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zfs/man/man5/zfs-module-parameters.5

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Document ZFS module parameters. This is a first draft of a zfs-module-parameters(5) man page. I have just extracted the parameter name and its description with modinfo, then checked the source what type it is and its default value. This will need more work, preferably someone that actually know these values and what to use them for. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1856
2013-11-16 06:52:54 +00:00
'\" te
.\" Copyright (c) 2013 by Turbo Fredriksson <turbo@bayour.com>. All rights reserved.
.\" The contents of this file are subject to the terms of the Common Development
.\" and Distribution License (the "License"). You may not use this file except
.\" in compliance with the License. You can obtain a copy of the license at
.\" usr/src/OPENSOLARIS.LICENSE or http://www.opensolaris.org/os/licensing.
.\"
.\" See the License for the specific language governing permissions and
.\" limitations under the License. When distributing Covered Code, include this
.\" CDDL HEADER in each file and include the License file at
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.\" CDDL HEADER, with the fields enclosed by brackets "[]" replaced with your
.\" own identifying information:
.\" Portions Copyright [yyyy] [name of copyright owner]
.TH ZFS-MODULE-PARAMETERS 5 "Nov 16, 2013"
.SH NAME
zfs\-module\-parameters \- ZFS module parameters
.SH DESCRIPTION
.sp
.LP
Description of the different parameters to the ZFS module.
.SS "Module parameters"
.sp
.LP
.sp
.ne 2
.na
\fBl2arc_feed_again\fR (int)
.ad
.RS 12n
Turbo L2ARC warmup
.sp
Use \fB1\fR for yes (default) and \fB0\fR to disable.
.RE
.sp
.ne 2
.na
\fBl2arc_feed_min_ms\fR (ulong)
.ad
.RS 12n
Min feed interval in milliseconds
.sp
Default value: \fB200\fR.
.RE
.sp
.ne 2
.na
\fBl2arc_feed_secs\fR (ulong)
.ad
.RS 12n
Seconds between L2ARC writing
.sp
Default value: \fB1\fR.
.RE
.sp
.ne 2
.na
\fBl2arc_headroom\fR (ulong)
.ad
.RS 12n
Number of max device writes to precache
.sp
Default value: \fB2\fR.
.RE
.sp
.ne 2
.na
\fBl2arc_headroom_boost\fR (ulong)
.ad
.RS 12n
Compressed l2arc_headroom multiplier
.sp
Default value: \fB200\fR.
.RE
.sp
.ne 2
.na
\fBl2arc_nocompress\fR (int)
.ad
.RS 12n
Skip compressing L2ARC buffers
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE
.sp
.ne 2
.na
\fBl2arc_noprefetch\fR (int)
.ad
.RS 12n
Skip caching prefetched buffers
.sp
Use \fB1\fR for yes (default) and \fB0\fR to disable.
.RE
.sp
.ne 2
.na
\fBl2arc_norw\fR (int)
.ad
.RS 12n
No reads during writes
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE
.sp
.ne 2
.na
\fBl2arc_write_boost\fR (ulong)
.ad
.RS 12n
Extra write bytes during device warmup
.sp
Default value: \fB8,388,608\fR.
.RE
.sp
.ne 2
.na
\fBl2arc_write_max\fR (ulong)
.ad
.RS 12n
Max write bytes per interval
.sp
Default value: \fB8,388,608\fR.
.RE
Illumos 4976-4984 - metaslab improvements 4976 zfs should only avoid writing to a failing non-redundant top-level vdev 4978 ztest fails in get_metaslab_refcount() 4979 extend free space histogram to device and pool 4980 metaslabs should have a fragmentation metric 4981 remove fragmented ops vector from block allocator 4982 space_map object should proactively upgrade when feature is enabled 4983 need to collect metaslab information via mdb 4984 device selection should use fragmentation metric Reviewed by: Matthew Ahrens <mahrens@delphix.com> Reviewed by: Adam Leventhal <adam.leventhal@delphix.com> Reviewed by: Christopher Siden <christopher.siden@delphix.com> Approved by: Garrett D'Amore <garrett@damore.org> References: https://www.illumos.org/issues/4976 https://www.illumos.org/issues/4978 https://www.illumos.org/issues/4979 https://www.illumos.org/issues/4980 https://www.illumos.org/issues/4981 https://www.illumos.org/issues/4982 https://www.illumos.org/issues/4983 https://www.illumos.org/issues/4984 https://github.com/illumos/illumos-gate/commit/2e4c998 Notes: The "zdb -M" option has been re-tasked to display the new metaslab fragmentation metric and the new "zdb -I" option is used to control the maximum number of in-flight I/Os. The new fragmentation metric is derived from the space map histogram which has been rolled up to the vdev and pool level and is presented to the user via "zpool list". Add a number of module parameters related to the new metaslab weighting logic. Ported by: Tim Chase <tim@chase2k.com> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #2595
2014-07-19 20:19:24 +00:00
.sp
.ne 2
.na
\fBmetaslab_bias_enabled\fR (int)
.ad
.RS 12n
Enable metaslab group biasing based on its vdev's over- or under-utilization
relative to the pool.
.sp
Use \fB1\fR for yes (default) and \fB0\fR for no.
.RE
Document ZFS module parameters. This is a first draft of a zfs-module-parameters(5) man page. I have just extracted the parameter name and its description with modinfo, then checked the source what type it is and its default value. This will need more work, preferably someone that actually know these values and what to use them for. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1856
2013-11-16 06:52:54 +00:00
.sp
.ne 2
.na
Illumos #4101 finer-grained control of metaslab_debug Today the metaslab_debug logic performs two tasks: - load all metaslabs on import/open - don't unload metaslabs at the end of spa_sync This change provides knobs for each of these independently. References: https://illumos.org/issues/4101 https://github.com/illumos/illumos-gate/commit/0713e23 Notes: 1) This is a small piece of the metaslab improvement patch from Illumos. It was worth bringing over before the rest, since it's low risk and it can be useful on fragmented pools (e.g. Lustre MDTs). metaslab_debug_unload would give the performance benefit of the old metaslab_debug option without causing unwanted delay during pool import. Ported-by: Ned Bass <bass6@llnl.gov> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #2227
2014-04-01 00:22:55 +00:00
\fBmetaslab_debug_load\fR (int)
Document ZFS module parameters. This is a first draft of a zfs-module-parameters(5) man page. I have just extracted the parameter name and its description with modinfo, then checked the source what type it is and its default value. This will need more work, preferably someone that actually know these values and what to use them for. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1856
2013-11-16 06:52:54 +00:00
.ad
.RS 12n
Illumos #4101 finer-grained control of metaslab_debug Today the metaslab_debug logic performs two tasks: - load all metaslabs on import/open - don't unload metaslabs at the end of spa_sync This change provides knobs for each of these independently. References: https://illumos.org/issues/4101 https://github.com/illumos/illumos-gate/commit/0713e23 Notes: 1) This is a small piece of the metaslab improvement patch from Illumos. It was worth bringing over before the rest, since it's low risk and it can be useful on fragmented pools (e.g. Lustre MDTs). metaslab_debug_unload would give the performance benefit of the old metaslab_debug option without causing unwanted delay during pool import. Ported-by: Ned Bass <bass6@llnl.gov> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #2227
2014-04-01 00:22:55 +00:00
Load all metaslabs during pool import.
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE
.sp
.ne 2
.na
\fBmetaslab_debug_unload\fR (int)
.ad
.RS 12n
Prevent metaslabs from being unloaded.
Document ZFS module parameters. This is a first draft of a zfs-module-parameters(5) man page. I have just extracted the parameter name and its description with modinfo, then checked the source what type it is and its default value. This will need more work, preferably someone that actually know these values and what to use them for. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1856
2013-11-16 06:52:54 +00:00
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE
Illumos 4976-4984 - metaslab improvements 4976 zfs should only avoid writing to a failing non-redundant top-level vdev 4978 ztest fails in get_metaslab_refcount() 4979 extend free space histogram to device and pool 4980 metaslabs should have a fragmentation metric 4981 remove fragmented ops vector from block allocator 4982 space_map object should proactively upgrade when feature is enabled 4983 need to collect metaslab information via mdb 4984 device selection should use fragmentation metric Reviewed by: Matthew Ahrens <mahrens@delphix.com> Reviewed by: Adam Leventhal <adam.leventhal@delphix.com> Reviewed by: Christopher Siden <christopher.siden@delphix.com> Approved by: Garrett D'Amore <garrett@damore.org> References: https://www.illumos.org/issues/4976 https://www.illumos.org/issues/4978 https://www.illumos.org/issues/4979 https://www.illumos.org/issues/4980 https://www.illumos.org/issues/4981 https://www.illumos.org/issues/4982 https://www.illumos.org/issues/4983 https://www.illumos.org/issues/4984 https://github.com/illumos/illumos-gate/commit/2e4c998 Notes: The "zdb -M" option has been re-tasked to display the new metaslab fragmentation metric and the new "zdb -I" option is used to control the maximum number of in-flight I/Os. The new fragmentation metric is derived from the space map histogram which has been rolled up to the vdev and pool level and is presented to the user via "zpool list". Add a number of module parameters related to the new metaslab weighting logic. Ported by: Tim Chase <tim@chase2k.com> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #2595
2014-07-19 20:19:24 +00:00
.sp
.ne 2
.na
\fBmetaslab_fragmentation_factor_enabled\fR (int)
.ad
.RS 12n
Enable use of the fragmentation metric in computing metaslab weights.
