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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 \fBl2arc_feed_again\fR (int)
33 Turbo L2ARC warm-up. When the L2ARC is cold the fill interval will be set as
36 Use \fB1\fR for yes (default) and \fB0\fR to disable.
42 \fBl2arc_feed_min_ms\fR (ulong)
45 Min feed interval in milliseconds. Requires \fBl2arc_feed_again=1\fR and only
46 applicable in related situations.
48 Default value: \fB200\fR.
54 \fBl2arc_feed_secs\fR (ulong)
57 Seconds between L2ARC writing
59 Default value: \fB1\fR.
65 \fBl2arc_headroom\fR (ulong)
68 How far through the ARC lists to search for L2ARC cacheable content, expressed
69 as a multiplier of \fBl2arc_write_max\fR
71 Default value: \fB2\fR.
77 \fBl2arc_headroom_boost\fR (ulong)
80 Scales \fBl2arc_headroom\fR by this percentage when L2ARC contents are being
81 successfully compressed before writing. A value of 100 disables this feature.
83 Default value: \fB200\fR.
89 \fBl2arc_max_block_size\fR (ulong)
92 The maximum block size which may be written to an L2ARC device, after
93 compression and other factors. This setting is used to prevent a small
94 number of large blocks from pushing a larger number of small blocks out
97 Default value: \fB16,777,216\fR.
103 \fBl2arc_nocompress\fR (int)
106 Skip compressing L2ARC buffers
108 Use \fB1\fR for yes and \fB0\fR for no (default).
114 \fBl2arc_noprefetch\fR (int)
117 Do not write buffers to L2ARC if they were prefetched but not used by
120 Use \fB1\fR for yes (default) and \fB0\fR to disable.
126 \fBl2arc_norw\fR (int)
129 No reads during writes
131 Use \fB1\fR for yes and \fB0\fR for no (default).
137 \fBl2arc_write_boost\fR (ulong)
140 Cold L2ARC devices will have \fBl2arc_write_nax\fR increased by this amount
141 while they remain cold.
143 Default value: \fB8,388,608\fR.
149 \fBl2arc_write_max\fR (ulong)
152 Max write bytes per interval
154 Default value: \fB8,388,608\fR.
160 \fBmetaslab_aliquot\fR (ulong)
163 Metaslab granularity, in bytes. This is roughly similar to what would be
164 referred to as the "stripe size" in traditional RAID arrays. In normal
165 operation, ZFS will try to write this amount of data to a top-level vdev
166 before moving on to the next one.
168 Default value: \fB524,288\fR.
174 \fBmetaslab_bias_enabled\fR (int)
177 Enable metaslab group biasing based on its vdev's over- or under-utilization
178 relative to the pool.
180 Use \fB1\fR for yes (default) and \fB0\fR for no.
186 \fBmetaslab_debug_load\fR (int)
189 Load all metaslabs during pool import.
191 Use \fB1\fR for yes and \fB0\fR for no (default).
197 \fBmetaslab_debug_unload\fR (int)
200 Prevent metaslabs from being unloaded.
202 Use \fB1\fR for yes and \fB0\fR for no (default).
208 \fBmetaslab_fragmentation_factor_enabled\fR (int)
211 Enable use of the fragmentation metric in computing metaslab weights.
213 Use \fB1\fR for yes (default) and \fB0\fR for no.
219 \fBmetaslabs_per_vdev\fR (int)
222 When a vdev is added, it will be divided into approximately (but no more than) this number of metaslabs.
224 Default value: \fB200\fR.
230 \fBmetaslab_preload_enabled\fR (int)
233 Enable metaslab group preloading.
235 Use \fB1\fR for yes (default) and \fB0\fR for no.
241 \fBmetaslab_lba_weighting_enabled\fR (int)
244 Give more weight to metaslabs with lower LBAs, assuming they have
245 greater bandwidth as is typically the case on a modern constant
246 angular velocity disk drive.
248 Use \fB1\fR for yes (default) and \fB0\fR for no.
254 \fBspa_config_path\fR (charp)
259 Default value: \fB/etc/zfs/zpool.cache\fR.
265 \fBspa_asize_inflation\fR (int)
268 Multiplication factor used to estimate actual disk consumption from the
269 size of data being written. The default value is a worst case estimate,
270 but lower values may be valid for a given pool depending on its
271 configuration. Pool administrators who understand the factors involved
272 may wish to specify a more realistic inflation factor, particularly if
273 they operate close to quota or capacity limits.
275 Default value: \fB24\fR.
281 \fBspa_load_verify_data\fR (int)
284 Whether to traverse data blocks during an "extreme rewind" (\fB-X\fR)
285 import. Use 0 to disable and 1 to enable.
287 An extreme rewind import normally performs a full traversal of all
288 blocks in the pool for verification. If this parameter is set to 0,
289 the traversal skips non-metadata blocks. It can be toggled once the
290 import has started to stop or start the traversal of non-metadata blocks.
292 Default value: \fB1\fR.
298 \fBspa_load_verify_metadata\fR (int)
301 Whether to traverse blocks during an "extreme rewind" (\fB-X\fR)
302 pool import. Use 0 to disable and 1 to enable.
304 An extreme rewind import normally performs a full traversal of all
305 blocks in the pool for verification. If this parameter is set to 0,
306 the traversal is not performed. It can be toggled once the import has
307 started to stop or start the traversal.
309 Default value: \fB1\fR.
315 \fBspa_load_verify_maxinflight\fR (int)
318 Maximum concurrent I/Os during the traversal performed during an "extreme
319 rewind" (\fB-X\fR) pool import.
321 Default value: \fB10000\fR.
327 \fBspa_slop_shift\fR (int)
330 Normally, we don't allow the last 3.2% (1/(2^spa_slop_shift)) of space
331 in the pool to be consumed. This ensures that we don't run the pool
332 completely out of space, due to unaccounted changes (e.g. to the MOS).
