
125 \fBl2arc_norw\fR (int)
128 No reads during writes
130 Use \fB1\fR for yes and \fB0\fR for no (default).
136 \fBl2arc_write_boost\fR (ulong)
139 Cold L2ARC devices will have \fBl2arc_write_max\fR increased by this amount
140 while they remain cold.
142 Default value: \fB8,388,608\fR.
148 \fBl2arc_write_max\fR (ulong)
151 Max write bytes per interval
153 Default value: \fB8,388,608\fR.
159 \fBmetaslab_aliquot\fR (ulong)
162 Metaslab granularity, in bytes. This is roughly similar to what would be
163 referred to as the "stripe size" in traditional RAID arrays. In normal
164 operation, ZFS will try to write this amount of data to a top-level vdev
165 before moving on to the next one.
167 Default value: \fB524,288\fR.
173 \fBmetaslab_bias_enabled\fR (int)
176 Enable metaslab group biasing based on its vdev's over- or under-utilization
177 relative to the pool.
179 Use \fB1\fR for yes (default) and \fB0\fR for no.
185 \fBzfs_metaslab_segment_weight_enabled\fR (int)
188 Enable/disable segment-based metaslab selection.
190 Use \fB1\fR for yes (default) and \fB0\fR for no.
196 \fBzfs_metaslab_switch_threshold\fR (int)
199 When using segment-based metaslab selection, continue allocating
200 from the active metaslab until \fBzfs_metaslab_switch_threshold\fR
201 worth of buckets have been exhausted.
203 Default value: \fB2\fR.
209 \fBmetaslab_debug_load\fR (int)
212 Load all metaslabs during pool import.
214 Use \fB1\fR for yes and \fB0\fR for no (default).
220 \fBmetaslab_debug_unload\fR (int)
223 Prevent metaslabs from being unloaded.
225 Use \fB1\fR for yes and \fB0\fR for no (default).
231 \fBmetaslab_fragmentation_factor_enabled\fR (int)
234 Enable use of the fragmentation metric in computing metaslab weights.
236 Use \fB1\fR for yes (default) and \fB0\fR for no.
242 \fBmetaslabs_per_vdev\fR (int)
245 When a vdev is added, it will be divided into approximately (but no more than) this number of metaslabs.
247 Default value: \fB200\fR.
253 \fBmetaslab_preload_enabled\fR (int)
256 Enable metaslab group preloading.
258 Use \fB1\fR for yes (default) and \fB0\fR for no.
264 \fBmetaslab_lba_weighting_enabled\fR (int)
267 Give more weight to metaslabs with lower LBAs, assuming they have
268 greater bandwidth as is typically the case on a modern constant
269 angular velocity disk drive.
271 Use \fB1\fR for yes (default) and \fB0\fR for no.
277 \fBspa_config_path\fR (charp)
282 Default value: \fB/etc/zfs/zpool.cache\fR.
288 \fBspa_asize_inflation\fR (int)
291 Multiplication factor used to estimate actual disk consumption from the
292 size of data being written. The default value is a worst case estimate,
293 but lower values may be valid for a given pool depending on its
294 configuration. Pool administrators who understand the factors involved
295 may wish to specify a more realistic inflation factor, particularly if
296 they operate close to quota or capacity limits.
298 Default value: \fB24\fR.
304 \fBspa_load_verify_data\fR (int)
307 Whether to traverse data blocks during an "extreme rewind" (\fB-X\fR)
308 import. Use 0 to disable and 1 to enable.
310 An extreme rewind import normally performs a full traversal of all
311 blocks in the pool for verification. If this parameter is set to 0,
312 the traversal skips non-metadata blocks. It can be toggled once the
313 import has started to stop or start the traversal of non-metadata blocks.
315 Default value: \fB1\fR.
321 \fBspa_load_verify_metadata\fR (int)
324 Whether to traverse blocks during an "extreme rewind" (\fB-X\fR)
325 pool import. Use 0 to disable and 1 to enable.
327 An extreme rewind import normally performs a full traversal of all
328 blocks in the pool for verification. If this parameter is set to 0,
329 the traversal is not performed. It can be toggled once the import has
330 started to stop or start the traversal.
332 Default value: \fB1\fR.
338 \fBspa_load_verify_maxinflight\fR (int)
341 Maximum concurrent I/Os during the traversal performed during an "extreme
342 rewind" (\fB-X\fR) pool import.
344 Default value: \fB10000\fR.
350 \fBspa_slop_shift\fR (int)
353 Normally, we don't allow the last 3.2% (1/(2^spa_slop_shift)) of space
354 in the pool to be consumed. This ensures that we don't run the pool
355 completely out of space, due to unaccounted changes (e.g. to the MOS).
356 It also limits the worst-case time to allocate space. If we have
357 less than this amount of free space, most ZPL operations (e.g. write,
358 create) will return ENOSPC.
360 Default value: \fB5\fR.
366 \fBzfetch_array_rd_sz\fR (ulong)
369 If prefetching is enabled, disable prefetching for reads larger than this size.
371 Default value: \fB1,048,576\fR.
377 \fBzfetch_max_distance\fR (uint)
380 Max bytes to prefetch per stream (default 8MB).
382 Default value: \fB8,388,608\fR.
388 \fBzfetch_max_streams\fR (uint)
391 Max number of streams per zfetch (prefetch streams per file).
393 Default value: \fB8\fR.
399 \fBzfetch_min_sec_reap\fR (uint)
402 Min time before an active prefetch stream can be reclaimed
404 Default value: \fB2\fR.
410 \fBzfs_arc_dnode_limit\fR (ulong)
413 When the number of bytes consumed by dnodes in the ARC exceeds this number of
414 bytes, try to unpin some of it in response to demand for non-metadata. This
415 value acts as a ceiling to the amount of dnode metadata, and defaults to 0 which
416 indicates that a percent which is based on \fBzfs_arc_dnode_limit_percent\fR of
417 the ARC meta buffers that may be used for dnodes.
419 See also \fBzfs_arc_meta_prune\fR which serves a similar purpose but is used
420 when the amount of metadata in the ARC exceeds \fBzfs_arc_meta_limit\fR rather
421 than in response to overall demand for non-metadata.
424 Default value: \fB0\fR.
430 \fBzfs_arc_dnode_limit_percent\fR (ulong)
433 Percentage that can be consumed by dnodes of ARC meta buffers.
435 See also \fBzfs_arc_dnode_limit\fR which serves a similar purpose but has a
436 higher priority if set to nonzero value.
438 Default value: \fB10\fR.
444 \fBzfs_arc_dnode_reduce_percent\fR (ulong)
447 Percentage of ARC dnodes to try to scan in response to demand for non-metadata
448 when the number of bytes consumed by dnodes exceeds \fBzfs_arc_dnode_limit\fR.