.sp
Use \fB1\fR for yes (default) and \fB0\fR for no.
.RE
Illumos 5161 - add tunable for number of metaslabs per vdev 5161 add tunable for number of metaslabs per vdev Reviewed by: Alex Reece <alex.reece@delphix.com> Reviewed by: Christopher Siden <christopher.siden@delphix.com> Reviewed by: George Wilson <george.wilson@delphix.com> Reviewed by: Paul Dagnelie <paul.dagnelie@delphix.com> Reviewed by: Saso Kiselkov <skiselkov.ml@gmail.com> Reviewed by: Richard Elling <richard.elling@gmail.com> Approved by: Richard Lowe <richlowe@richlowe.net> References: https://www.illumos.org/issues/5161 https://github.com/illumos/illumos-gate/commit/bf3e216 Ported by: Turbo Fredriksson <turbo@bayour.com> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #2698
2014-09-13 14:13:00 +00:00
.sp
.ne 2
.na
\fBmetaslabs_per_vdev\fR (int)
.ad
.RS 12n
When a vdev is added, it will be divided into approximately (but no more than) this number of metaslabs.
.sp
Default value: \fB200\fR.
.RE
Illumos 4976-4984 - metaslab improvements 4976 zfs should only avoid writing to a failing non-redundant top-level vdev 4978 ztest fails in get_metaslab_refcount() 4979 extend free space histogram to device and pool 4980 metaslabs should have a fragmentation metric 4981 remove fragmented ops vector from block allocator 4982 space_map object should proactively upgrade when feature is enabled 4983 need to collect metaslab information via mdb 4984 device selection should use fragmentation metric Reviewed by: Matthew Ahrens <mahrens@delphix.com> Reviewed by: Adam Leventhal <adam.leventhal@delphix.com> Reviewed by: Christopher Siden <christopher.siden@delphix.com> Approved by: Garrett D'Amore <garrett@damore.org> References: https://www.illumos.org/issues/4976 https://www.illumos.org/issues/4978 https://www.illumos.org/issues/4979 https://www.illumos.org/issues/4980 https://www.illumos.org/issues/4981 https://www.illumos.org/issues/4982 https://www.illumos.org/issues/4983 https://www.illumos.org/issues/4984 https://github.com/illumos/illumos-gate/commit/2e4c998 Notes: The "zdb -M" option has been re-tasked to display the new metaslab fragmentation metric and the new "zdb -I" option is used to control the maximum number of in-flight I/Os. The new fragmentation metric is derived from the space map histogram which has been rolled up to the vdev and pool level and is presented to the user via "zpool list". Add a number of module parameters related to the new metaslab weighting logic. Ported by: Tim Chase <tim@chase2k.com> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #2595
2014-07-19 20:19:24 +00:00
.sp
.ne 2
.na
\fBmetaslab_preload_enabled\fR (int)
.ad
.RS 12n
Enable metaslab group preloading.
.sp
Use \fB1\fR for yes (default) and \fB0\fR for no.
.RE
.sp
.ne 2
.na
\fBmetaslab_lba_weighting_enabled\fR (int)
.ad
.RS 12n
Give more weight to metaslabs with lower LBAs, assuming they have
greater bandwidth as is typically the case on a modern constant
angular velocity disk drive.
.sp
Use \fB1\fR for yes (default) and \fB0\fR for no.
.RE
Document ZFS module parameters. This is a first draft of a zfs-module-parameters(5) man page. I have just extracted the parameter name and its description with modinfo, then checked the source what type it is and its default value. This will need more work, preferably someone that actually know these values and what to use them for. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1856
2013-11-16 06:52:54 +00:00
.sp
.ne 2
.na
\fBspa_config_path\fR (charp)
.ad
.RS 12n
SPA config file
.sp
Default value: \fB/etc/zfs/zpool.cache\fR.
.RE
Illumos #4045 write throttle & i/o scheduler performance work 4045 zfs write throttle & i/o scheduler performance work 1. The ZFS i/o scheduler (vdev_queue.c) now divides i/os into 5 classes: sync read, sync write, async read, async write, and scrub/resilver. The scheduler issues a number of concurrent i/os from each class to the device. Once a class has been selected, an i/o is selected from this class using either an elevator algorithem (async, scrub classes) or FIFO (sync classes). The number of concurrent async write i/os is tuned dynamically based on i/o load, to achieve good sync i/o latency when there is not a high load of writes, and good write throughput when there is. See the block comment in vdev_queue.c (reproduced below) for more details. 2. The write throttle (dsl_pool_tempreserve_space() and txg_constrain_throughput()) is rewritten to produce much more consistent delays when under constant load. The new write throttle is based on the amount of dirty data, rather than guesses about future performance of the system. When there is a lot of dirty data, each transaction (e.g. write() syscall) will be delayed by the same small amount. This eliminates the "brick wall of wait" that the old write throttle could hit, causing all transactions to wait several seconds until the next txg opens. One of the keys to the new write throttle is decrementing the amount of dirty data as i/o completes, rather than at the end of spa_sync(). Note that the write throttle is only applied once the i/o scheduler is issuing the maximum number of outstanding async writes. See the block comments in dsl_pool.c and above dmu_tx_delay() (reproduced below) for more details. This diff has several other effects, including: * the commonly-tuned global variable zfs_vdev_max_pending has been removed; use per-class zfs_vdev_*_max_active values or zfs_vdev_max_active instead. * the size of each txg (meaning the amount of dirty data written, and thus the time it takes to write out) is now controlled differently. There is no longer an explicit time goal; the primary determinant is amount of dirty data. Systems that are under light or medium load will now often see that a txg is always syncing, but the impact to performance (e.g. read latency) is minimal. Tune zfs_dirty_data_max and zfs_dirty_data_sync to control this. * zio_taskq_batch_pct = 75 -- Only use 75% of all CPUs for compression, checksum, etc. This improves latency by not allowing these CPU-intensive tasks to consume all CPU (on machines with at least 4 CPU's; the percentage is rounded up). --matt APPENDIX: problems with the current i/o scheduler The current ZFS i/o scheduler (vdev_queue.c) is deadline based. The problem with this is that if there are always i/os pending, then certain classes of i/os can see very long delays. For example, if there are always synchronous reads outstanding, then no async writes will be serviced until they become "past due". One symptom of this situation is that each pass of the txg sync takes at least several seconds (typically 3 seconds). If many i/os become "past due" (their deadline is in the past), then we must service all of these overdue i/os before any new i/os. This happens when we enqueue a batch of async writes for the txg sync, with deadlines 2.5 seconds in the future. If we can't complete all the i/os in 2.5 seconds (e.g. because there were always reads pending), then these i/os will become past due. Now we must service all the "async" writes (which could be hundreds of megabytes) before we service any reads, introducing considerable latency to synchronous i/os (reads or ZIL writes). Notes on porting to ZFS on Linux: - zio_t gained new members io_physdone and io_phys_children. Because object caches in the Linux port call the constructor only once at allocation time, objects may contain residual data when retrieved from the cache. Therefore zio_create() was updated to zero out the two new fields. - vdev_mirror_pending() relied on the depth of the per-vdev pending queue (vq->vq_pending_tree) to select the least-busy leaf vdev to read from. This tree has been replaced by vq->vq_active_tree which is now used for the same purpose. - vdev_queue_init() used the value of zfs_vdev_max_pending to determine the number of vdev I/O buffers to pre-allocate. That global no longer exists, so we instead use the sum of the *_max_active values for each of the five I/O classes described above. - The Illumos implementation of dmu_tx_delay() delays a transaction by sleeping in condition variable embedded in the thread (curthread->t_delay_cv). We do not have an equivalent CV to use in Linux, so this change replaced the delay logic with a wrapper called zfs_sleep_until(). This wrapper could be adopted upstream and in other downstream ports to abstract away operating system-specific delay logic. - These tunables are added as module parameters, and descriptions added to the zfs-module-parameters.5 man page. spa_asize_inflation zfs_deadman_synctime_ms zfs_vdev_max_active zfs_vdev_async_write_active_min_dirty_percent zfs_vdev_async_write_active_max_dirty_percent zfs_vdev_async_read_max_active zfs_vdev_async_read_min_active zfs_vdev_async_write_max_active zfs_vdev_async_write_min_active zfs_vdev_scrub_max_active zfs_vdev_scrub_min_active zfs_vdev_sync_read_max_active zfs_vdev_sync_read_min_active zfs_vdev_sync_write_max_active zfs_vdev_sync_write_min_active zfs_dirty_data_max_percent zfs_delay_min_dirty_percent zfs_dirty_data_max_max_percent zfs_dirty_data_max zfs_dirty_data_max_max zfs_dirty_data_sync zfs_delay_scale The latter four have type unsigned long, whereas they are uint64_t in Illumos. This accommodates Linux's module_param() supported types, but means they may overflow on 32-bit architectures. The values zfs_dirty_data_max and zfs_dirty_data_max_max are the most likely to overflow on 32-bit systems, since they express physical RAM sizes in bytes. In fact, Illumos initializes zfs_dirty_data_max_max to 2^32 which does overflow. To resolve that, this port instead initializes it in arc_init() to 25% of physical RAM, and adds the tunable zfs_dirty_data_max_max_percent to override that percentage. While this solution doesn't completely avoid the overflow issue, it should be a reasonable default for most systems, and the minority of affected systems can work around the issue by overriding the defaults. - Fixed reversed logic in comment above zfs_delay_scale declaration. - Clarified comments in vdev_queue.c regarding when per-queue minimums take effect. - Replaced dmu_tx_write_limit in the dmu_tx kstat file with dmu_tx_dirty_delay and dmu_tx_dirty_over_max. The first counts how many times a transaction has been delayed because the pool dirty data has exceeded zfs_delay_min_dirty_percent. The latter counts how many times the pool dirty data has exceeded zfs_dirty_data_max (which we expect to never happen). - The original patch would have regressed the bug fixed in zfsonlinux/zfs@c418410, which prevented users from setting the zfs_vdev_aggregation_limit tuning larger than SPA_MAXBLOCKSIZE. A similar fix is added to vdev_queue_aggregate(). - In vdev_queue_io_to_issue(), dynamically allocate 'zio_t search' on the heap instead of the stack. In Linux we can't afford such large structures on the stack. Reviewed by: George Wilson <george.wilson@delphix.com> Reviewed by: Adam Leventhal <ahl@delphix.com> Reviewed by: Christopher Siden <christopher.siden@delphix.com> Reviewed by: Ned Bass <bass6@llnl.gov> Reviewed by: Brendan Gregg <brendan.gregg@joyent.com> Approved by: Robert Mustacchi <rm@joyent.com> References: http://www.illumos.org/issues/4045 illumos/illumos-gate@69962b5647e4a8b9b14998733b765925381b727e Ported-by: Ned Bass <bass6@llnl.gov> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1913
2013-08-29 03:01:20 +00:00
.sp
.ne 2
.na
\fBspa_asize_inflation\fR (int)
.ad
.RS 12n
Multiplication factor used to estimate actual disk consumption from the
size of data being written. The default value is a worst case estimate,
but lower values may be valid for a given pool depending on its
configuration. Pool administrators who understand the factors involved
may wish to specify a more realistic inflation factor, particularly if
they operate close to quota or capacity limits.