333 It also limits the worst-case time to allocate space. If we have
334 less than this amount of free space, most ZPL operations (e.g. write,
335 create) will return ENOSPC.
337 Default value: \fB5\fR.
343 \fBzfetch_array_rd_sz\fR (ulong)
346 If prefetching is enabled, disable prefetching for reads larger than this size.
348 Default value: \fB1,048,576\fR.
354 \fBzfetch_max_distance\fR (uint)
357 Max bytes to prefetch per stream (default 8MB).
359 Default value: \fB8,388,608\fR.
365 \fBzfetch_max_streams\fR (uint)
368 Max number of streams per zfetch (prefetch streams per file).
370 Default value: \fB8\fR.
376 \fBzfetch_min_sec_reap\fR (uint)
379 Min time before an active prefetch stream can be reclaimed
381 Default value: \fB2\fR.
387 \fBzfs_arc_average_blocksize\fR (int)
390 The ARC's buffer hash table is sized based on the assumption of an average
391 block size of \fBzfs_arc_average_blocksize\fR (default 8K). This works out
392 to roughly 1MB of hash table per 1GB of physical memory with 8-byte pointers.
393 For configurations with a known larger average block size this value can be
394 increased to reduce the memory footprint.
397 Default value: \fB8192\fR.
403 \fBzfs_arc_evict_batch_limit\fR (int)
406 Number ARC headers to evict per sub-list before proceeding to another sub-list.
407 This batch-style operation prevents entire sub-lists from being evicted at once
408 but comes at a cost of additional unlocking and locking.
410 Default value: \fB10\fR.
416 \fBzfs_arc_grow_retry\fR (int)
419 After a memory pressure event the ARC will wait this many seconds before trying
422 Default value: \fB5\fR.
428 \fBzfs_arc_lotsfree_percent\fR (int)
431 Throttle I/O when free system memory drops below this percentage of total
432 system memory. Setting this value to 0 will disable the throttle.
434 Default value: \fB10\fR.
440 \fBzfs_arc_max\fR (ulong)
443 Max arc size of ARC in bytes. If set to 0 then it will consume 1/2 of system
444 RAM. This value must be at least 67108864 (64 megabytes).
446 This value can be changed dynamically with some caveats. It cannot be set back
447 to 0 while running and reducing it below the current ARC size will not cause
448 the ARC to shrink without memory pressure to induce shrinking.
450 Default value: \fB0\fR.
456 \fBzfs_arc_meta_limit\fR (ulong)
459 The maximum allowed size in bytes that meta data buffers are allowed to
460 consume in the ARC. When this limit is reached meta data buffers will
461 be reclaimed even if the overall arc_c_max has not been reached. This
462 value defaults to 0 which indicates that 3/4 of the ARC may be used
465 This value my be changed dynamically except that it cannot be set back to 0
466 for 3/4 of the ARC; it must be set to an explicit value.
468 Default value: \fB0\fR.
474 \fBzfs_arc_meta_min\fR (ulong)
477 The minimum allowed size in bytes that meta data buffers may consume in
478 the ARC. This value defaults to 0 which disables a floor on the amount
479 of the ARC devoted meta data.
481 Default value: \fB0\fR.
487 \fBzfs_arc_meta_prune\fR (int)
490 The number of dentries and inodes to be scanned looking for entries
491 which can be dropped. This may be required when the ARC reaches the
492 \fBzfs_arc_meta_limit\fR because dentries and inodes can pin buffers
493 in the ARC. Increasing this value will cause to dentry and inode caches
494 to be pruned more aggressively. Setting this value to 0 will disable
495 pruning the inode and dentry caches.
497 Default value: \fB10,000\fR.
503 \fBzfs_arc_meta_adjust_restarts\fR (ulong)
506 The number of restart passes to make while scanning the ARC attempting
507 the free buffers in order to stay below the \fBzfs_arc_meta_limit\fR.
508 This value should not need to be tuned but is available to facilitate
509 performance analysis.
511 Default value: \fB4096\fR.
517 \fBzfs_arc_min\fR (ulong)
522 Default value: \fB100\fR.
528 \fBzfs_arc_min_prefetch_lifespan\fR (int)
531 Minimum time prefetched blocks are locked in the ARC, specified in jiffies.
532 A value of 0 will default to 1 second.
534 Default value: \fB0\fR.
540 \fBzfs_arc_num_sublists_per_state\fR (int)
543 To allow more fine-grained locking, each ARC state contains a series
544 of lists for both data and meta data objects. Locking is performed at
545 the level of these "sub-lists". This parameters controls the number of
546 sub-lists per ARC state.
548 Default value: \fR1\fB or the number of online CPUs, whichever is greater
554 \fBzfs_arc_overflow_shift\fR (int)
557 The ARC size is considered to be overflowing if it exceeds the current
558 ARC target size (arc_c) by a threshold determined by this parameter.
559 The threshold is calculated as a fraction of arc_c using the formula
560 "arc_c >> \fBzfs_arc_overflow_shift\fR".
562 The default value of 8 causes the ARC to be considered to be overflowing
563 if it exceeds the target size by 1/256th (0.3%) of the target size.
565 When the ARC is overflowing, new buffer allocations are stalled until
566 the reclaim thread catches up and the overflow condition no longer exists.