451 Default value: \fB10% of the number of dnodes in the ARC\fR.
457 \fBzfs_arc_average_blocksize\fR (int)
460 The ARC's buffer hash table is sized based on the assumption of an average
461 block size of \fBzfs_arc_average_blocksize\fR (default 8K). This works out
462 to roughly 1MB of hash table per 1GB of physical memory with 8-byte pointers.
463 For configurations with a known larger average block size this value can be
464 increased to reduce the memory footprint.
467 Default value: \fB8192\fR.
473 \fBzfs_arc_evict_batch_limit\fR (int)
476 Number ARC headers to evict per sub-list before proceeding to another sub-list.
477 This batch-style operation prevents entire sub-lists from being evicted at once
478 but comes at a cost of additional unlocking and locking.
480 Default value: \fB10\fR.
486 \fBzfs_arc_grow_retry\fR (int)
489 After a memory pressure event the ARC will wait this many seconds before trying
492 Default value: \fB5\fR.
498 \fBzfs_arc_lotsfree_percent\fR (int)
501 Throttle I/O when free system memory drops below this percentage of total
502 system memory. Setting this value to 0 will disable the throttle.
504 Default value: \fB10\fR.
510 \fBzfs_arc_max\fR (ulong)
513 Max arc size of ARC in bytes. If set to 0 then it will consume 1/2 of system
514 RAM. This value must be at least 67108864 (64 megabytes).
516 This value can be changed dynamically with some caveats. It cannot be set back
517 to 0 while running and reducing it below the current ARC size will not cause
518 the ARC to shrink without memory pressure to induce shrinking.
520 Default value: \fB0\fR.
526 \fBzfs_arc_meta_limit\fR (ulong)
529 The maximum allowed size in bytes that meta data buffers are allowed to
530 consume in the ARC. When this limit is reached meta data buffers will
531 be reclaimed even if the overall arc_c_max has not been reached. This
532 value defaults to 0 which indicates that a percent which is based on
533 \fBzfs_arc_meta_limit_percent\fR of the ARC may be used for meta data.
535 This value my be changed dynamically except that it cannot be set back to 0
536 for a specific percent of the ARC; it must be set to an explicit value.
538 Default value: \fB0\fR.
544 \fBzfs_arc_meta_limit_percent\fR (ulong)
547 Percentage of ARC buffers that can be used for meta data.
549 See also \fBzfs_arc_meta_limit\fR which serves a similar purpose but has a
550 higher priority if set to nonzero value.
553 Default value: \fB75\fR.
559 \fBzfs_arc_meta_min\fR (ulong)
562 The minimum allowed size in bytes that meta data buffers may consume in
563 the ARC. This value defaults to 0 which disables a floor on the amount
564 of the ARC devoted meta data.
566 Default value: \fB0\fR.
572 \fBzfs_arc_meta_prune\fR (int)
575 The number of dentries and inodes to be scanned looking for entries
576 which can be dropped. This may be required when the ARC reaches the
577 \fBzfs_arc_meta_limit\fR because dentries and inodes can pin buffers
578 in the ARC. Increasing this value will cause to dentry and inode caches
579 to be pruned more aggressively. Setting this value to 0 will disable
580 pruning the inode and dentry caches.
582 Default value: \fB10,000\fR.
588 \fBzfs_arc_meta_adjust_restarts\fR (ulong)
591 The number of restart passes to make while scanning the ARC attempting
592 the free buffers in order to stay below the \fBzfs_arc_meta_limit\fR.
593 This value should not need to be tuned but is available to facilitate
594 performance analysis.
596 Default value: \fB4096\fR.
602 \fBzfs_arc_min\fR (ulong)
607 Default value: \fB100\fR.
613 \fBzfs_arc_min_prefetch_lifespan\fR (int)
616 Minimum time prefetched blocks are locked in the ARC, specified in jiffies.
617 A value of 0 will default to 1 second.
619 Default value: \fB0\fR.
625 \fBzfs_multilist_num_sublists\fR (int)
628 To allow more fine-grained locking, each ARC state contains a series
629 of lists for both data and meta data objects. Locking is performed at
630 the level of these "sub-lists". This parameters controls the number of
631 sub-lists per ARC state, and also applies to other uses of the
632 multilist data structure.
634 Default value: \fB4\fR or the number of online CPUs, whichever is greater
640 \fBzfs_arc_overflow_shift\fR (int)
643 The ARC size is considered to be overflowing if it exceeds the current
644 ARC target size (arc_c) by a threshold determined by this parameter.
645 The threshold is calculated as a fraction of arc_c using the formula
646 "arc_c >> \fBzfs_arc_overflow_shift\fR".
648 The default value of 8 causes the ARC to be considered to be overflowing
649 if it exceeds the target size by 1/256th (0.3%) of the target size.
651 When the ARC is overflowing, new buffer allocations are stalled until
652 the reclaim thread catches up and the overflow condition no longer exists.
654 Default value: \fB8\fR.
661 \fBzfs_arc_p_min_shift\fR (int)
664 arc_c shift to calc min/max arc_p
666 Default value: \fB4\fR.
672 \fBzfs_arc_p_aggressive_disable\fR (int)
675 Disable aggressive arc_p growth
677 Use \fB1\fR for yes (default) and \fB0\fR to disable.
683 \fBzfs_arc_p_dampener_disable\fR (int)
686 Disable arc_p adapt dampener
688 Use \fB1\fR for yes (default) and \fB0\fR to disable.
694 \fBzfs_arc_shrink_shift\fR (int)
697 log2(fraction of arc to reclaim)
699 Default value: \fB5\fR.
705 \fBzfs_arc_pc_percent\fR (uint)
708 Percent of pagecache to reclaim arc to
710 This tunable allows ZFS arc to play more nicely with the kernel's LRU
711 pagecache. It can guarantee that the arc size won't collapse under scanning
712 pressure on the pagecache, yet still allows arc to be reclaimed down to
713 zfs_arc_min if necessary. This value is specified as percent of pagecache
714 size (as measured by NR_FILE_PAGES) where that percent may exceed 100. This
715 only operates during memory pressure/reclaim.
717 Default value: \fB0\fR (disabled).
723 \fBzfs_arc_sys_free\fR (ulong)
726 The target number of bytes the ARC should leave as free memory on the system.
727 Defaults to the larger of 1/64 of physical memory or 512K. Setting this
728 option to a non-zero value will override the default.
730 Default value: \fB0\fR.
736 \fBzfs_autoimport_disable\fR (int)
739 Disable pool import at module load by ignoring the cache file (typically \fB/etc/zfs/zpool.cache\fR).
741 Use \fB1\fR for yes (default) and \fB0\fR for no.
747 \fBzfs_dbgmsg_enable\fR (int)
750 Internally ZFS keeps a small log to facilitate debugging. By default the log
751 is disabled, to enable it set this option to 1. The contents of the log can
752 be accessed by reading the /proc/spl/kstat/zfs/dbgmsg file. Writing 0 to
753 this proc file clears the log.