.sp
Default value: 24
.RE
Illumos 4970-4974 - extreme rewind enhancements 4970 need controls on i/o issued by zpool import -XF 4971 zpool import -T should accept hex values 4972 zpool import -T implies extreme rewind, and thus a scrub 4973 spa_load_retry retries the same txg 4974 spa_load_verify() reads all data twice Reviewed by: Christopher Siden <christopher.siden@delphix.com> Reviewed by: Dan McDonald <danmcd@omniti.com> Reviewed by: George Wilson <george.wilson@delphix.com> Approved by: Robert Mustacchi <rm@joyent.com> References: https://www.illumos.org/issues/4970 https://www.illumos.org/issues/4971 https://www.illumos.org/issues/4972 https://www.illumos.org/issues/4973 https://www.illumos.org/issues/4974 https://github.com/illumos/illumos-gate/commit/e42d205 Notes: This set of patches adds a set of tunable parameters for the "extreme rewind" mode of pool import which allows control over the traversal performed during such an import. Ported by: Tim Chase <tim@chase2k.com> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #2598
2014-07-15 18:58:41 +00:00
.sp
.ne 2
.na
\fBspa_load_verify_data\fR (int)
.ad
.RS 12n
Whether to traverse data blocks during an "extreme rewind" (\fB-X\fR)
import. Use 0 to disable and 1 to enable.
An extreme rewind import normally performs a full traversal of all
blocks in the pool for verification. If this parameter is set to 0,
the traversal skips non-metadata blocks. It can be toggled once the
import has started to stop or start the traversal of non-metadata blocks.
.sp
Default value: 1
.RE
.sp
.ne 2
.na
\fBspa_load_verify_metadata\fR (int)
.ad
.RS 12n
Whether to traverse blocks during an "extreme rewind" (\fB-X\fR)
pool import. Use 0 to disable and 1 to enable.
An extreme rewind import normally performs a full traversal of all
blocks in the pool for verification. If this parameter is set to 1,
the traversal is not performed. It can be toggled once the import has
started to stop or start the traversal.
.sp
Default value: 1
.RE
.sp
.ne 2
.na
\fBspa_load_verify_maxinflight\fR (int)
.ad
.RS 12n
Maximum concurrent I/Os during the traversal performed during an "extreme
rewind" (\fB-X\fR) pool import.
.sp
Default value: 10000
.RE
Document ZFS module parameters. This is a first draft of a zfs-module-parameters(5) man page. I have just extracted the parameter name and its description with modinfo, then checked the source what type it is and its default value. This will need more work, preferably someone that actually know these values and what to use them for. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1856
2013-11-16 06:52:54 +00:00
.sp
.ne 2
.na
\fBzfetch_array_rd_sz\fR (ulong)
.ad
.RS 12n
Expand the description of scan-related and other parameters. Document that the scan-related parameters are, in fact, applicable only to scrub and/or resilver operations as appropriate. Expand a few of the prefetch-related descriptions. Add clarification to other module parameters. Signed-off-by: Tim Chase <tim@chase2k.com> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #2361
2014-06-04 12:23:31 +00:00
If prefetching is enabled, disable prefetching for reads larger than this size.
Document ZFS module parameters. This is a first draft of a zfs-module-parameters(5) man page. I have just extracted the parameter name and its description with modinfo, then checked the source what type it is and its default value. This will need more work, preferably someone that actually know these values and what to use them for. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1856
2013-11-16 06:52:54 +00:00
.sp
Default value: \fB1,048,576\fR.
.RE
.sp
.ne 2
.na
\fBzfetch_block_cap\fR (uint)
.ad
.RS 12n
Expand the description of scan-related and other parameters. Document that the scan-related parameters are, in fact, applicable only to scrub and/or resilver operations as appropriate. Expand a few of the prefetch-related descriptions. Add clarification to other module parameters. Signed-off-by: Tim Chase <tim@chase2k.com> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #2361
2014-06-04 12:23:31 +00:00
Max number of blocks to prefetch at a time
Document ZFS module parameters. This is a first draft of a zfs-module-parameters(5) man page. I have just extracted the parameter name and its description with modinfo, then checked the source what type it is and its default value. This will need more work, preferably someone that actually know these values and what to use them for. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1856
2013-11-16 06:52:54 +00:00
.sp
Default value: \fB256\fR.
.RE
.sp
.ne 2
.na
\fBzfetch_max_streams\fR (uint)
.ad
.RS 12n
Expand the description of scan-related and other parameters. Document that the scan-related parameters are, in fact, applicable only to scrub and/or resilver operations as appropriate. Expand a few of the prefetch-related descriptions. Add clarification to other module parameters. Signed-off-by: Tim Chase <tim@chase2k.com> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #2361
2014-06-04 12:23:31 +00:00
Max number of streams per zfetch (prefetch streams per file).
Document ZFS module parameters. This is a first draft of a zfs-module-parameters(5) man page. I have just extracted the parameter name and its description with modinfo, then checked the source what type it is and its default value. This will need more work, preferably someone that actually know these values and what to use them for. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1856
2013-11-16 06:52:54 +00:00
.sp
Default value: \fB8\fR.
.RE
.sp
.ne 2
.na
\fBzfetch_min_sec_reap\fR (uint)
.ad
.RS 12n
Expand the description of scan-related and other parameters. Document that the scan-related parameters are, in fact, applicable only to scrub and/or resilver operations as appropriate. Expand a few of the prefetch-related descriptions. Add clarification to other module parameters. Signed-off-by: Tim Chase <tim@chase2k.com> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #2361
2014-06-04 12:23:31 +00:00
Min time before an active prefetch stream can be reclaimed
Document ZFS module parameters. This is a first draft of a zfs-module-parameters(5) man page. I have just extracted the parameter name and its description with modinfo, then checked the source what type it is and its default value. This will need more work, preferably someone that actually know these values and what to use them for. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1856
2013-11-16 06:52:54 +00:00
.sp
Default value: \fB2\fR.
.RE
Illumos 5034 - ARC's buf_hash_table is too small 5034 ARC's buf_hash_table is too small Reviewed by: Christopher Siden <christopher.siden@delphix.com> Reviewed by: George Wilson <george.wilson@delphix.com> Reviewed by: Saso Kiselkov <skiselkov.ml@gmail.com> Reviewed by: Richard Elling <richard.elling@gmail.com> Approved by: Gordon Ross <gwr@nexenta.com> References: https://www.illumos.org/issues/5034 https://github.com/illumos/illumos-gate/commit/63e911b Ported-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #2615
2014-08-20 17:09:40 +00:00
.sp
.ne 2
.na
\fBzfs_arc_average_blocksize\fR (int)
.ad
.RS 12n
The ARC's buffer hash table is sized based on the assumption of an average
block size of \fBzfs_arc_average_blocksize\fR (default 8K). This works out
to roughly 1MB of hash table per 1GB of physical memory with 8-byte pointers.