568 Default value: \fB8\fR.
575 \fBzfs_arc_p_min_shift\fR (int)
578 arc_c shift to calc min/max arc_p
580 Default value: \fB4\fR.
586 \fBzfs_arc_p_aggressive_disable\fR (int)
589 Disable aggressive arc_p growth
591 Use \fB1\fR for yes (default) and \fB0\fR to disable.
597 \fBzfs_arc_p_dampener_disable\fR (int)
600 Disable arc_p adapt dampener
602 Use \fB1\fR for yes (default) and \fB0\fR to disable.
608 \fBzfs_arc_shrink_shift\fR (int)
611 log2(fraction of arc to reclaim)
613 Default value: \fB5\fR.
619 \fBzfs_arc_sys_free\fR (ulong)
622 The target number of bytes the ARC should leave as free memory on the system.
623 Defaults to the larger of 1/64 of physical memory or 512K. Setting this
624 option to a non-zero value will override the default.
626 Default value: \fB0\fR.
632 \fBzfs_autoimport_disable\fR (int)
635 Disable pool import at module load by ignoring the cache file (typically \fB/etc/zfs/zpool.cache\fR).
637 Use \fB1\fR for yes (default) and \fB0\fR for no.
643 \fBzfs_dbgmsg_enable\fR (int)
646 Internally ZFS keeps a small log to facilitate debugging. By default the log
647 is disabled, to enable it set this option to 1. The contents of the log can
648 be accessed by reading the /proc/spl/kstat/zfs/dbgmsg file. Writing 0 to
649 this proc file clears the log.
651 Default value: \fB0\fR.
657 \fBzfs_dbgmsg_maxsize\fR (int)
660 The maximum size in bytes of the internal ZFS debug log.
662 Default value: \fB4M\fR.
668 \fBzfs_dbuf_state_index\fR (int)
671 This feature is currently unused. It is normally used for controlling what
672 reporting is available under /proc/spl/kstat/zfs.
674 Default value: \fB0\fR.
680 \fBzfs_deadman_enabled\fR (int)
683 Enable deadman timer. See description below.
685 Use \fB1\fR for yes (default) and \fB0\fR to disable.
691 \fBzfs_deadman_synctime_ms\fR (ulong)
694 Expiration time in milliseconds. This value has two meanings. First it is
695 used to determine when the spa_deadman() logic should fire. By default the
696 spa_deadman() will fire if spa_sync() has not completed in 1000 seconds.
697 Secondly, the value determines if an I/O is considered "hung". Any I/O that
698 has not completed in zfs_deadman_synctime_ms is considered "hung" resulting
699 in a zevent being logged.
701 Default value: \fB1,000,000\fR.
707 \fBzfs_dedup_prefetch\fR (int)
710 Enable prefetching dedup-ed blks
712 Use \fB1\fR for yes and \fB0\fR to disable (default).
718 \fBzfs_delay_min_dirty_percent\fR (int)
721 Start to delay each transaction once there is this amount of dirty data,
722 expressed as a percentage of \fBzfs_dirty_data_max\fR.
723 This value should be >= zfs_vdev_async_write_active_max_dirty_percent.
724 See the section "ZFS TRANSACTION DELAY".
726 Default value: \fB60\fR.
732 \fBzfs_delay_scale\fR (int)
735 This controls how quickly the transaction delay approaches infinity.
736 Larger values cause longer delays for a given amount of dirty data.
738 For the smoothest delay, this value should be about 1 billion divided
739 by the maximum number of operations per second. This will smoothly
740 handle between 10x and 1/10th this number.
742 See the section "ZFS TRANSACTION DELAY".
744 Note: \fBzfs_delay_scale\fR * \fBzfs_dirty_data_max\fR must be < 2^64.
746 Default value: \fB500,000\fR.
752 \fBzfs_delete_blocks\fR (ulong)
755 This is the used to define a large file for the purposes of delete. Files
756 containing more than \fBzfs_delete_blocks\fR will be deleted asynchronously
757 while smaller files are deleted synchronously. Decreasing this value will
758 reduce the time spent in an unlink(2) system call at the expense of a longer
759 delay before the freed space is available.
761 Default value: \fB20,480\fR.
767 \fBzfs_dirty_data_max\fR (int)
770 Determines the dirty space limit in bytes. Once this limit is exceeded, new
771 writes are halted until space frees up. This parameter takes precedence
772 over \fBzfs_dirty_data_max_percent\fR.
773 See the section "ZFS TRANSACTION DELAY".
775 Default value: 10 percent of all memory, capped at \fBzfs_dirty_data_max_max\fR.
781 \fBzfs_dirty_data_max_max\fR (int)
784 Maximum allowable value of \fBzfs_dirty_data_max\fR, expressed in bytes.
785 This limit is only enforced at module load time, and will be ignored if
786 \fBzfs_dirty_data_max\fR is later changed. This parameter takes
787 precedence over \fBzfs_dirty_data_max_max_percent\fR. See the section
788 "ZFS TRANSACTION DELAY".
790 Default value: 25% of physical RAM.
796 \fBzfs_dirty_data_max_max_percent\fR (int)
799 Maximum allowable value of \fBzfs_dirty_data_max\fR, expressed as a
800 percentage of physical RAM. This limit is only enforced at module load
801 time, and will be ignored if \fBzfs_dirty_data_max\fR is later changed.
802 The parameter \fBzfs_dirty_data_max_max\fR takes precedence over this
803 one. See the section "ZFS TRANSACTION DELAY".
805 Default value: \fN25\fR.
811 \fBzfs_dirty_data_max_percent\fR (int)
814 Determines the dirty space limit, expressed as a percentage of all
815 memory. Once this limit is exceeded, new writes are halted until space frees
816 up. The parameter \fBzfs_dirty_data_max\fR takes precedence over this
817 one. See the section "ZFS TRANSACTION DELAY".
819 Default value: 10%, subject to \fBzfs_dirty_data_max_max\fR.
825 \fBzfs_dirty_data_sync\fR (int)
828 Start syncing out a transaction group if there is at least this much dirty data.
830 Default value: \fB67,108,864\fR.
836 \fBzfs_fletcher_4_impl\fR (string)
839 Select a fletcher 4 implementation.
841 Supported selectors are: \fBfastest\fR, \fBscalar\fR, and \fBavx2\fR when
842 AVX2 is supported by the processor. If multiple implementations of fletcher 4
843 are available the \fBfastest\fR will be chosen using a micro benchmark.