755 Default value: \fB0\fR.
761 \fBzfs_dbgmsg_maxsize\fR (int)
764 The maximum size in bytes of the internal ZFS debug log.
766 Default value: \fB4M\fR.
772 \fBzfs_dbuf_state_index\fR (int)
775 This feature is currently unused. It is normally used for controlling what
776 reporting is available under /proc/spl/kstat/zfs.
778 Default value: \fB0\fR.
784 \fBzfs_deadman_enabled\fR (int)
787 When a pool sync operation takes longer than \fBzfs_deadman_synctime_ms\fR
788 milliseconds, a "slow spa_sync" message is logged to the debug log
789 (see \fBzfs_dbgmsg_enable\fR). If \fBzfs_deadman_enabled\fR is set,
790 all pending IO operations are also checked and if any haven't completed
791 within \fBzfs_deadman_synctime_ms\fR milliseconds, a "SLOW IO" message
792 is logged to the debug log and a "delay" system event with the details of
793 the hung IO is posted.
795 Use \fB1\fR (default) to enable the slow IO check and \fB0\fR to disable.
801 \fBzfs_deadman_checktime_ms\fR (int)
804 Once a pool sync operation has taken longer than
805 \fBzfs_deadman_synctime_ms\fR milliseconds, continue to check for slow
806 operations every \fBzfs_deadman_checktime_ms\fR milliseconds.
808 Default value: \fB5,000\fR.
814 \fBzfs_deadman_synctime_ms\fR (ulong)
817 Interval in milliseconds after which the deadman is triggered and also
818 the interval after which an IO operation is considered to be "hung"
819 if \fBzfs_deadman_enabled\fR is set.
821 See \fBzfs_deadman_enabled\fR.
823 Default value: \fB1,000,000\fR.
829 \fBzfs_dedup_prefetch\fR (int)
832 Enable prefetching dedup-ed blks
834 Use \fB1\fR for yes and \fB0\fR to disable (default).
840 \fBzfs_delay_min_dirty_percent\fR (int)
843 Start to delay each transaction once there is this amount of dirty data,
844 expressed as a percentage of \fBzfs_dirty_data_max\fR.
845 This value should be >= zfs_vdev_async_write_active_max_dirty_percent.
846 See the section "ZFS TRANSACTION DELAY".
848 Default value: \fB60\fR.
854 \fBzfs_delay_scale\fR (int)
857 This controls how quickly the transaction delay approaches infinity.
858 Larger values cause longer delays for a given amount of dirty data.
860 For the smoothest delay, this value should be about 1 billion divided
861 by the maximum number of operations per second. This will smoothly
862 handle between 10x and 1/10th this number.
864 See the section "ZFS TRANSACTION DELAY".
866 Note: \fBzfs_delay_scale\fR * \fBzfs_dirty_data_max\fR must be < 2^64.
868 Default value: \fB500,000\fR.
874 \fBzfs_delete_blocks\fR (ulong)
877 This is the used to define a large file for the purposes of delete. Files
878 containing more than \fBzfs_delete_blocks\fR will be deleted asynchronously
879 while smaller files are deleted synchronously. Decreasing this value will
880 reduce the time spent in an unlink(2) system call at the expense of a longer
881 delay before the freed space is available.
883 Default value: \fB20,480\fR.
889 \fBzfs_dirty_data_max\fR (int)
892 Determines the dirty space limit in bytes. Once this limit is exceeded, new
893 writes are halted until space frees up. This parameter takes precedence
894 over \fBzfs_dirty_data_max_percent\fR.
895 See the section "ZFS TRANSACTION DELAY".
897 Default value: 10 percent of all memory, capped at \fBzfs_dirty_data_max_max\fR.
903 \fBzfs_dirty_data_max_max\fR (int)
906 Maximum allowable value of \fBzfs_dirty_data_max\fR, expressed in bytes.
907 This limit is only enforced at module load time, and will be ignored if
908 \fBzfs_dirty_data_max\fR is later changed. This parameter takes
909 precedence over \fBzfs_dirty_data_max_max_percent\fR. See the section
910 "ZFS TRANSACTION DELAY".
912 Default value: 25% of physical RAM.
918 \fBzfs_dirty_data_max_max_percent\fR (int)
921 Maximum allowable value of \fBzfs_dirty_data_max\fR, expressed as a
922 percentage of physical RAM. This limit is only enforced at module load
923 time, and will be ignored if \fBzfs_dirty_data_max\fR is later changed.
924 The parameter \fBzfs_dirty_data_max_max\fR takes precedence over this
925 one. See the section "ZFS TRANSACTION DELAY".
927 Default value: \fB25\fR.
933 \fBzfs_dirty_data_max_percent\fR (int)
936 Determines the dirty space limit, expressed as a percentage of all
937 memory. Once this limit is exceeded, new writes are halted until space frees
938 up. The parameter \fBzfs_dirty_data_max\fR takes precedence over this
939 one. See the section "ZFS TRANSACTION DELAY".
941 Default value: 10%, subject to \fBzfs_dirty_data_max_max\fR.
947 \fBzfs_dirty_data_sync\fR (int)
950 Start syncing out a transaction group if there is at least this much dirty data.
952 Default value: \fB67,108,864\fR.
958 \fBzfs_fletcher_4_impl\fR (string)
961 Select a fletcher 4 implementation.
963 Supported selectors are: \fBfastest\fR, \fBscalar\fR, \fBsse2\fR, \fBssse3\fR,
964 \fBavx2\fR, \fBavx512f\fR, and \fBaarch64_neon\fR.
965 All of the selectors except \fBfastest\fR and \fBscalar\fR require instruction
966 set extensions to be available and will only appear if ZFS detects that they are
967 present at runtime. If multiple implementations of fletcher 4 are available,
968 the \fBfastest\fR will be chosen using a micro benchmark. Selecting \fBscalar\fR
969 results in the original, CPU based calculation, being used. Selecting any option
970 other than \fBfastest\fR and \fBscalar\fR results in vector instructions from
971 the respective CPU instruction set being used.
973 Default value: \fBfastest\fR.
979 \fBzfs_free_bpobj_enabled\fR (int)
982 Enable/disable the processing of the free_bpobj object.
984 Default value: \fB1\fR.
990 \fBzfs_free_max_blocks\fR (ulong)
993 Maximum number of blocks freed in a single txg.
995 Default value: \fB100,000\fR.
1001 \fBzfs_vdev_async_read_max_active\fR (int)
1004 Maximum asynchronous read I/Os active to each device.
1005 See the section "ZFS I/O SCHEDULER".
1007 Default value: \fB3\fR.
1013 \fBzfs_vdev_async_read_min_active\fR (int)
1016 Minimum asynchronous read I/Os active to each device.
1017 See the section "ZFS I/O SCHEDULER".
1019 Default value: \fB1\fR.
1025 \fBzfs_vdev_async_write_active_max_dirty_percent\fR (int)
1028 When the pool has more than
1029 \fBzfs_vdev_async_write_active_max_dirty_percent\fR dirty data, use
1030 \fBzfs_vdev_async_write_max_active\fR to limit active async writes. If
1031 the dirty data is between min and max, the active I/O limit is linearly
1032 interpolated. See the section "ZFS I/O SCHEDULER".