For configurations with a known larger average block size this value can be
increased to reduce the memory footprint.
.sp
Default value: \fB8192\fR.
.RE
Document ZFS module parameters. This is a first draft of a zfs-module-parameters(5) man page. I have just extracted the parameter name and its description with modinfo, then checked the source what type it is and its default value. This will need more work, preferably someone that actually know these values and what to use them for. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1856
2013-11-16 06:52:54 +00:00
.sp
.ne 2
.na
\fBzfs_arc_grow_retry\fR (int)
.ad
.RS 12n
Seconds before growing arc size
.sp
Default value: \fB5\fR.
.RE
.sp
.ne 2
.na
\fBzfs_arc_max\fR (ulong)
.ad
.RS 12n
Max arc size
.sp
Default value: \fB0\fR.
.RE
.sp
.ne 2
.na
\fBzfs_arc_memory_throttle_disable\fR (int)
.ad
.RS 12n
Disable memory throttle
.sp
Use \fB1\fR for yes (default) and \fB0\fR to disable.
.RE
.sp
.ne 2
.na
\fBzfs_arc_meta_limit\fR (ulong)
.ad
.RS 12n
Restructure per-filesystem reclaim Originally when the ARC prune callback was introduced the idea was to register a single callback for the ZPL. The ARC could invoke this call back if it needed the ZPL to drop dentries, inodes, or other cache objects which might be pinning buffers in the ARC. The ZPL would iterate over all ZFS super blocks and perform the reclaim. For the most part this design has worked well but due to limitations in 2.6.35 and earlier kernels there were some problems. This patch is designed to address those issues. 1) iterate_supers_type() is not provided by all kernels which makes it impossible to safely iterate over all zpl_fs_type filesystems in a single callback. The most straight forward and portable way to resolve this is to register a callback per-filesystem during mount. The arc_*_prune_callback() functions have always supported multiple callbacks so this is functionally a very small change. 2) Commit 050d22b removed the non-portable shrink_dcache_memory() and shrink_icache_memory() functions and didn't replace them with equivalent functionality. This meant that for Linux 3.1 and older kernels the ARC had no mechanism to drop dentries and inodes from the caches if needed. This patch adds that missing functionality by calling shrink_dcache_parent() to release dentries which may be pinning inodes. This will result in all unused cache entries being dropped which is a bit heavy handed but it's the only interface available for old kernels. 3) A zpl_drop_inode() callback is registered for kernels older than 2.6.35 which do not support the .evict_inode callback. This ensures that when the last reference on an inode is dropped it is immediately removed from the cache. If this isn't done than inode can end up on the global unused LRU with no mechanism available to ZFS to drop them. Since the ARC buffers are not dropped the hottest inodes can still be recreated without performing disk IO. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Signed-off-by: Pavel Snajdr <snajpa@snajpa.net> Issue #3160
2015-03-17 22:07:47 +00:00
The maximum allowed size in bytes that meta data buffers are allowed to
consume in the ARC. When this limit is reached meta data buffers will
be reclaimed even if the overall arc_c_max has not been reached. This
value defaults to 0 which indicates that 3/4 of the ARC may be used
for meta data.
Document ZFS module parameters. This is a first draft of a zfs-module-parameters(5) man page. I have just extracted the parameter name and its description with modinfo, then checked the source what type it is and its default value. This will need more work, preferably someone that actually know these values and what to use them for. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1856
2013-11-16 06:52:54 +00:00
.sp
Default value: \fB0\fR.
.RE
.sp
.ne 2
.na
\fBzfs_arc_meta_prune\fR (int)
.ad
.RS 12n
Restructure per-filesystem reclaim Originally when the ARC prune callback was introduced the idea was to register a single callback for the ZPL. The ARC could invoke this call back if it needed the ZPL to drop dentries, inodes, or other cache objects which might be pinning buffers in the ARC. The ZPL would iterate over all ZFS super blocks and perform the reclaim. For the most part this design has worked well but due to limitations in 2.6.35 and earlier kernels there were some problems. This patch is designed to address those issues. 1) iterate_supers_type() is not provided by all kernels which makes it impossible to safely iterate over all zpl_fs_type filesystems in a single callback. The most straight forward and portable way to resolve this is to register a callback per-filesystem during mount. The arc_*_prune_callback() functions have always supported multiple callbacks so this is functionally a very small change. 2) Commit 050d22b removed the non-portable shrink_dcache_memory() and shrink_icache_memory() functions and didn't replace them with equivalent functionality. This meant that for Linux 3.1 and older kernels the ARC had no mechanism to drop dentries and inodes from the caches if needed. This patch adds that missing functionality by calling shrink_dcache_parent() to release dentries which may be pinning inodes. This will result in all unused cache entries being dropped which is a bit heavy handed but it's the only interface available for old kernels. 3) A zpl_drop_inode() callback is registered for kernels older than 2.6.35 which do not support the .evict_inode callback. This ensures that when the last reference on an inode is dropped it is immediately removed from the cache. If this isn't done than inode can end up on the global unused LRU with no mechanism available to ZFS to drop them. Since the ARC buffers are not dropped the hottest inodes can still be recreated without performing disk IO. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Signed-off-by: Pavel Snajdr <snajpa@snajpa.net> Issue #3160
2015-03-17 22:07:47 +00:00
The number of dentries and inodes to be scanned looking for entries
which can be dropped. This may be required when the ARC reaches the
\fBzfs_arc_meta_limit\fR because dentries and inodes can pin buffers
in the ARC. Increasing this value will cause to dentry and inode caches
to be pruned more aggressively. Setting this value to 0 will disable
pruning the inode and dentry caches.
Document ZFS module parameters. This is a first draft of a zfs-module-parameters(5) man page. I have just extracted the parameter name and its description with modinfo, then checked the source what type it is and its default value. This will need more work, preferably someone that actually know these values and what to use them for. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1856
2013-11-16 06:52:54 +00:00
.sp
Restructure per-filesystem reclaim Originally when the ARC prune callback was introduced the idea was to register a single callback for the ZPL. The ARC could invoke this call back if it needed the ZPL to drop dentries, inodes, or other cache objects which might be pinning buffers in the ARC. The ZPL would iterate over all ZFS super blocks and perform the reclaim. For the most part this design has worked well but due to limitations in 2.6.35 and earlier kernels there were some problems. This patch is designed to address those issues. 1) iterate_supers_type() is not provided by all kernels which makes it impossible to safely iterate over all zpl_fs_type filesystems in a single callback. The most straight forward and portable way to resolve this is to register a callback per-filesystem during mount. The arc_*_prune_callback() functions have always supported multiple callbacks so this is functionally a very small change. 2) Commit 050d22b removed the non-portable shrink_dcache_memory() and shrink_icache_memory() functions and didn't replace them with equivalent functionality. This meant that for Linux 3.1 and older kernels the ARC had no mechanism to drop dentries and inodes from the caches if needed. This patch adds that missing functionality by calling shrink_dcache_parent() to release dentries which may be pinning inodes. This will result in all unused cache entries being dropped which is a bit heavy handed but it's the only interface available for old kernels. 3) A zpl_drop_inode() callback is registered for kernels older than 2.6.35 which do not support the .evict_inode callback. This ensures that when the last reference on an inode is dropped it is immediately removed from the cache. If this isn't done than inode can end up on the global unused LRU with no mechanism available to ZFS to drop them. Since the ARC buffers are not dropped the hottest inodes can still be recreated without performing disk IO. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Signed-off-by: Pavel Snajdr <snajpa@snajpa.net> Issue #3160
2015-03-17 22:07:47 +00:00
Default value: \fB10,000\fR.
Document ZFS module parameters. This is a first draft of a zfs-module-parameters(5) man page. I have just extracted the parameter name and its description with modinfo, then checked the source what type it is and its default value. This will need more work, preferably someone that actually know these values and what to use them for. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1856
2013-11-16 06:52:54 +00:00
.RE
.sp
.ne 2
.na
\fBzfs_arc_min\fR (ulong)
.ad
.RS 12n
Min arc size
.sp
Default value: \fB100\fR.
.RE
.sp
.ne 2
.na
\fBzfs_arc_min_prefetch_lifespan\fR (int)
.ad
.RS 12n
Min life of prefetch block
.sp
Default value: \fB100\fR.