844 Selecting \fBscalar\fR results in the original CPU based calculation being
845 used, \fBavx2\fR uses the AVX2 vector instructions to compute a fletcher 4.
847 Default value: \fBfastest\fR.
853 \fBzfs_free_bpobj_enabled\fR (int)
856 Enable/disable the processing of the free_bpobj object.
858 Default value: \fB1\fR.
864 \fBzfs_free_max_blocks\fR (ulong)
867 Maximum number of blocks freed in a single txg.
869 Default value: \fB100,000\fR.
875 \fBzfs_vdev_async_read_max_active\fR (int)
878 Maximum asynchronous read I/Os active to each device.
879 See the section "ZFS I/O SCHEDULER".
881 Default value: \fB3\fR.
887 \fBzfs_vdev_async_read_min_active\fR (int)
890 Minimum asynchronous read I/Os active to each device.
891 See the section "ZFS I/O SCHEDULER".
893 Default value: \fB1\fR.
899 \fBzfs_vdev_async_write_active_max_dirty_percent\fR (int)
902 When the pool has more than
903 \fBzfs_vdev_async_write_active_max_dirty_percent\fR dirty data, use
904 \fBzfs_vdev_async_write_max_active\fR to limit active async writes. If
905 the dirty data is between min and max, the active I/O limit is linearly
906 interpolated. See the section "ZFS I/O SCHEDULER".
908 Default value: \fB60\fR.
914 \fBzfs_vdev_async_write_active_min_dirty_percent\fR (int)
917 When the pool has less than
918 \fBzfs_vdev_async_write_active_min_dirty_percent\fR dirty data, use
919 \fBzfs_vdev_async_write_min_active\fR to limit active async writes. If
920 the dirty data is between min and max, the active I/O limit is linearly
921 interpolated. See the section "ZFS I/O SCHEDULER".
923 Default value: \fB30\fR.
929 \fBzfs_vdev_async_write_max_active\fR (int)
932 Maximum asynchronous write I/Os active to each device.
933 See the section "ZFS I/O SCHEDULER".
935 Default value: \fB10\fR.
941 \fBzfs_vdev_async_write_min_active\fR (int)
944 Minimum asynchronous write I/Os active to each device.
945 See the section "ZFS I/O SCHEDULER".
947 Default value: \fB1\fR.
953 \fBzfs_vdev_max_active\fR (int)
956 The maximum number of I/Os active to each device. Ideally, this will be >=
957 the sum of each queue's max_active. It must be at least the sum of each
958 queue's min_active. See the section "ZFS I/O SCHEDULER".
960 Default value: \fB1,000\fR.
966 \fBzfs_vdev_scrub_max_active\fR (int)
969 Maximum scrub I/Os active to each device.
970 See the section "ZFS I/O SCHEDULER".
972 Default value: \fB2\fR.
978 \fBzfs_vdev_scrub_min_active\fR (int)
981 Minimum scrub I/Os active to each device.
982 See the section "ZFS I/O SCHEDULER".
984 Default value: \fB1\fR.
990 \fBzfs_vdev_sync_read_max_active\fR (int)
993 Maximum synchronous read I/Os active to each device.
994 See the section "ZFS I/O SCHEDULER".
996 Default value: \fB10\fR.
1002 \fBzfs_vdev_sync_read_min_active\fR (int)
1005 Minimum synchronous read I/Os active to each device.
1006 See the section "ZFS I/O SCHEDULER".
1008 Default value: \fB10\fR.
1014 \fBzfs_vdev_sync_write_max_active\fR (int)
1017 Maximum synchronous write I/Os active to each device.
1018 See the section "ZFS I/O SCHEDULER".
1020 Default value: \fB10\fR.
1026 \fBzfs_vdev_sync_write_min_active\fR (int)
1029 Minimum synchronous write I/Os active to each device.
1030 See the section "ZFS I/O SCHEDULER".
1032 Default value: \fB10\fR.
1038 \fBzfs_disable_dup_eviction\fR (int)
1041 Disable duplicate buffer eviction
1043 Use \fB1\fR for yes and \fB0\fR for no (default).
1049 \fBzfs_expire_snapshot\fR (int)
1052 Seconds to expire .zfs/snapshot
1054 Default value: \fB300\fR.
1060 \fBzfs_admin_snapshot\fR (int)
1063 Allow the creation, removal, or renaming of entries in the .zfs/snapshot
1064 directory to cause the creation, destruction, or renaming of snapshots.
1065 When enabled this functionality works both locally and over NFS exports
1066 which have the 'no_root_squash' option set. This functionality is disabled
1069 Use \fB1\fR for yes and \fB0\fR for no (default).
1075 \fBzfs_flags\fR (int)
1078 Set additional debugging flags. The following flags may be bitwise-or'd
1090 Enable dprintf entries in the debug log.
1092 2 ZFS_DEBUG_DBUF_VERIFY *
1093 Enable extra dbuf verifications.
1095 4 ZFS_DEBUG_DNODE_VERIFY *
1096 Enable extra dnode verifications.
1098 8 ZFS_DEBUG_SNAPNAMES
1099 Enable snapshot name verification.
1102 Check for illegally modified ARC buffers.
1105 Enable spa_dbgmsg entries in the debug log.
1107 64 ZFS_DEBUG_ZIO_FREE
1108 Enable verification of block frees.
1110 128 ZFS_DEBUG_HISTOGRAM_VERIFY
1111 Enable extra spacemap histogram verifications.
1114 * Requires debug build.