1034 Default value: \fB60\fR.
1040 \fBzfs_vdev_async_write_active_min_dirty_percent\fR (int)
1043 When the pool has less than
1044 \fBzfs_vdev_async_write_active_min_dirty_percent\fR dirty data, use
1045 \fBzfs_vdev_async_write_min_active\fR to limit active async writes. If
1046 the dirty data is between min and max, the active I/O limit is linearly
1047 interpolated. See the section "ZFS I/O SCHEDULER".
1049 Default value: \fB30\fR.
1055 \fBzfs_vdev_async_write_max_active\fR (int)
1058 Maximum asynchronous write I/Os active to each device.
1059 See the section "ZFS I/O SCHEDULER".
1061 Default value: \fB10\fR.
1067 \fBzfs_vdev_async_write_min_active\fR (int)
1070 Minimum asynchronous write I/Os active to each device.
1071 See the section "ZFS I/O SCHEDULER".
1073 Lower values are associated with better latency on rotational media but poorer
1074 resilver performance. The default value of 2 was chosen as a compromise. A
1075 value of 3 has been shown to improve resilver performance further at a cost of
1076 further increasing latency.
1078 Default value: \fB2\fR.
1084 \fBzfs_vdev_max_active\fR (int)
1087 The maximum number of I/Os active to each device. Ideally, this will be >=
1088 the sum of each queue's max_active. It must be at least the sum of each
1089 queue's min_active. See the section "ZFS I/O SCHEDULER".
1091 Default value: \fB1,000\fR.
1097 \fBzfs_vdev_scrub_max_active\fR (int)
1100 Maximum scrub I/Os active to each device.
1101 See the section "ZFS I/O SCHEDULER".
1103 Default value: \fB2\fR.
1109 \fBzfs_vdev_scrub_min_active\fR (int)
1112 Minimum scrub I/Os active to each device.
1113 See the section "ZFS I/O SCHEDULER".
1115 Default value: \fB1\fR.
1121 \fBzfs_vdev_sync_read_max_active\fR (int)
1124 Maximum synchronous read I/Os active to each device.
1125 See the section "ZFS I/O SCHEDULER".
1127 Default value: \fB10\fR.
1133 \fBzfs_vdev_sync_read_min_active\fR (int)
1136 Minimum synchronous read I/Os active to each device.
1137 See the section "ZFS I/O SCHEDULER".
1139 Default value: \fB10\fR.
1145 \fBzfs_vdev_sync_write_max_active\fR (int)
1148 Maximum synchronous write I/Os active to each device.
1149 See the section "ZFS I/O SCHEDULER".
1151 Default value: \fB10\fR.
1157 \fBzfs_vdev_sync_write_min_active\fR (int)
1160 Minimum synchronous write I/Os active to each device.
1161 See the section "ZFS I/O SCHEDULER".
1163 Default value: \fB10\fR.
1169 \fBzfs_vdev_queue_depth_pct\fR (int)
1172 Maximum number of queued allocations per top-level vdev expressed as
1173 a percentage of \fBzfs_vdev_async_write_max_active\fR which allows the
1174 system to detect devices that are more capable of handling allocations
1175 and to allocate more blocks to those devices. It allows for dynamic
1176 allocation distribution when devices are imbalanced as fuller devices
1177 will tend to be slower than empty devices.
1179 See also \fBzio_dva_throttle_enabled\fR.
1181 Default value: \fB1000\fR.
1187 \fBzfs_disable_dup_eviction\fR (int)
1190 Disable duplicate buffer eviction
1192 Use \fB1\fR for yes and \fB0\fR for no (default).
1198 \fBzfs_expire_snapshot\fR (int)
1201 Seconds to expire .zfs/snapshot
1203 Default value: \fB300\fR.
1209 \fBzfs_admin_snapshot\fR (int)
1212 Allow the creation, removal, or renaming of entries in the .zfs/snapshot
1213 directory to cause the creation, destruction, or renaming of snapshots.
1214 When enabled this functionality works both locally and over NFS exports
1215 which have the 'no_root_squash' option set. This functionality is disabled
1218 Use \fB1\fR for yes and \fB0\fR for no (default).
1224 \fBzfs_flags\fR (int)
1227 Set additional debugging flags. The following flags may be bitwise-or'd
1239 Enable dprintf entries in the debug log.
1241 2 ZFS_DEBUG_DBUF_VERIFY *
1242 Enable extra dbuf verifications.
1244 4 ZFS_DEBUG_DNODE_VERIFY *
1245 Enable extra dnode verifications.
1247 8 ZFS_DEBUG_SNAPNAMES
1248 Enable snapshot name verification.
1251 Check for illegally modified ARC buffers.
1254 Enable spa_dbgmsg entries in the debug log.
1256 64 ZFS_DEBUG_ZIO_FREE
1257 Enable verification of block frees.
1259 128 ZFS_DEBUG_HISTOGRAM_VERIFY
1260 Enable extra spacemap histogram verifications.
1262 256 ZFS_DEBUG_METASLAB_VERIFY
1263 Verify space accounting on disk matches in-core range_trees.
1265 512 ZFS_DEBUG_SET_ERROR
1266 Enable SET_ERROR and dprintf entries in the debug log.
1269 * Requires debug build.
1271 Default value: \fB0\fR.
1277 \fBzfs_free_leak_on_eio\fR (int)
1280 If destroy encounters an EIO while reading metadata (e.g. indirect
1281 blocks), space referenced by the missing metadata can not be freed.
1282 Normally this causes the background destroy to become "stalled", as
1283 it is unable to make forward progress. While in this stalled state,
1284 all remaining space to free from the error-encountering filesystem is
1285 "temporarily leaked". Set this flag to cause it to ignore the EIO,
1286 permanently leak the space from indirect blocks that can not be read,
1287 and continue to free everything else that it can.
1289 The default, "stalling" behavior is useful if the storage partially
1290 fails (i.e. some but not all i/os fail), and then later recovers. In
1291 this case, we will be able to continue pool operations while it is
1292 partially failed, and when it recovers, we can continue to free the
1293 space, with no leaks. However, note that this case is actually
1296 Typically pools either (a) fail completely (but perhaps temporarily,
1297 e.g. a top-level vdev going offline), or (b) have localized,
1298 permanent errors (e.g. disk returns the wrong data due to bit flip or
1299 firmware bug). In case (a), this setting does not matter because the
1300 pool will be suspended and the sync thread will not be able to make
1301 forward progress regardless. In case (b), because the error is
1302 permanent, the best we can do is leak the minimum amount of space,
1303 which is what setting this flag will do. Therefore, it is reasonable
1304 for this flag to normally be set, but we chose the more conservative
1305 approach of not setting it, so that there is no possibility of
1306 leaking space in the "partial temporary" failure case.