.RE
Disable aggressive arc_p growth by default For specific workloads consisting mainly of mfu data and new anon data buffers, the aggressive growth of arc_p found in the arc_get_data_buf() function can have detrimental effects on the mfu list size and ghost list hit rate. Running a workload consisting of two processes: * Process 1 is creating many small files * Process 2 is tar'ing a directory consisting of many small files I've seen arc_p and the mru grow to their maximum size, while the mru ghost list receives 100K times fewer hits than the mfu ghost list. Ideally, as the mfu ghost list receives hits, arc_p should be driven down and the size of the mfu should increase. Given the specific workload I was testing with, the mfu list size should grow to a point where almost no mfu ghost list hits would occur. Unfortunately, this does not happen because the newly dirtied anon buffers constancy drive arc_p to its maximum value and keep it there (effectively prioritizing the mru list and starving the mfu list down to a negligible size). The logic to increment arc_p from within the arc_get_data_buf() function was introduced many years ago in this upstream commit: commit 641fbdae3a027d12b3c3dcd18927ccafae6d58bc Author: maybee <none@none> Date: Wed Dec 20 15:46:12 2006 -0800 6505658 target MRU size (arc.p) needs to be adjusted more aggressively and since I don't fully understand the motivation for the change, I am reluctant to completely remove it. As a way to test out how it's removal might affect performance, I've disabled that code by default, but left it tunable via a module option. Thus, if its removal is found to be grossly detrimental for certain workloads, it can be re-enabled on the fly, without a code change. Signed-off-by: Prakash Surya <surya1@llnl.gov> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Issue #2110
2013-12-11 17:40:13 +00:00
.sp
.ne 2
.na
\fBzfs_arc_p_aggressive_disable\fR (int)
.ad
.RS 12n
Disable aggressive arc_p growth
.sp
Use \fB1\fR for yes (default) and \fB0\fR to disable.
.RE
Disable arc_p adapt dampener by default It's unclear why adjustments to arc_p need to be dampened as they are in arc_adjust. With that said, it's removal significantly improves the arc's ability to "warm up" to a given workload. Thus, I'm disabling by default until its usefulness is better understood. Signed-off-by: Prakash Surya <surya1@llnl.gov> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Issue #2110
2014-01-03 18:36:26 +00:00
.sp
.ne 2
.na
\fBzfs_arc_p_dampener_disable\fR (int)
.ad
.RS 12n
Disable arc_p adapt dampener
.sp
Use \fB1\fR for yes (default) and \fB0\fR to disable.
.RE
Document ZFS module parameters. This is a first draft of a zfs-module-parameters(5) man page. I have just extracted the parameter name and its description with modinfo, then checked the source what type it is and its default value. This will need more work, preferably someone that actually know these values and what to use them for. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1856
2013-11-16 06:52:54 +00:00
.sp
.ne 2
.na
\fBzfs_arc_shrink_shift\fR (int)
.ad
.RS 12n
log2(fraction of arc to reclaim)
.sp
Default value: \fB5\fR.
.RE
.sp
.ne 2
.na
\fBzfs_autoimport_disable\fR (int)
.ad
.RS 12n
Expand the description of scan-related and other parameters. Document that the scan-related parameters are, in fact, applicable only to scrub and/or resilver operations as appropriate. Expand a few of the prefetch-related descriptions. Add clarification to other module parameters. Signed-off-by: Tim Chase <tim@chase2k.com> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #2361
2014-06-04 12:23:31 +00:00
Disable pool import at module load by ignoring the cache file (typically \fB/etc/zfs/zpool.cache\fR).
Document ZFS module parameters. This is a first draft of a zfs-module-parameters(5) man page. I have just extracted the parameter name and its description with modinfo, then checked the source what type it is and its default value. This will need more work, preferably someone that actually know these values and what to use them for. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1856
2013-11-16 06:52:54 +00:00
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE
.sp
.ne 2
.na
\fBzfs_dbuf_state_index\fR (int)
.ad
.RS 12n
Calculate arc header index
.sp
Default value: \fB0\fR.
.RE
.sp
.ne 2
.na
\fBzfs_deadman_enabled\fR (int)
.ad
.RS 12n
Enable deadman timer
.sp
Use \fB1\fR for yes (default) and \fB0\fR to disable.
.RE
.sp
.ne 2
.na
Illumos #4045 write throttle & i/o scheduler performance work 4045 zfs write throttle & i/o scheduler performance work 1. The ZFS i/o scheduler (vdev_queue.c) now divides i/os into 5 classes: sync read, sync write, async read, async write, and scrub/resilver. The scheduler issues a number of concurrent i/os from each class to the device. Once a class has been selected, an i/o is selected from this class using either an elevator algorithem (async, scrub classes) or FIFO (sync classes). The number of concurrent async write i/os is tuned dynamically based on i/o load, to achieve good sync i/o latency when there is not a high load of writes, and good write throughput when there is. See the block comment in vdev_queue.c (reproduced below) for more details. 2. The write throttle (dsl_pool_tempreserve_space() and txg_constrain_throughput()) is rewritten to produce much more consistent delays when under constant load. The new write throttle is based on the amount of dirty data, rather than guesses about future performance of the system. When there is a lot of dirty data, each transaction (e.g. write() syscall) will be delayed by the same small amount. This eliminates the "brick wall of wait" that the old write throttle could hit, causing all transactions to wait several seconds until the next txg opens. One of the keys to the new write throttle is decrementing the amount of dirty data as i/o completes, rather than at the end of spa_sync(). Note that the write throttle is only applied once the i/o scheduler is issuing the maximum number of outstanding async writes. See the block comments in dsl_pool.c and above dmu_tx_delay() (reproduced below) for more details. This diff has several other effects, including: * the commonly-tuned global variable zfs_vdev_max_pending has been removed; use per-class zfs_vdev_*_max_active values or zfs_vdev_max_active instead. * the size of each txg (meaning the amount of dirty data written, and thus the time it takes to write out) is now controlled differently. There is no longer an explicit time goal; the primary determinant is amount of dirty data. Systems that are under light or medium load will now often see that a txg is always syncing, but the impact to performance (e.g. read latency) is minimal. Tune zfs_dirty_data_max and zfs_dirty_data_sync to control this. * zio_taskq_batch_pct = 75 -- Only use 75% of all CPUs for compression, checksum, etc. This improves latency by not allowing these CPU-intensive tasks to consume all CPU (on machines with at least 4 CPU's; the percentage is rounded up). --matt APPENDIX: problems with the current i/o scheduler The current ZFS i/o scheduler (vdev_queue.c) is deadline based. The problem with this is that if there are always i/os pending, then certain classes of i/os can see very long delays. For example, if there are always synchronous reads outstanding, then no async writes will be serviced until they become "past due". One symptom of this situation is that each pass of the txg sync takes at least several seconds (typically 3 seconds). If many i/os become "past due" (their deadline is in the past), then we must service all of these overdue i/os before any new i/os. This happens when we enqueue a batch of async writes for the txg sync, with deadlines 2.5 seconds in the future. If we can't complete all the i/os in 2.5 seconds (e.g. because there were always reads pending), then these i/os will become past due. Now we must service all the "async" writes (which could be hundreds of megabytes) before we service any reads, introducing considerable latency to synchronous i/os (reads or ZIL writes). Notes on porting to ZFS on Linux: - zio_t gained new members io_physdone and io_phys_children. Because object caches in the Linux port call the constructor only once at allocation time, objects may contain residual data when retrieved from the cache. Therefore zio_create() was updated to zero out the two new fields. - vdev_mirror_pending() relied on the depth of the per-vdev pending queue (vq->vq_pending_tree) to select the least-busy leaf vdev to read from. This tree has been replaced by vq->vq_active_tree which is now used for the same purpose. - vdev_queue_init() used the value of zfs_vdev_max_pending to determine the number of vdev I/O buffers to pre-allocate. That global no longer exists, so we instead use the sum of the *_max_active values for each of the five I/O classes described above. - The Illumos implementation of dmu_tx_delay() delays a transaction by sleeping in condition variable embedded in the thread (curthread->t_delay_cv). We do not have an equivalent CV to use in Linux, so this change replaced the delay logic with a wrapper called zfs_sleep_until(). This wrapper could be adopted upstream and in other downstream ports to abstract away operating system-specific delay logic. - These tunables are added as module parameters, and descriptions added to the zfs-module-parameters.5 man page. spa_asize_inflation zfs_deadman_synctime_ms zfs_vdev_max_active zfs_vdev_async_write_active_min_dirty_percent zfs_vdev_async_write_active_max_dirty_percent zfs_vdev_async_read_max_active zfs_vdev_async_read_min_active zfs_vdev_async_write_max_active zfs_vdev_async_write_min_active zfs_vdev_scrub_max_active zfs_vdev_scrub_min_active zfs_vdev_sync_read_max_active zfs_vdev_sync_read_min_active zfs_vdev_sync_write_max_active zfs_vdev_sync_write_min_active zfs_dirty_data_max_percent zfs_delay_min_dirty_percent zfs_dirty_data_max_max_percent zfs_dirty_data_max zfs_dirty_data_max_max zfs_dirty_data_sync zfs_delay_scale The latter four have type unsigned long, whereas they are uint64_t in Illumos. This accommodates Linux's module_param() supported types, but means they may overflow on 32-bit architectures. The values zfs_dirty_data_max and zfs_dirty_data_max_max are the most likely to overflow on 32-bit systems, since they express physical RAM sizes in