1116 Default value: \fB0\fR.
1122 \fBzfs_free_leak_on_eio\fR (int)
1125 If destroy encounters an EIO while reading metadata (e.g. indirect
1126 blocks), space referenced by the missing metadata can not be freed.
1127 Normally this causes the background destroy to become "stalled", as
1128 it is unable to make forward progress. While in this stalled state,
1129 all remaining space to free from the error-encountering filesystem is
1130 "temporarily leaked". Set this flag to cause it to ignore the EIO,
1131 permanently leak the space from indirect blocks that can not be read,
1132 and continue to free everything else that it can.
1134 The default, "stalling" behavior is useful if the storage partially
1135 fails (i.e. some but not all i/os fail), and then later recovers. In
1136 this case, we will be able to continue pool operations while it is
1137 partially failed, and when it recovers, we can continue to free the
1138 space, with no leaks. However, note that this case is actually
1141 Typically pools either (a) fail completely (but perhaps temporarily,
1142 e.g. a top-level vdev going offline), or (b) have localized,
1143 permanent errors (e.g. disk returns the wrong data due to bit flip or
1144 firmware bug). In case (a), this setting does not matter because the
1145 pool will be suspended and the sync thread will not be able to make
1146 forward progress regardless. In case (b), because the error is
1147 permanent, the best we can do is leak the minimum amount of space,
1148 which is what setting this flag will do. Therefore, it is reasonable
1149 for this flag to normally be set, but we chose the more conservative
1150 approach of not setting it, so that there is no possibility of
1151 leaking space in the "partial temporary" failure case.
1153 Default value: \fB0\fR.
1159 \fBzfs_free_min_time_ms\fR (int)
1162 During a \fRzfs destroy\fB operation using \fRfeature@async_destroy\fB a minimum
1163 of this much time will be spent working on freeing blocks per txg.
1165 Default value: \fB1,000\fR.
1171 \fBzfs_immediate_write_sz\fR (long)
1174 Largest data block to write to zil. Larger blocks will be treated as if the
1175 dataset being written to had the property setting \fRlogbias=throughput\fB.
1177 Default value: \fB32,768\fR.
1183 \fBzfs_max_recordsize\fR (int)
1186 We currently support block sizes from 512 bytes to 16MB. The benefits of
1187 larger blocks, and thus larger IO, need to be weighed against the cost of
1188 COWing a giant block to modify one byte. Additionally, very large blocks
1189 can have an impact on i/o latency, and also potentially on the memory
1190 allocator. Therefore, we do not allow the recordsize to be set larger than
1191 zfs_max_recordsize (default 1MB). Larger blocks can be created by changing
1192 this tunable, and pools with larger blocks can always be imported and used,
1193 regardless of this setting.
1195 Default value: \fB1,048,576\fR.
1201 \fBzfs_mdcomp_disable\fR (int)
1204 Disable meta data compression
1206 Use \fB1\fR for yes and \fB0\fR for no (default).
1212 \fBzfs_metaslab_fragmentation_threshold\fR (int)
1215 Allow metaslabs to keep their active state as long as their fragmentation
1216 percentage is less than or equal to this value. An active metaslab that
1217 exceeds this threshold will no longer keep its active status allowing
1218 better metaslabs to be selected.
1220 Default value: \fB70\fR.
1226 \fBzfs_mg_fragmentation_threshold\fR (int)
1229 Metaslab groups are considered eligible for allocations if their
1230 fragmentation metric (measured as a percentage) is less than or equal to
1231 this value. If a metaslab group exceeds this threshold then it will be
1232 skipped unless all metaslab groups within the metaslab class have also
1233 crossed this threshold.
1235 Default value: \fB85\fR.
1241 \fBzfs_mg_noalloc_threshold\fR (int)
1244 Defines a threshold at which metaslab groups should be eligible for
1245 allocations. The value is expressed as a percentage of free space
1246 beyond which a metaslab group is always eligible for allocations.
1247 If a metaslab group's free space is less than or equal to the
1248 threshold, the allocator will avoid allocating to that group
1249 unless all groups in the pool have reached the threshold. Once all
1250 groups have reached the threshold, all groups are allowed to accept
1251 allocations. The default value of 0 disables the feature and causes
1252 all metaslab groups to be eligible for allocations.
1254 This parameter allows to deal with pools having heavily imbalanced
1255 vdevs such as would be the case when a new vdev has been added.
1256 Setting the threshold to a non-zero percentage will stop allocations
1257 from being made to vdevs that aren't filled to the specified percentage
1258 and allow lesser filled vdevs to acquire more allocations than they
1259 otherwise would under the old \fBzfs_mg_alloc_failures\fR facility.
1261 Default value: \fB0\fR.
1267 \fBzfs_no_scrub_io\fR (int)
1270 Set for no scrub I/O. This results in scrubs not actually scrubbing data and
1271 simply doing a metadata crawl of the pool instead.
1273 Use \fB1\fR for yes and \fB0\fR for no (default).
1279 \fBzfs_no_scrub_prefetch\fR (int)
1282 Set to disable block prefetching for scrubs.
1284 Use \fB1\fR for yes and \fB0\fR for no (default).
1290 \fBzfs_nocacheflush\fR (int)
1293 Disable cache flush operations on disks when writing. Beware, this may cause
1294 corruption if disks re-order writes.
1296 Use \fB1\fR for yes and \fB0\fR for no (default).
1302 \fBzfs_nopwrite_enabled\fR (int)
1307 Use \fB1\fR for yes (default) and \fB0\fR to disable.
1313 \fBzfs_pd_bytes_max\fR (int)
1316 The number of bytes which should be prefetched during a pool traversal
1317 (eg: \fRzfs send\fB or other data crawling operations)
1319 Default value: \fB52,428,800\fR.
1325 \fBzfs_prefetch_disable\fR (int)
1328 This tunable disables predictive prefetch. Note that it leaves "prescient"
1329 prefetch (e.g. prefetch for zfs send) intact. Unlike predictive prefetch,
1330 prescient prefetch never issues i/os that end up not being needed, so it
1331 can't hurt performance.