1308 Default value: \fB0\fR.
1314 \fBzfs_free_min_time_ms\fR (int)
1317 During a \fBzfs destroy\fR operation using \fBfeature@async_destroy\fR a minimum
1318 of this much time will be spent working on freeing blocks per txg.
1320 Default value: \fB1,000\fR.
1326 \fBzfs_immediate_write_sz\fR (long)
1329 Largest data block to write to zil. Larger blocks will be treated as if the
1330 dataset being written to had the property setting \fBlogbias=throughput\fR.
1332 Default value: \fB32,768\fR.
1338 \fBzfs_max_recordsize\fR (int)
1341 We currently support block sizes from 512 bytes to 16MB. The benefits of
1342 larger blocks, and thus larger IO, need to be weighed against the cost of
1343 COWing a giant block to modify one byte. Additionally, very large blocks
1344 can have an impact on i/o latency, and also potentially on the memory
1345 allocator. Therefore, we do not allow the recordsize to be set larger than
1346 zfs_max_recordsize (default 1MB). Larger blocks can be created by changing
1347 this tunable, and pools with larger blocks can always be imported and used,
1348 regardless of this setting.
1350 Default value: \fB1,048,576\fR.
1356 \fBzfs_mdcomp_disable\fR (int)
1359 Disable meta data compression
1361 Use \fB1\fR for yes and \fB0\fR for no (default).
1367 \fBzfs_metaslab_fragmentation_threshold\fR (int)
1370 Allow metaslabs to keep their active state as long as their fragmentation
1371 percentage is less than or equal to this value. An active metaslab that
1372 exceeds this threshold will no longer keep its active status allowing
1373 better metaslabs to be selected.
1375 Default value: \fB70\fR.
1381 \fBzfs_mg_fragmentation_threshold\fR (int)
1384 Metaslab groups are considered eligible for allocations if their
1385 fragmentation metric (measured as a percentage) is less than or equal to
1386 this value. If a metaslab group exceeds this threshold then it will be
1387 skipped unless all metaslab groups within the metaslab class have also
1388 crossed this threshold.
1390 Default value: \fB85\fR.
1396 \fBzfs_mg_noalloc_threshold\fR (int)
1399 Defines a threshold at which metaslab groups should be eligible for
1400 allocations. The value is expressed as a percentage of free space
1401 beyond which a metaslab group is always eligible for allocations.
1402 If a metaslab group's free space is less than or equal to the
1403 threshold, the allocator will avoid allocating to that group
1404 unless all groups in the pool have reached the threshold. Once all
1405 groups have reached the threshold, all groups are allowed to accept
1406 allocations. The default value of 0 disables the feature and causes
1407 all metaslab groups to be eligible for allocations.
1409 This parameter allows to deal with pools having heavily imbalanced
1410 vdevs such as would be the case when a new vdev has been added.
1411 Setting the threshold to a non-zero percentage will stop allocations
1412 from being made to vdevs that aren't filled to the specified percentage
1413 and allow lesser filled vdevs to acquire more allocations than they
1414 otherwise would under the old \fBzfs_mg_alloc_failures\fR facility.
1416 Default value: \fB0\fR.
1422 \fBzfs_multihost_history\fR (int)
1425 Historical statistics for the last N multihost updates will be available in
1426 \fB/proc/spl/kstat/zfs/<pool>/multihost\fR
1428 Default value: \fB0\fR.
1434 \fBzfs_multihost_interval\fR (ulong)
1437 Used to control the frequency of multihost writes which are performed when the
1438 \fBmultihost\fR pool property is on. This is one factor used to determine
1439 the length of the activity check during import.
1441 The multihost write period is \fBzfs_multihost_interval / leaf-vdevs\fR milliseconds.
1442 This means that on average a multihost write will be issued for each leaf vdev every
1443 \fBzfs_multihost_interval\fR milliseconds. In practice, the observed period can
1444 vary with the I/O load and this observed value is the delay which is stored in
1447 On import the activity check waits a minimum amount of time determined by
1448 \fBzfs_multihost_interval * zfs_multihost_import_intervals\fR. The activity
1449 check time may be further extended if the value of mmp delay found in the best
1450 uberblock indicates actual multihost updates happened at longer intervals than
1451 \fBzfs_multihost_interval\fR. A minimum value of \fB100ms\fR is enforced.
1453 Default value: \fB1000\fR.
1459 \fBzfs_multihost_import_intervals\fR (uint)
1462 Used to control the duration of the activity test on import. Smaller values of
1463 \fBzfs_multihost_import_intervals\fR will reduce the import time but increase
1464 the risk of failing to detect an active pool. The total activity check time is
1465 never allowed to drop below one second. A value of 0 is ignored and treated as
1468 Default value: \fB10\fR.
1474 \fBzfs_multihost_fail_intervals\fR (uint)
1477 Controls the behavior of the pool when multihost write failures are detected.
1479 When \fBzfs_multihost_fail_intervals = 0\fR then multihost write failures are ignored.
1480 The failures will still be reported to the ZED which depending on its
1481 configuration may take action such as suspending the pool or offlining a device.
1483 When \fBzfs_multihost_fail_intervals > 0\fR then sequential multihost write failures
1484 will cause the pool to be suspended. This occurs when
1485 \fBzfs_multihost_fail_intervals * zfs_multihost_interval\fR milliseconds have
1486 passed since the last successful multihost write. This guarantees the activity test
1487 will see multihost writes if the pool is imported.
1489 Default value: \fB5\fR.
1495 \fBzfs_no_scrub_io\fR (int)
1498 Set for no scrub I/O. This results in scrubs not actually scrubbing data and
1499 simply doing a metadata crawl of the pool instead.
1501 Use \fB1\fR for yes and \fB0\fR for no (default).
1507 \fBzfs_no_scrub_prefetch\fR (int)
1510 Set to disable block prefetching for scrubs.
1512 Use \fB1\fR for yes and \fB0\fR for no (default).
1518 \fBzfs_nocacheflush\fR (int)
1521 Disable cache flush operations on disks when writing. Beware, this may cause
1522 corruption if disks re-order writes.