bytes. In fact, Illumos initializes zfs_dirty_data_max_max to 2^32 which does overflow. To resolve that, this port instead initializes it in arc_init() to 25% of physical RAM, and adds the tunable zfs_dirty_data_max_max_percent to override that percentage. While this solution doesn't completely avoid the overflow issue, it should be a reasonable default for most systems, and the minority of affected systems can work around the issue by overriding the defaults. - Fixed reversed logic in comment above zfs_delay_scale declaration. - Clarified comments in vdev_queue.c regarding when per-queue minimums take effect. - Replaced dmu_tx_write_limit in the dmu_tx kstat file with dmu_tx_dirty_delay and dmu_tx_dirty_over_max. The first counts how many times a transaction has been delayed because the pool dirty data has exceeded zfs_delay_min_dirty_percent. The latter counts how many times the pool dirty data has exceeded zfs_dirty_data_max (which we expect to never happen). - The original patch would have regressed the bug fixed in zfsonlinux/zfs@c418410, which prevented users from setting the zfs_vdev_aggregation_limit tuning larger than SPA_MAXBLOCKSIZE. A similar fix is added to vdev_queue_aggregate(). - In vdev_queue_io_to_issue(), dynamically allocate 'zio_t search' on the heap instead of the stack. In Linux we can't afford such large structures on the stack. Reviewed by: George Wilson <george.wilson@delphix.com> Reviewed by: Adam Leventhal <ahl@delphix.com> Reviewed by: Christopher Siden <christopher.siden@delphix.com> Reviewed by: Ned Bass <bass6@llnl.gov> Reviewed by: Brendan Gregg <brendan.gregg@joyent.com> Approved by: Robert Mustacchi <rm@joyent.com> References: http://www.illumos.org/issues/4045 illumos/illumos-gate@69962b5647e4a8b9b14998733b765925381b727e Ported-by: Ned Bass <bass6@llnl.gov> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1913
2013-08-29 03:01:20 +00:00
\fBzfs_deadman_synctime_ms\fR (ulong)
Document ZFS module parameters. This is a first draft of a zfs-module-parameters(5) man page. I have just extracted the parameter name and its description with modinfo, then checked the source what type it is and its default value. This will need more work, preferably someone that actually know these values and what to use them for. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1856
2013-11-16 06:52:54 +00:00
.ad
.RS 12n
Illumos #4045 write throttle & i/o scheduler performance work 4045 zfs write throttle & i/o scheduler performance work 1. The ZFS i/o scheduler (vdev_queue.c) now divides i/os into 5 classes: sync read, sync write, async read, async write, and scrub/resilver. The scheduler issues a number of concurrent i/os from each class to the device. Once a class has been selected, an i/o is selected from this class using either an elevator algorithem (async, scrub classes) or FIFO (sync classes). The number of concurrent async write i/os is tuned dynamically based on i/o load, to achieve good sync i/o latency when there is not a high load of writes, and good write throughput when there is. See the block comment in vdev_queue.c (reproduced below) for more details. 2. The write throttle (dsl_pool_tempreserve_space() and txg_constrain_throughput()) is rewritten to produce much more consistent delays when under constant load. The new write throttle is based on the amount of dirty data, rather than guesses about future performance of the system. When there is a lot of dirty data, each transaction (e.g. write() syscall) will be delayed by the same small amount. This eliminates the "brick wall of wait" that the old write throttle could hit, causing all transactions to wait several seconds until the next txg opens. One of the keys to the new write throttle is decrementing the amount of dirty data as i/o completes, rather than at the end of spa_sync(). Note that the write throttle is only applied once the i/o scheduler is issuing the maximum number of outstanding async writes. See the block comments in dsl_pool.c and above dmu_tx_delay() (reproduced below) for more details. This diff has several other effects, including: * the commonly-tuned global variable zfs_vdev_max_pending has been removed; use per-class zfs_vdev_*_max_active values or zfs_vdev_max_active instead. * the size of each txg (meaning the amount of dirty data written, and thus the time it takes to write out) is now controlled differently. There is no longer an explicit time goal; the primary determinant is amount of dirty data. Systems that are under light or medium load will now often see that a txg is always syncing, but the impact to performance (e.g. read latency) is minimal. Tune zfs_dirty_data_max and zfs_dirty_data_sync to control this. * zio_taskq_batch_pct = 75 -- Only use 75% of all CPUs for compression, checksum, etc. This improves latency by not allowing these CPU-intensive tasks to consume all CPU (on machines with at least 4 CPU's; the percentage is rounded up). --matt APPENDIX: problems with the current i/o scheduler The current ZFS i/o scheduler (vdev_queue.c) is deadline based. The problem with this is that if there are always i/os pending, then certain classes of i/os can see very long delays. For example, if there are always synchronous reads outstanding, then no async writes will be serviced until they become "past due". One symptom of this situation is that each pass of the txg sync takes at least several seconds (typically 3 seconds). If many i/os become "past due" (their deadline is in the past), then we must service all of these overdue i/os before any new i/os. This happens when we enqueue a batch of async writes for the txg sync, with deadlines 2.5 seconds in the future. If we can't complete all the i/os in 2.5 seconds (e.g. because there were always reads pending), then these i/os will become past due. Now we must service all the "async" writes (which could be hundreds of megabytes) before we service any reads, introducing considerable latency to synchronous i/os (reads or ZIL writes). Notes on porting to ZFS on Linux: - zio_t gained new members io_physdone and io_phys_children. Because object caches in the Linux port call the constructor only once at allocation time, objects may contain residual data when retrieved from the cache. Therefore zio_create() was updated to zero out the two new fields. - vdev_mirror_pending() relied on the depth of the per-vdev pending queue (vq->vq_pending_tree) to select the least-busy leaf vdev to read from. This tree has been replaced by vq->vq_active_tree which is now used for the same purpose. - vdev_queue_init() used the value of zfs_vdev_max_pending to determine the number of vdev I/O buffers to pre-allocate. That global no longer exists, so we instead use the sum of the *_max_active values for each of the five I/O classes described above. - The Illumos implementation of dmu_tx_delay() delays a transaction by sleeping in condition variable embedded in the thread (curthread->t_delay_cv). We do not have an equivalent CV to use in Linux, so this change replaced the delay logic with a wrapper called zfs_sleep_until(). This wrapper could be adopted upstream and in other downstream ports to abstract away operating system-specific delay logic. - These tunables are added as module parameters, and descriptions added to the zfs-module-parameters.5 man page. spa_asize_inflation zfs_deadman_synctime_ms zfs_vdev_max_active zfs_vdev_async_write_active_min_dirty_percent zfs_vdev_async_write_active_max_dirty_percent zfs_vdev_async_read_max_active zfs_vdev_async_read_min_active zfs_vdev_async_write_max_active zfs_vdev_async_write_min_active zfs_vdev_scrub_max_active zfs_vdev_scrub_min_active zfs_vdev_sync_read_max_active zfs_vdev_sync_read_min_active zfs_vdev_sync_write_max_active zfs_vdev_sync_write_min_active zfs_dirty_data_max_percent zfs_delay_min_dirty_percent zfs_dirty_data_max_max_percent zfs_dirty_data_max zfs_dirty_data_max_max zfs_dirty_data_sync zfs_delay_scale The latter four have type unsigned long, whereas they are uint64_t in Illumos. This accommodates Linux's module_param() supported types, but means they may overflow on 32-bit architectures. The values zfs_dirty_data_max and zfs_dirty_data_max_max are the most likely to overflow on 32-bit systems, since they express physical RAM sizes in bytes. In fact, Illumos initializes zfs_dirty_data_max_max to 2^32 which does overflow. To resolve that, this port instead initializes it in arc_init() to 25% of physical RAM, and adds the tunable zfs_dirty_data_max_max_percent to override that percentage. While this solution doesn't completely avoid the overflow issue, it should be a reasonable default for most systems, and the minority of affected systems can work around the issue by overriding the defaults. - Fixed reversed logic in comment above zfs_delay_scale declaration. - Clarified comments in vdev_queue.c regarding when per-queue minimums take effect. - Replaced dmu_tx_write_limit in the dmu_tx kstat file with dmu_tx_dirty_delay and dmu_tx_dirty_over_max. The first counts how many times a transaction has been delayed because the pool dirty data has exceeded zfs_delay_min_dirty_percent. The latter counts how many times the pool dirty data has exceeded zfs_dirty_data_max (which we expect to never happen). - The original patch would have regressed the bug fixed in zfsonlinux/zfs@c418410, which prevented users from setting the zfs_vdev_aggregation_limit tuning larger than SPA_MAXBLOCKSIZE. A similar fix is added to vdev_queue_aggregate(). - In vdev_queue_io_to_issue(), dynamically allocate 'zio_t search' on the heap instead of the stack. In Linux we can't afford such large structures on the stack. Reviewed by: George Wilson <george.wilson@delphix.com> Reviewed by: Adam Leventhal <ahl@delphix.com> Reviewed by: Christopher Siden <christopher.siden@delphix.com> Reviewed by: Ned Bass <bass6@llnl.gov> Reviewed by: Brendan Gregg <brendan.gregg@joyent.com> Approved by: Robert Mustacchi <rm@joyent.com> References: http://www.illumos.org/issues/4045 illumos/illumos-gate@69962b5647e4a8b9b14998733b765925381b727e Ported-by: Ned Bass <bass6@llnl.gov> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1913
2013-08-29 03:01:20 +00:00
Expiration time in milliseconds. This value has two meanings. First it is
used to determine when the spa_deadman() logic should fire. By default the
spa_deadman() will fire if spa_sync() has not completed in 1000 seconds.