1333 Use \fB1\fR for yes and \fB0\fR for no (default).
1339 \fBzfs_read_chunk_size\fR (long)
1342 Bytes to read per chunk
1344 Default value: \fB1,048,576\fR.
1350 \fBzfs_read_history\fR (int)
1353 Historic statistics for the last N reads will be available in
1354 \fR/proc/spl/kstat/zfs/POOLNAME/reads\fB
1356 Default value: \fB0\fR (no data is kept).
1362 \fBzfs_read_history_hits\fR (int)
1365 Include cache hits in read history
1367 Use \fB1\fR for yes and \fB0\fR for no (default).
1373 \fBzfs_recover\fR (int)
1376 Set to attempt to recover from fatal errors. This should only be used as a
1377 last resort, as it typically results in leaked space, or worse.
1379 Use \fB1\fR for yes and \fB0\fR for no (default).
1385 \fBzfs_resilver_delay\fR (int)
1388 Number of ticks to delay prior to issuing a resilver I/O operation when
1389 a non-resilver or non-scrub I/O operation has occurred within the past
1390 \fBzfs_scan_idle\fR ticks.
1392 Default value: \fB2\fR.
1398 \fBzfs_resilver_min_time_ms\fR (int)
1401 Resilvers are processed by the sync thread. While resilvering it will spend
1402 at least this much time working on a resilver between txg flushes.
1404 Default value: \fB3,000\fR.
1410 \fBzfs_scan_idle\fR (int)
1413 Idle window in clock ticks. During a scrub or a resilver, if
1414 a non-scrub or non-resilver I/O operation has occurred during this
1415 window, the next scrub or resilver operation is delayed by, respectively
1416 \fBzfs_scrub_delay\fR or \fBzfs_resilver_delay\fR ticks.
1418 Default value: \fB50\fR.
1424 \fBzfs_scan_min_time_ms\fR (int)
1427 Scrubs are processed by the sync thread. While scrubbing it will spend
1428 at least this much time working on a scrub between txg flushes.
1430 Default value: \fB1,000\fR.
1436 \fBzfs_scrub_delay\fR (int)
1439 Number of ticks to delay prior to issuing a scrub I/O operation when
1440 a non-scrub or non-resilver I/O operation has occurred within the past
1441 \fBzfs_scan_idle\fR ticks.
1443 Default value: \fB4\fR.
1449 \fBzfs_send_corrupt_data\fR (int)
1452 Allow sending of corrupt data (ignore read/checksum errors when sending data)
1454 Use \fB1\fR for yes and \fB0\fR for no (default).
1460 \fBzfs_sync_pass_deferred_free\fR (int)
1463 Flushing of data to disk is done in passes. Defer frees starting in this pass
1465 Default value: \fB2\fR.
1471 \fBzfs_sync_pass_dont_compress\fR (int)
1474 Don't compress starting in this pass
1476 Default value: \fB5\fR.
1482 \fBzfs_sync_pass_rewrite\fR (int)
1485 Rewrite new block pointers starting in this pass
1487 Default value: \fB2\fR.
1493 \fBzfs_top_maxinflight\fR (int)
1496 Max concurrent I/Os per top-level vdev (mirrors or raidz arrays) allowed during
1497 scrub or resilver operations.
1499 Default value: \fB32\fR.
1505 \fBzfs_txg_history\fR (int)
1508 Historic statistics for the last N txgs will be available in
1509 \fR/proc/spl/kstat/zfs/POOLNAME/txgs\fB
1511 Default value: \fB0\fR.
1517 \fBzfs_txg_timeout\fR (int)
1520 Flush dirty data to disk at least every N seconds (maximum txg duration)
1522 Default value: \fB5\fR.
1528 \fBzfs_vdev_aggregation_limit\fR (int)
1531 Max vdev I/O aggregation size
1533 Default value: \fB131,072\fR.
1539 \fBzfs_vdev_cache_bshift\fR (int)
1542 Shift size to inflate reads too
1544 Default value: \fB16\fR (effectively 65536).
1550 \fBzfs_vdev_cache_max\fR (int)
1553 Inflate reads small than this value to meet the \fBzfs_vdev_cache_bshift\fR
1556 Default value: \fB16384\fR.
1562 \fBzfs_vdev_cache_size\fR (int)
1565 Total size of the per-disk cache in bytes.
1567 Currently this feature is disabled as it has been found to not be helpful
1568 for performance and in some cases harmful.
1570 Default value: \fB0\fR.
1576 \fBzfs_vdev_mirror_rotating_inc\fR (int)
1579 A number by which the balancing algorithm increments the load calculation for
1580 the purpose of selecting the least busy mirror member when an I/O immediately
1581 follows its predecessor on rotational vdevs for the purpose of making decisions
1584 Default value: \fB0\fR.
1590 \fBzfs_vdev_mirror_rotating_seek_inc\fR (int)
1593 A number by which the balancing algorithm increments the load calculation for
1594 the purpose of selecting the least busy mirror member when an I/O lacks
1595 locality as defined by the zfs_vdev_mirror_rotating_seek_offset. I/Os within
1596 this that are not immediately following the previous I/O are incremented by
1599 Default value: \fB5\fR.
1605 \fBzfs_vdev_mirror_rotating_seek_offset\fR (int)
1608 The maximum distance for the last queued I/O in which the balancing algorithm
1609 considers an I/O to have locality.
1610 See the section "ZFS I/O SCHEDULER".
1612 Default value: \fB1048576\fR.
1618 \fBzfs_vdev_mirror_non_rotating_inc\fR (int)
1621 A number by which the balancing algorithm increments the load calculation for
1622 the purpose of selecting the least busy mirror member on non-rotational vdevs
1623 when I/Os do not immediately follow one another.