1524 Use \fB1\fR for yes and \fB0\fR for no (default).
1530 \fBzfs_nopwrite_enabled\fR (int)
1535 Use \fB1\fR for yes (default) and \fB0\fR to disable.
1541 \fBzfs_dmu_offset_next_sync\fR (int)
1544 Enable forcing txg sync to find holes. When enabled forces ZFS to act
1545 like prior versions when SEEK_HOLE or SEEK_DATA flags are used, which
1546 when a dnode is dirty causes txg's to be synced so that this data can be
1549 Use \fB1\fR for yes and \fB0\fR to disable (default).
1555 \fBzfs_pd_bytes_max\fR (int)
1558 The number of bytes which should be prefetched during a pool traversal
1559 (eg: \fBzfs send\fR or other data crawling operations)
1561 Default value: \fB52,428,800\fR.
1567 \fBzfs_per_txg_dirty_frees_percent \fR (ulong)
1570 Tunable to control percentage of dirtied blocks from frees in one TXG.
1571 After this threshold is crossed, additional dirty blocks from frees
1572 wait until the next TXG.
1573 A value of zero will disable this throttle.
1575 Default value: \fB30\fR and \fB0\fR to disable.
1583 \fBzfs_prefetch_disable\fR (int)
1586 This tunable disables predictive prefetch. Note that it leaves "prescient"
1587 prefetch (e.g. prefetch for zfs send) intact. Unlike predictive prefetch,
1588 prescient prefetch never issues i/os that end up not being needed, so it
1589 can't hurt performance.
1591 Use \fB1\fR for yes and \fB0\fR for no (default).
1597 \fBzfs_read_chunk_size\fR (long)
1600 Bytes to read per chunk
1602 Default value: \fB1,048,576\fR.
1608 \fBzfs_read_history\fR (int)
1611 Historical statistics for the last N reads will be available in
1612 \fB/proc/spl/kstat/zfs/<pool>/reads\fR
1614 Default value: \fB0\fR (no data is kept).
1620 \fBzfs_read_history_hits\fR (int)
1623 Include cache hits in read history
1625 Use \fB1\fR for yes and \fB0\fR for no (default).
1631 \fBzfs_recover\fR (int)
1634 Set to attempt to recover from fatal errors. This should only be used as a
1635 last resort, as it typically results in leaked space, or worse.
1637 Use \fB1\fR for yes and \fB0\fR for no (default).
1643 \fBzfs_resilver_delay\fR (int)
1646 Number of ticks to delay prior to issuing a resilver I/O operation when
1647 a non-resilver or non-scrub I/O operation has occurred within the past
1648 \fBzfs_scan_idle\fR ticks.
1650 Default value: \fB2\fR.
1656 \fBzfs_resilver_min_time_ms\fR (int)
1659 Resilvers are processed by the sync thread. While resilvering it will spend
1660 at least this much time working on a resilver between txg flushes.
1662 Default value: \fB3,000\fR.
1668 \fBzfs_scan_idle\fR (int)
1671 Idle window in clock ticks. During a scrub or a resilver, if
1672 a non-scrub or non-resilver I/O operation has occurred during this
1673 window, the next scrub or resilver operation is delayed by, respectively
1674 \fBzfs_scrub_delay\fR or \fBzfs_resilver_delay\fR ticks.
1676 Default value: \fB50\fR.
1682 \fBzfs_scan_min_time_ms\fR (int)
1685 Scrubs are processed by the sync thread. While scrubbing it will spend
1686 at least this much time working on a scrub between txg flushes.
1688 Default value: \fB1,000\fR.
1694 \fBzfs_scrub_delay\fR (int)
1697 Number of ticks to delay prior to issuing a scrub I/O operation when
1698 a non-scrub or non-resilver I/O operation has occurred within the past
1699 \fBzfs_scan_idle\fR ticks.
1701 Default value: \fB4\fR.
1707 \fBzfs_send_corrupt_data\fR (int)
1710 Allow sending of corrupt data (ignore read/checksum errors when sending data)
1712 Use \fB1\fR for yes and \fB0\fR for no (default).
1718 \fBzfs_sync_pass_deferred_free\fR (int)
1721 Flushing of data to disk is done in passes. Defer frees starting in this pass
1723 Default value: \fB2\fR.
1729 \fBzfs_sync_pass_dont_compress\fR (int)
1732 Don't compress starting in this pass
1734 Default value: \fB5\fR.
1740 \fBzfs_sync_pass_rewrite\fR (int)
1743 Rewrite new block pointers starting in this pass
1745 Default value: \fB2\fR.
1751 \fBzfs_top_maxinflight\fR (int)
1754 Max concurrent I/Os per top-level vdev (mirrors or raidz arrays) allowed during
1755 scrub or resilver operations.
1757 Default value: \fB32\fR.
1763 \fBzfs_txg_history\fR (int)
1766 Historical statistics for the last N txgs will be available in
1767 \fB/proc/spl/kstat/zfs/<pool>/txgs\fR
1769 Default value: \fB0\fR.
1775 \fBzfs_txg_timeout\fR (int)
1778 Flush dirty data to disk at least every N seconds (maximum txg duration)
1780 Default value: \fB5\fR.
1786 \fBzfs_vdev_aggregation_limit\fR (int)
1789 Max vdev I/O aggregation size
1791 Default value: \fB131,072\fR.
1797 \fBzfs_vdev_cache_bshift\fR (int)
1800 Shift size to inflate reads too
1802 Default value: \fB16\fR (effectively 65536).
1808 \fBzfs_vdev_cache_max\fR (int)
1811 Inflate reads small than this value to meet the \fBzfs_vdev_cache_bshift\fR
1814 Default value: \fB16384\fR.
1820 \fBzfs_vdev_cache_size\fR (int)
1823 Total size of the per-disk cache in bytes.
1825 Currently this feature is disabled as it has been found to not be helpful
1826 for performance and in some cases harmful.
1828 Default value: \fB0\fR.
1834 \fBzfs_vdev_mirror_rotating_inc\fR (int)
1837 A number by which the balancing algorithm increments the load calculation for
1838 the purpose of selecting the least busy mirror member when an I/O immediately
1839 follows its predecessor on rotational vdevs for the purpose of making decisions
1842 Default value: \fB0\fR.
1848 \fBzfs_vdev_mirror_rotating_seek_inc\fR (int)
1851 A number by which the balancing algorithm increments the load calculation for
1852 the purpose of selecting the least busy mirror member when an I/O lacks
1853 locality as defined by the zfs_vdev_mirror_rotating_seek_offset. I/Os within
1854 this that are not immediately following the previous I/O are incremented by
1857 Default value: \fB5\fR.
1863 \fBzfs_vdev_mirror_rotating_seek_offset\fR (int)
1866 The maximum distance for the last queued I/O in which the balancing algorithm
1867 considers an I/O to have locality.
1868 See the section "ZFS I/O SCHEDULER".
1870 Default value: \fB1048576\fR.
1876 \fBzfs_vdev_mirror_non_rotating_inc\fR (int)
1879 A number by which the balancing algorithm increments the load calculation for
1880 the purpose of selecting the least busy mirror member on non-rotational vdevs
1881 when I/Os do not immediately follow one another.