Secondly, the value determines if an I/O is considered "hung". Any I/O that
has not completed in zfs_deadman_synctime_ms is considered "hung" resulting
in a zevent being logged.
Document ZFS module parameters. This is a first draft of a zfs-module-parameters(5) man page. I have just extracted the parameter name and its description with modinfo, then checked the source what type it is and its default value. This will need more work, preferably someone that actually know these values and what to use them for. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1856
2013-11-16 06:52:54 +00:00
.sp
Illumos #4045 write throttle & i/o scheduler performance work 4045 zfs write throttle & i/o scheduler performance work 1. The ZFS i/o scheduler (vdev_queue.c) now divides i/os into 5 classes: sync read, sync write, async read, async write, and scrub/resilver. The scheduler issues a number of concurrent i/os from each class to the device. Once a class has been selected, an i/o is selected from this class using either an elevator algorithem (async, scrub classes) or FIFO (sync classes). The number of concurrent async write i/os is tuned dynamically based on i/o load, to achieve good sync i/o latency when there is not a high load of writes, and good write throughput when there is. See the block comment in vdev_queue.c (reproduced below) for more details. 2. The write throttle (dsl_pool_tempreserve_space() and txg_constrain_throughput()) is rewritten to produce much more consistent delays when under constant load. The new write throttle is based on the amount of dirty data, rather than guesses about future performance of the system. When there is a lot of dirty data, each transaction (e.g. write() syscall) will be delayed by the same small amount. This eliminates the "brick wall of wait" that the old write throttle could hit, causing all transactions to wait several seconds until the next txg opens. One of the keys to the new write throttle is decrementing the amount of dirty data as i/o completes, rather than at the end of spa_sync(). Note that the write throttle is only applied once the i/o scheduler is issuing the maximum number of outstanding async writes. See the block comments in dsl_pool.c and above dmu_tx_delay() (reproduced below) for more details. This diff has several other effects, including: * the commonly-tuned global variable zfs_vdev_max_pending has been removed; use per-class zfs_vdev_*_max_active values or zfs_vdev_max_active instead. * the size of each txg (meaning the amount of dirty data written, and thus the time it takes to write out) is now controlled differently. There is no longer an explicit time goal; the primary determinant is amount of dirty data. Systems that are under light or medium load will now often see that a txg is always syncing, but the impact to performance (e.g. read latency) is minimal. Tune zfs_dirty_data_max and zfs_dirty_data_sync to control this. * zio_taskq_batch_pct = 75 -- Only use 75% of all CPUs for compression, checksum, etc. This improves latency by not allowing these CPU-intensive tasks to consume all CPU (on machines with at least 4 CPU's; the percentage is rounded up). --matt APPENDIX: problems with the current i/o scheduler The current ZFS i/o scheduler (vdev_queue.c) is deadline based. The problem with this is that if there are always i/os pending, then certain classes of i/os can see very long delays. For example, if there are always synchronous reads outstanding, then no async writes will be serviced until they become "past due". One symptom of this situation is that each pass of the txg sync takes at least several seconds (typically 3 seconds). If many i/os become "past due" (their deadline is in the past), then we must service all of these overdue i/os before any new i/os. This happens when we enqueue a batch of async writes for the txg sync, with deadlines 2.5 seconds in the future. If we can't complete all the i/os in 2.5 seconds (e.g. because there were always reads pending), then these i/os will become past due. Now we must service all the "async" writes (which could be hundreds of megabytes) before we service any reads, introducing considerable latency to synchronous i/os (reads or ZIL writes). Notes on porting to ZFS on Linux: - zio_t gained new members io_physdone and io_phys_children. Because object caches in the Linux port call the constructor only once at allocation time, objects may contain residual data when retrieved from the cache. Therefore zio_create() was updated to zero out the two new fields. - vdev_mirror_pending() relied on the depth of the per-vdev pending queue (vq->vq_pending_tree) to select the least-busy leaf vdev to read from. This tree has been replaced by vq->vq_active_tree which is now used for the same purpose. - vdev_queue_init() used the value of zfs_vdev_max_pending to determine the number of vdev I/O buffers to pre-allocate. That global no longer exists, so we instead use the sum of the *_max_active values for each of the five I/O classes described above. - The Illumos implementation of dmu_tx_delay() delays a transaction by sleeping in condition variable embedded in the thread (curthread->t_delay_cv). We do not have an equivalent CV to use in Linux, so this change replaced the delay logic with a wrapper called zfs_sleep_until(). This wrapper could be adopted upstream and in other downstream ports to abstract away operating system-specific delay logic. - These tunables are added as module parameters, and descriptions added to the zfs-module-parameters.5 man page. spa_asize_inflation zfs_deadman_synctime_ms zfs_vdev_max_active zfs_vdev_async_write_active_min_dirty_percent zfs_vdev_async_write_active_max_dirty_percent zfs_vdev_async_read_max_active zfs_vdev_async_read_min_active zfs_vdev_async_write_max_active zfs_vdev_async_write_min_active zfs_vdev_scrub_max_active zfs_vdev_scrub_min_active zfs_vdev_sync_read_max_active zfs_vdev_sync_read_min_active zfs_vdev_sync_write_max_active zfs_vdev_sync_write_min_active zfs_dirty_data_max_percent zfs_delay_min_dirty_percent zfs_dirty_data_max_max_percent zfs_dirty_data_max zfs_dirty_data_max_max zfs_dirty_data_sync zfs_delay_scale The latter four have type unsigned long, whereas they are uint64_t in Illumos. This accommodates Linux's module_param() supported types, but means they may overflow on 32-bit architectures. The values zfs_dirty_data_max and zfs_dirty_data_max_max are the most likely to overflow on 32-bit systems, since they express physical RAM sizes in bytes. In fact, Illumos initializes zfs_dirty_data_max_max to 2^32 which does overflow. To resolve that, this port instead initializes it in arc_init() to 25% of physical RAM, and adds the tunable zfs_dirty_data_max_max_percent to override that percentage. While this solution doesn't completely avoid the overflow issue, it should be a reasonable default for most systems, and the minority of affected systems can work around the issue by overriding the defaults. - Fixed reversed logic in comment above zfs_delay_scale declaration. - Clarified comments in vdev_queue.c regarding when per-queue minimums take effect. - Replaced dmu_tx_write_limit in the dmu_tx kstat file with dmu_tx_dirty_delay and dmu_tx_dirty_over_max. The first counts how many times a transaction has been delayed because the pool dirty data has exceeded zfs_delay_min_dirty_percent. The latter counts how many times the pool dirty data has exceeded zfs_dirty_data_max (which we expect to never happen). - The original patch would have regressed the bug fixed in zfsonlinux/zfs@c418410, which prevented users from setting the zfs_vdev_aggregation_limit tuning larger than SPA_MAXBLOCKSIZE. A similar fix is added to vdev_queue_aggregate(). - In vdev_queue_io_to_issue(), dynamically allocate 'zio_t search' on the heap instead of the stack. In Linux we can't afford such large structures on the stack. Reviewed by: George Wilson <george.wilson@delphix.com> Reviewed by: Adam Leventhal <ahl@delphix.com> Reviewed by: Christopher Siden <christopher.siden@delphix.com> Reviewed by: Ned Bass <bass6@llnl.gov> Reviewed by: Brendan Gregg <brendan.gregg@joyent.com> Approved by: Robert Mustacchi <rm@joyent.com> References: http://www.illumos.org/issues/4045 illumos/illumos-gate@69962b5647e4a8b9b14998733b765925381b727e Ported-by: Ned Bass <bass6@llnl.gov> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1913
2013-08-29 03:01:20 +00:00
Default value: \fB1,000,000\fR.
Document ZFS module parameters. This is a first draft of a zfs-module-parameters(5) man page. I have just extracted the parameter name and its description with modinfo, then checked the source what type it is and its default value. This will need more work, preferably someone that actually know these values and what to use them for. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1856
2013-11-16 06:52:54 +00:00
.RE
.sp
.ne 2
.na
\fBzfs_dedup_prefetch\fR (int)
.ad
.RS 12n
Enable prefetching dedup-ed blks
.sp
Change the default 'zfs_dedup_prefetch' value to '0' This gives a huge performance improvement in operations with deduped datasets especially when the bottleneck is the amount of ram available for zfs. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #2639
2014-08-30 02:13:26 +00:00
Use \fB1\fR for yes and \fB0\fR to disable (default).