1625 Default value: \fB0\fR.
1631 \fBzfs_vdev_mirror_non_rotating_seek_inc\fR (int)
1634 A number by which the balancing algorithm increments the load calculation for
1635 the purpose of selecting the least busy mirror member when an I/O lacks
1636 locality as defined by the zfs_vdev_mirror_rotating_seek_offset. I/Os within
1637 this that are not immediately following the previous I/O are incremented by
1640 Default value: \fB1\fR.
1646 \fBzfs_vdev_read_gap_limit\fR (int)
1649 Aggregate read I/O operations if the gap on-disk between them is within this
1652 Default value: \fB32,768\fR.
1658 \fBzfs_vdev_scheduler\fR (charp)
1661 Set the Linux I/O scheduler on whole disk vdevs to this scheduler
1663 Default value: \fBnoop\fR.
1669 \fBzfs_vdev_write_gap_limit\fR (int)
1672 Aggregate write I/O over gap
1674 Default value: \fB4,096\fR.
1680 \fBzfs_vdev_raidz_impl\fR (string)
1683 Parameter for selecting raidz implementation to use.
1685 Options marked (always) below may be selected on module load as they are
1686 supported on all systems.
1687 The remaining options may only be set after the module is loaded, as they
1688 are available only if the implementations are compiled in and supported
1689 on the running system.
1691 Once the module is loaded, the content of
1692 /sys/module/zfs/parameters/zfs_vdev_raidz_impl will show available options
1693 with the currently selected one enclosed in [].
1694 Possible options are:
1695 fastest - (always) implementation selected using built-in benchmark
1696 original - (always) original raidz implementation
1697 scalar - (always) scalar raidz implementation
1698 sse - implementation using SSE instruction set (64bit x86 only)
1699 avx2 - implementation using AVX2 instruction set (64bit x86 only)
1701 Default value: \fBfastest\fR.
1707 \fBzfs_zevent_cols\fR (int)
1710 When zevents are logged to the console use this as the word wrap width.
1712 Default value: \fB80\fR.
1718 \fBzfs_zevent_console\fR (int)
1721 Log events to the console
1723 Use \fB1\fR for yes and \fB0\fR for no (default).
1729 \fBzfs_zevent_len_max\fR (int)
1732 Max event queue length. A value of 0 will result in a calculated value which
1733 increases with the number of CPUs in the system (minimum 64 events). Events
1734 in the queue can be viewed with the \fBzpool events\fR command.
1736 Default value: \fB0\fR.
1742 \fBzil_replay_disable\fR (int)
1745 Disable intent logging replay. Can be disabled for recovery from corrupted
1748 Use \fB1\fR for yes and \fB0\fR for no (default).
1754 \fBzil_slog_limit\fR (ulong)
1757 Max commit bytes to separate log device
1759 Default value: \fB1,048,576\fR.
1765 \fBzio_delay_max\fR (int)
1768 A zevent will be logged if a ZIO operation takes more than N milliseconds to
1769 complete. Note that this is only a logging facility, not a timeout on
1772 Default value: \fB30,000\fR.
1778 \fBzio_requeue_io_start_cut_in_line\fR (int)
1781 Prioritize requeued I/O
1783 Default value: \fB0\fR.
1789 \fBzio_taskq_batch_pct\fR (uint)
1792 Percentage of online CPUs (or CPU cores, etc) which will run a worker thread
1793 for IO. These workers are responsible for IO work such as compression and
1794 checksum calculations. Fractional number of CPUs will be rounded down.
1796 The default value of 75 was chosen to avoid using all CPUs which can result in
1797 latency issues and inconsistent application performance, especially when high
1798 compression is enabled.
1800 Default value: \fB75\fR.
1806 \fBzvol_inhibit_dev\fR (uint)
1809 Do not create zvol device nodes. This may slightly improve startup time on
1810 systems with a very large number of zvols.
1812 Use \fB1\fR for yes and \fB0\fR for no (default).
1818 \fBzvol_major\fR (uint)
1821 Major number for zvol block devices
1823 Default value: \fB230\fR.
1829 \fBzvol_max_discard_blocks\fR (ulong)
1832 Discard (aka TRIM) operations done on zvols will be done in batches of this
1833 many blocks, where block size is determined by the \fBvolblocksize\fR property
1836 Default value: \fB16,384\fR.
1842 \fBzvol_prefetch_bytes\fR (uint)
1845 When adding a zvol to the system prefetch \fBzvol_prefetch_bytes\fR
1846 from the start and end of the volume. Prefetching these regions
1847 of the volume is desirable because they are likely to be accessed
1848 immediately by \fBblkid(8)\fR or by the kernel scanning for a partition
1851 Default value: \fB131,072\fR.
1854 .SH ZFS I/O SCHEDULER
1855 ZFS issues I/O operations to leaf vdevs to satisfy and complete I/Os.
1856 The I/O scheduler determines when and in what order those operations are
1857 issued. The I/O scheduler divides operations into five I/O classes
1858 prioritized in the following order: sync read, sync write, async read,
1859 async write, and scrub/resilver. Each queue defines the minimum and
1860 maximum number of concurrent operations that may be issued to the
1861 device. In addition, the device has an aggregate maximum,
1862 \fBzfs_vdev_max_active\fR. Note that the sum of the per-queue minimums
1863 must not exceed the aggregate maximum. If the sum of the per-queue
1864 maximums exceeds the aggregate maximum, then the number of active I/Os
1865 may reach \fBzfs_vdev_max_active\fR, in which case no further I/Os will
1866 be issued regardless of whether all per-queue minimums have been met.