1883 Default value: \fB0\fR.
1889 \fBzfs_vdev_mirror_non_rotating_seek_inc\fR (int)
1892 A number by which the balancing algorithm increments the load calculation for
1893 the purpose of selecting the least busy mirror member when an I/O lacks
1894 locality as defined by the zfs_vdev_mirror_rotating_seek_offset. I/Os within
1895 this that are not immediately following the previous I/O are incremented by
1898 Default value: \fB1\fR.
1904 \fBzfs_vdev_read_gap_limit\fR (int)
1907 Aggregate read I/O operations if the gap on-disk between them is within this
1910 Default value: \fB32,768\fR.
1916 \fBzfs_vdev_scheduler\fR (charp)
1919 Set the Linux I/O scheduler on whole disk vdevs to this scheduler
1921 Default value: \fBnoop\fR.
1927 \fBzfs_vdev_write_gap_limit\fR (int)
1930 Aggregate write I/O over gap
1932 Default value: \fB4,096\fR.
1938 \fBzfs_vdev_raidz_impl\fR (string)
1941 Parameter for selecting raidz parity implementation to use.
1943 Options marked (always) below may be selected on module load as they are
1944 supported on all systems.
1945 The remaining options may only be set after the module is loaded, as they
1946 are available only if the implementations are compiled in and supported
1947 on the running system.
1949 Once the module is loaded, the content of
1950 /sys/module/zfs/parameters/zfs_vdev_raidz_impl will show available options
1951 with the currently selected one enclosed in [].
1952 Possible options are:
1953 fastest - (always) implementation selected using built-in benchmark
1954 original - (always) original raidz implementation
1955 scalar - (always) scalar raidz implementation
1956 sse2 - implementation using SSE2 instruction set (64bit x86 only)
1957 ssse3 - implementation using SSSE3 instruction set (64bit x86 only)
1958 avx2 - implementation using AVX2 instruction set (64bit x86 only)
1959 avx512f - implementation using AVX512F instruction set (64bit x86 only)
1960 avx512bw - implementation using AVX512F & AVX512BW instruction sets (64bit x86 only)
1961 aarch64_neon - implementation using NEON (Aarch64/64 bit ARMv8 only)
1962 aarch64_neonx2 - implementation using NEON with more unrolling (Aarch64/64 bit ARMv8 only)
1964 Default value: \fBfastest\fR.
1970 \fBzfs_zevent_cols\fR (int)
1973 When zevents are logged to the console use this as the word wrap width.
1975 Default value: \fB80\fR.
1981 \fBzfs_zevent_console\fR (int)
1984 Log events to the console
1986 Use \fB1\fR for yes and \fB0\fR for no (default).
1992 \fBzfs_zevent_len_max\fR (int)
1995 Max event queue length. A value of 0 will result in a calculated value which
1996 increases with the number of CPUs in the system (minimum 64 events). Events
1997 in the queue can be viewed with the \fBzpool events\fR command.
1999 Default value: \fB0\fR.
2005 \fBzil_replay_disable\fR (int)
2008 Disable intent logging replay. Can be disabled for recovery from corrupted
2011 Use \fB1\fR for yes and \fB0\fR for no (default).
2017 \fBzil_slog_bulk\fR (ulong)
2020 Limit SLOG write size per commit executed with synchronous priority.
2021 Any writes above that will be executed with lower (asynchronous) priority
2022 to limit potential SLOG device abuse by single active ZIL writer.
2024 Default value: \fB786,432\fR.
2030 \fBzio_delay_max\fR (int)
2033 A zevent will be logged if a ZIO operation takes more than N milliseconds to
2034 complete. Note that this is only a logging facility, not a timeout on
2037 Default value: \fB30,000\fR.
2043 \fBzio_dva_throttle_enabled\fR (int)
2046 Throttle block allocations in the ZIO pipeline. This allows for
2047 dynamic allocation distribution when devices are imbalanced.
2048 When enabled, the maximum number of pending allocations per top-level vdev
2049 is limited by \fBzfs_vdev_queue_depth_pct\fR.
2051 Default value: \fB1\fR.
2057 \fBzio_requeue_io_start_cut_in_line\fR (int)
2060 Prioritize requeued I/O
2062 Default value: \fB0\fR.
2068 \fBzio_taskq_batch_pct\fR (uint)
2071 Percentage of online CPUs (or CPU cores, etc) which will run a worker thread
2072 for IO. These workers are responsible for IO work such as compression and
2073 checksum calculations. Fractional number of CPUs will be rounded down.
2075 The default value of 75 was chosen to avoid using all CPUs which can result in
2076 latency issues and inconsistent application performance, especially when high
2077 compression is enabled.
2079 Default value: \fB75\fR.
2085 \fBzvol_inhibit_dev\fR (uint)
2088 Do not create zvol device nodes. This may slightly improve startup time on
2089 systems with a very large number of zvols.
2091 Use \fB1\fR for yes and \fB0\fR for no (default).
2097 \fBzvol_major\fR (uint)
2100 Major number for zvol block devices
2102 Default value: \fB230\fR.
2108 \fBzvol_max_discard_blocks\fR (ulong)
2111 Discard (aka TRIM) operations done on zvols will be done in batches of this
2112 many blocks, where block size is determined by the \fBvolblocksize\fR property
2115 Default value: \fB16,384\fR.
2121 \fBzvol_prefetch_bytes\fR (uint)
2124 When adding a zvol to the system prefetch \fBzvol_prefetch_bytes\fR
2125 from the start and end of the volume. Prefetching these regions
2126 of the volume is desirable because they are likely to be accessed
2127 immediately by \fBblkid(8)\fR or by the kernel scanning for a partition
2130 Default value: \fB131,072\fR.
2136 \fBzvol_request_sync\fR (uint)
2139 When processing I/O requests for a zvol submit them synchronously. This
2140 effectively limits the queue depth to 1 for each I/O submitter. When set
2141 to 0 requests are handled asynchronously by a thread pool. The number of
2142 requests which can be handled concurrently is controller by \fBzvol_threads\fR.
2144 Default value: \fB0\fR.
2150 \fBzvol_threads\fR (uint)
2153 Max number of threads which can handle zvol I/O requests concurrently.
2155 Default value: \fB32\fR.
2161 \fBzvol_volmode\fR (uint)
2164 Defines zvol block devices behaviour when \fBvolmode\fR is set to \fBdefault\fR.
2165 Valid values are \fB1\fR (full), \fB2\fR (dev) and \fB3\fR (none).
2167 Default value: \fB1\fR.
2173 \fBzfs_qat_disable\fR (int)
2176 This tunable disables qat hardware acceleration for gzip compression.
2177 It is available only if qat acceleration is compiled in and qat driver
2180 Use \fB1\fR for yes and \fB0\fR for no (default).
2183 .SH ZFS I/O SCHEDULER
2184 ZFS issues I/O operations to leaf vdevs to satisfy and complete I/Os.
2185 The I/O scheduler determines when and in what order those operations are
2186 issued. The I/O scheduler divides operations into five I/O classes
2187 prioritized in the following order: sync read, sync write, async read,
2188 async write, and scrub/resilver. Each queue defines the minimum and
2189 maximum number of concurrent operations that may be issued to the
2190 device. In addition, the device has an aggregate maximum,
2191 \fBzfs_vdev_max_active\fR. Note that the sum of the per-queue minimums
2192 must not exceed the aggregate maximum. If the sum of the per-queue
2193 maximums exceeds the aggregate maximum, then the number of active I/Os
2194 may reach \fBzfs_vdev_max_active\fR, in which case no further I/Os will
2195 be issued regardless of whether all per-queue minimums have been met.
2197 For many physical devices, throughput increases with the number of
2198 concurrent operations, but latency typically suffers. Further, physical
2199 devices typically have a limit at which more concurrent operations have no
2200 effect on throughput or can actually cause it to decrease.
2202 The scheduler selects the next operation to issue by first looking for an
2203 I/O class whose minimum has not been satisfied. Once all are satisfied and
2204 the aggregate maximum has not been hit, the scheduler looks for classes
2205 whose maximum has not been satisfied. Iteration through the I/O classes is
2206 done in the order specified above. No further operations are issued if the
2207 aggregate maximum number of concurrent operations has been hit or if there
2208 are no operations queued for an I/O class that has not hit its maximum.
2209 Every time an I/O is queued or an operation completes, the I/O scheduler
2210 looks for new operations to issue.
2212 In general, smaller max_active's will lead to lower latency of synchronous
2213 operations. Larger max_active's may lead to higher overall throughput,
2214 depending on underlying storage.
2216 The ratio of the queues' max_actives determines the balance of performance
2217 between reads, writes, and scrubs. E.g., increasing
2218 \fBzfs_vdev_scrub_max_active\fR will cause the scrub or resilver to complete
2219 more quickly, but reads and writes to have higher latency and lower throughput.
2221 All I/O classes have a fixed maximum number of outstanding operations
2222 except for the async write class. Asynchronous writes represent the data
2223 that is committed to stable storage during the syncing stage for
2224 transaction groups. Transaction groups enter the syncing state
2225 periodically so the number of queued async writes will quickly burst up
2226 and then bleed down to zero. Rather than servicing them as quickly as
2227 possible, the I/O scheduler changes the maximum number of active async
2228 write I/Os according to the amount of dirty data in the pool. Since
2229 both throughput and latency typically increase with the number of
2230 concurrent operations issued to physical devices, reducing the
2231 burstiness in the number of concurrent operations also stabilizes the
2232 response time of operations from other -- and in particular synchronous
2233 -- queues. In broad strokes, the I/O scheduler will issue more
2234 concurrent operations from the async write queue as there's more dirty
2239 The number of concurrent operations issued for the async write I/O class
2240 follows a piece-wise linear function defined by a few adjustable points.
2243 | o---------| <-- zfs_vdev_async_write_max_active
2250 |-------o | | <-- zfs_vdev_async_write_min_active
2251 0|_______^______|_________|
2252 0% | | 100% of zfs_dirty_data_max
2254 | `-- zfs_vdev_async_write_active_max_dirty_percent
2255 `--------- zfs_vdev_async_write_active_min_dirty_percent
2258 Until the amount of dirty data exceeds a minimum percentage of the dirty
2259 data allowed in the pool, the I/O scheduler will limit the number of
2260 concurrent operations to the minimum. As that threshold is crossed, the
2261 number of concurrent operations issued increases linearly to the maximum at
2262 the specified maximum percentage of the dirty data allowed in the pool.
2264 Ideally, the amount of dirty data on a busy pool will stay in the sloped
2265 part of the function between \fBzfs_vdev_async_write_active_min_dirty_percent\fR
2266 and \fBzfs_vdev_async_write_active_max_dirty_percent\fR. If it exceeds the
2267 maximum percentage, this indicates that the rate of incoming data is
2268 greater than the rate that the backend storage can handle. In this case, we
2269 must further throttle incoming writes, as described in the next section.
2271 .SH ZFS TRANSACTION DELAY
2272 We delay transactions when we've determined that the backend storage
2273 isn't able to accommodate the rate of incoming writes.
2275 If there is already a transaction waiting, we delay relative to when
2276 that transaction will finish waiting. This way the calculated delay time
2277 is independent of the number of threads concurrently executing
2280 If we are the only waiter, wait relative to when the transaction
2281 started, rather than the current time. This credits the transaction for
2282 "time already served", e.g. reading indirect blocks.
2284 The minimum time for a transaction to take is calculated as:
2286 min_time = zfs_delay_scale * (dirty - min) / (max - dirty)
2287 min_time is then capped at 100 milliseconds.
2290 The delay has two degrees of freedom that can be adjusted via tunables. The
2291 percentage of dirty data at which we start to delay is defined by
2292 \fBzfs_delay_min_dirty_percent\fR. This should typically be at or above
2293 \fBzfs_vdev_async_write_active_max_dirty_percent\fR so that we only start to
2294 delay after writing at full speed has failed to keep up with the incoming write
2295 rate. The scale of the curve is defined by \fBzfs_delay_scale\fR. Roughly speaking,
2296 this variable determines the amount of delay at the midpoint of the curve.
2300 10ms +-------------------------------------------------------------*+
2316 2ms + (midpoint) * +
2319 | zfs_delay_scale ----------> ******** |
2320 0 +-------------------------------------*********----------------+
2321 0% <- zfs_dirty_data_max -> 100%
2324 Note that since the delay is added to the outstanding time remaining on the
2325 most recent transaction, the delay is effectively the inverse of IOPS.
2326 Here the midpoint of 500us translates to 2000 IOPS. The shape of the curve
2327 was chosen such that small changes in the amount of accumulated dirty data
2328 in the first 3/4 of the curve yield relatively small differences in the
2331 The effects can be easier to understand when the amount of delay is
2332 represented on a log scale:
2336 100ms +-------------------------------------------------------------++
2345 + zfs_delay_scale ----------> ***** +
2356 +--------------------------------------------------------------+
2357 0% <- zfs_dirty_data_max -> 100%
2360 Note here that only as the amount of dirty data approaches its limit does
2361 the delay start to increase rapidly. The goal of a properly tuned system
2362 should be to keep the amount of dirty data out of that range by first
2363 ensuring that the appropriate limits are set for the I/O scheduler to reach
2364 optimal throughput on the backend storage, and then by changing the value
2365 of \fBzfs_delay_scale\fR to increase the steepness of the curve.
projects / zfs / blob