1868 For many physical devices, throughput increases with the number of
1869 concurrent operations, but latency typically suffers. Further, physical
1870 devices typically have a limit at which more concurrent operations have no
1871 effect on throughput or can actually cause it to decrease.
1873 The scheduler selects the next operation to issue by first looking for an
1874 I/O class whose minimum has not been satisfied. Once all are satisfied and
1875 the aggregate maximum has not been hit, the scheduler looks for classes
1876 whose maximum has not been satisfied. Iteration through the I/O classes is
1877 done in the order specified above. No further operations are issued if the
1878 aggregate maximum number of concurrent operations has been hit or if there
1879 are no operations queued for an I/O class that has not hit its maximum.
1880 Every time an I/O is queued or an operation completes, the I/O scheduler
1881 looks for new operations to issue.
1883 In general, smaller max_active's will lead to lower latency of synchronous
1884 operations. Larger max_active's may lead to higher overall throughput,
1885 depending on underlying storage.
1887 The ratio of the queues' max_actives determines the balance of performance
1888 between reads, writes, and scrubs. E.g., increasing
1889 \fBzfs_vdev_scrub_max_active\fR will cause the scrub or resilver to complete
1890 more quickly, but reads and writes to have higher latency and lower throughput.
1892 All I/O classes have a fixed maximum number of outstanding operations
1893 except for the async write class. Asynchronous writes represent the data
1894 that is committed to stable storage during the syncing stage for
1895 transaction groups. Transaction groups enter the syncing state
1896 periodically so the number of queued async writes will quickly burst up
1897 and then bleed down to zero. Rather than servicing them as quickly as
1898 possible, the I/O scheduler changes the maximum number of active async
1899 write I/Os according to the amount of dirty data in the pool. Since
1900 both throughput and latency typically increase with the number of
1901 concurrent operations issued to physical devices, reducing the
1902 burstiness in the number of concurrent operations also stabilizes the
1903 response time of operations from other -- and in particular synchronous
1904 -- queues. In broad strokes, the I/O scheduler will issue more
1905 concurrent operations from the async write queue as there's more dirty
1910 The number of concurrent operations issued for the async write I/O class
1911 follows a piece-wise linear function defined by a few adjustable points.
1914 | o---------| <-- zfs_vdev_async_write_max_active
1921 |-------o | | <-- zfs_vdev_async_write_min_active
1922 0|_______^______|_________|
1923 0% | | 100% of zfs_dirty_data_max
1925 | `-- zfs_vdev_async_write_active_max_dirty_percent
1926 `--------- zfs_vdev_async_write_active_min_dirty_percent
1929 Until the amount of dirty data exceeds a minimum percentage of the dirty
1930 data allowed in the pool, the I/O scheduler will limit the number of
1931 concurrent operations to the minimum. As that threshold is crossed, the
1932 number of concurrent operations issued increases linearly to the maximum at
1933 the specified maximum percentage of the dirty data allowed in the pool.
1935 Ideally, the amount of dirty data on a busy pool will stay in the sloped
1936 part of the function between \fBzfs_vdev_async_write_active_min_dirty_percent\fR
1937 and \fBzfs_vdev_async_write_active_max_dirty_percent\fR. If it exceeds the
1938 maximum percentage, this indicates that the rate of incoming data is
1939 greater than the rate that the backend storage can handle. In this case, we
1940 must further throttle incoming writes, as described in the next section.
1942 .SH ZFS TRANSACTION DELAY
1943 We delay transactions when we've determined that the backend storage
1944 isn't able to accommodate the rate of incoming writes.
1946 If there is already a transaction waiting, we delay relative to when
1947 that transaction will finish waiting. This way the calculated delay time
1948 is independent of the number of threads concurrently executing
1951 If we are the only waiter, wait relative to when the transaction
1952 started, rather than the current time. This credits the transaction for
1953 "time already served", e.g. reading indirect blocks.
1955 The minimum time for a transaction to take is calculated as:
1957 min_time = zfs_delay_scale * (dirty - min) / (max - dirty)
1958 min_time is then capped at 100 milliseconds.
1961 The delay has two degrees of freedom that can be adjusted via tunables. The
1962 percentage of dirty data at which we start to delay is defined by
1963 \fBzfs_delay_min_dirty_percent\fR. This should typically be at or above
1964 \fBzfs_vdev_async_write_active_max_dirty_percent\fR so that we only start to
1965 delay after writing at full speed has failed to keep up with the incoming write
1966 rate. The scale of the curve is defined by \fBzfs_delay_scale\fR. Roughly speaking,
1967 this variable determines the amount of delay at the midpoint of the curve.
1971 10ms +-------------------------------------------------------------*+
1987 2ms + (midpoint) * +
1990 | zfs_delay_scale ----------> ******** |
1991 0 +-------------------------------------*********----------------+
1992 0% <- zfs_dirty_data_max -> 100%
1995 Note that since the delay is added to the outstanding time remaining on the
1996 most recent transaction, the delay is effectively the inverse of IOPS.
1997 Here the midpoint of 500us translates to 2000 IOPS. The shape of the curve
1998 was chosen such that small changes in the amount of accumulated dirty data
1999 in the first 3/4 of the curve yield relatively small differences in the
2002 The effects can be easier to understand when the amount of delay is
2003 represented on a log scale:
2007 100ms +-------------------------------------------------------------++
2016 + zfs_delay_scale ----------> ***** +
2027 +--------------------------------------------------------------+
2028 0% <- zfs_dirty_data_max -> 100%
2031 Note here that only as the amount of dirty data approaches its limit does
2032 the delay start to increase rapidly. The goal of a properly tuned system
2033 should be to keep the amount of dirty data out of that range by first
2034 ensuring that the appropriate limits are set for the I/O scheduler to reach
2035 optimal throughput on the backend storage, and then by changing the value
2036 of \fBzfs_delay_scale\fR to increase the steepness of the curve.
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