Document ZFS module parameters. This is a first draft of a zfs-module-parameters(5) man page. I have just extracted the parameter name and its description with modinfo, then checked the source what type it is and its default value. This will need more work, preferably someone that actually know these values and what to use them for. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1856
2013-11-16 06:52:54 +00:00
.RE
Illumos #4045 write throttle & i/o scheduler performance work 4045 zfs write throttle & i/o scheduler performance work 1. The ZFS i/o scheduler (vdev_queue.c) now divides i/os into 5 classes: sync read, sync write, async read, async write, and scrub/resilver. The scheduler issues a number of concurrent i/os from each class to the device. Once a class has been selected, an i/o is selected from this class using either an elevator algorithem (async, scrub classes) or FIFO (sync classes). The number of concurrent async write i/os is tuned dynamically based on i/o load, to achieve good sync i/o latency when there is not a high load of writes, and good write throughput when there is. See the block comment in vdev_queue.c (reproduced below) for more details. 2. The write throttle (dsl_pool_tempreserve_space() and txg_constrain_throughput()) is rewritten to produce much more consistent delays when under constant load. The new write throttle is based on the amount of dirty data, rather than guesses about future performance of the system. When there is a lot of dirty data, each transaction (e.g. write() syscall) will be delayed by the same small amount. This eliminates the "brick wall of wait" that the old write throttle could hit, causing all transactions to wait several seconds until the next txg opens. One of the keys to the new write throttle is decrementing the amount of dirty data as i/o completes, rather than at the end of spa_sync(). Note that the write throttle is only applied once the i/o scheduler is issuing the maximum number of outstanding async writes. See the block comments in dsl_pool.c and above dmu_tx_delay() (reproduced below) for more details. This diff has several other effects, including: * the commonly-tuned global variable zfs_vdev_max_pending has been removed; use per-class zfs_vdev_*_max_active values or zfs_vdev_max_active instead. * the size of each txg (meaning the amount of dirty data written, and thus the time it takes to write out) is now controlled differently. There is no longer an explicit time goal; the primary determinant is amount of dirty data. Systems that are under light or medium load will now often see that a txg is always syncing, but the impact to performance (e.g. read latency) is minimal. Tune zfs_dirty_data_max and zfs_dirty_data_sync to control this. * zio_taskq_batch_pct = 75 -- Only use 75% of all CPUs for compression, checksum, etc. This improves latency by not allowing these CPU-intensive tasks to consume all CPU (on machines with at least 4 CPU's; the percentage is rounded up). --matt APPENDIX: problems with the current i/o scheduler The current ZFS i/o scheduler (vdev_queue.c) is deadline based. The problem with this is that if there are always i/os pending, then certain classes of i/os can see very long delays. For example, if there are always synchronous reads outstanding, then no async writes will be serviced until they become "past due". One symptom of this situation is that each pass of the txg sync takes at least several seconds (typically 3 seconds). If many i/os become "past due" (their deadline is in the past), then we must service all of these overdue i/os before any new i/os. This happens when we enqueue a batch of async writes for the txg sync, with deadlines 2.5 seconds in the future. If we can't complete all the i/os in 2.5 seconds (e.g. because there were always reads pending), then these i/os will become past due. Now we must service all the "async" writes (which could be hundreds of megabytes) before we service any reads, introducing considerable latency to synchronous i/os (reads or ZIL writes). Notes on porting to ZFS on Linux: - zio_t gained new members io_physdone and io_phys_children. Because object caches in the Linux port call the constructor only once at allocation time, objects may contain residual data when retrieved from the cache. Therefore zio_create() was updated to zero out the two new fields. - vdev_mirror_pending() relied on the depth of the per-vdev pending queue (vq->vq_pending_tree) to select the least-busy leaf vdev to read from. This tree has been replaced by vq->vq_active_tree which is now used for the same purpose. - vdev_queue_init() used the value of zfs_vdev_max_pending to determine the number of vdev I/O buffers to pre-allocate. That global no longer exists, so we instead use the sum of the *_max_active values for each of the five I/O classes described above. - The Illumos implementation of dmu_tx_delay() delays a transaction by sleeping in condition variable embedded in the thread (curthread->t_delay_cv). We do not have an equivalent CV to use in Linux, so this change replaced the delay logic with a wrapper called zfs_sleep_until(). This wrapper could be adopted upstream and in other downstream ports to abstract away operating system-specific delay logic. - These tunables are added as module parameters, and descriptions added to the zfs-module-parameters.5 man page. spa_asize_inflation zfs_deadman_synctime_ms zfs_vdev_max_active zfs_vdev_async_write_active_min_dirty_percent zfs_vdev_async_write_active_max_dirty_percent zfs_vdev_async_read_max_active zfs_vdev_async_read_min_active zfs_vdev_async_write_max_active zfs_vdev_async_write_min_active zfs_vdev_scrub_max_active zfs_vdev_scrub_min_active zfs_vdev_sync_read_max_active zfs_vdev_sync_read_min_active zfs_vdev_sync_write_max_active zfs_vdev_sync_write_min_active zfs_dirty_data_max_percent zfs_delay_min_dirty_percent zfs_dirty_data_max_max_percent zfs_dirty_data_max zfs_dirty_data_max_max zfs_dirty_data_sync zfs_delay_scale The latter four have type unsigned long, whereas they are uint64_t in Illumos. This accommodates Linux's module_param() supported types, but means they may overflow on 32-bit architectures. The values zfs_dirty_data_max and zfs_dirty_data_max_max are the most likely to overflow on 32-bit systems, since they express physical RAM sizes in bytes. In fact, Illumos initializes zfs_dirty_data_max_max to 2^32 which does overflow. To resolve that, this port instead initializes it in arc_init() to 25% of physical RAM, and adds the tunable zfs_dirty_data_max_max_percent to override that percentage. While this solution doesn't completely avoid the overflow issue, it should be a reasonable default for most systems, and the minority of affected systems can work around the issue by overriding the defaults. - Fixed reversed logic in comment above zfs_delay_scale declaration. - Clarified comments in vdev_queue.c regarding when per-queue minimums take effect. - Replaced dmu_tx_write_limit in the dmu_tx kstat file with dmu_tx_dirty_delay and dmu_tx_dirty_over_max. The first counts how many times a transaction has been delayed because the pool dirty data has exceeded zfs_delay_min_dirty_percent. The latter counts how many times the pool dirty data has exceeded zfs_dirty_data_max (which we expect to never happen). - The original patch would have regressed the bug fixed in zfsonlinux/zfs@c418410, which prevented users from setting the zfs_vdev_aggregation_limit tuning larger than SPA_MAXBLOCKSIZE. A similar fix is added to vdev_queue_aggregate(). - In vdev_queue_io_to_issue(), dynamically allocate 'zio_t search' on the heap instead of the stack. In Linux we can't afford such large structures on the stack. Reviewed by: George Wilson <george.wilson@delphix.com> Reviewed by: Adam Leventhal <ahl@delphix.com> Reviewed by: Christopher Siden <christopher.siden@delphix.com> Reviewed by: Ned Bass <bass6@llnl.gov> Reviewed by: Brendan Gregg <brendan.gregg@joyent.com> Approved by: Robert Mustacchi <rm@joyent.com> References: http://www.illumos.org/issues/4045 illumos/illumos-gate@69962b5647e4a8b9b14998733b765925381b727e Ported-by: Ned Bass <bass6@llnl.gov> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1913
2013-08-29 03:01:20 +00:00
.sp
.ne 2
.na
\fBzfs_delay_min_dirty_percent\fR (int)
.ad
.RS 12n
Start to delay each transaction once there is this amount of dirty data,
expressed as a percentage of \fBzfs_dirty_data_max\fR.
This value should be >= zfs_vdev_async_write_active_max_dirty_percent.
See the section "ZFS TRANSACTION DELAY".
.sp
Default value: \fB60\fR.
.RE
.sp
.ne 2
.na
\fBzfs_delay_scale\fR (int)
.ad
.RS 12n
This controls how quickly the transaction delay approaches infinity.
Larger values cause longer delays for a given amount of dirty data.
.sp
For the smoothest delay, this value should be about 1 billion divided
by the maximum number of operations per second. This will smoothly
handle between 10x and 1/10th this number.
.sp
See the section "ZFS TRANSACTION DELAY".
.sp
Note: \fBzfs_delay_scale\fR * \fBzfs_dirty_data_max\fR must be < 2^64.
.sp
Default value: \fB500,000\fR.
.RE
.sp
.ne 2
.na
\fBzfs_dirty_data_max\fR (int)
.ad
.RS 12n
Determines the dirty space limit in bytes. Once this limit is exceeded, new
writes are halted until space frees up. This parameter takes precedence
over \fBzfs_dirty_data_max_percent\fR.
See the section "ZFS TRANSACTION DELAY".
.sp
Default value: 10 percent of all memory, capped at \fBzfs_dirty_data_max_max\fR.
.RE
.sp
.ne 2
.na
\fBzfs_dirty_data_max_max\fR (int)
.ad
.RS 12n
Maximum allowable value of \fBzfs_dirty_data_max\fR, expressed in bytes.
This limit is only enforced at module load time, and will be ignored if
\fBzfs_dirty_data_max\fR is later changed. This parameter takes
precedence over \fBzfs_dirty_data_max_max_percent\fR. See the section
"ZFS TRANSACTION DELAY".
.sp
Default value: 25% of physical RAM.
.RE
.sp
.ne 2
.na
\fBzfs_dirty_data_max_max_percent\fR (int)
.ad
.RS 12n
Maximum allowable value of \fBzfs_dirty_data_max\fR, expressed as a
percentage of physical RAM. This limit is only enforced at module load
time, and will be ignored if \fBzfs_dirty_data_max\fR is later changed.
The parameter \fBzfs_dirty_data_max_max\fR takes precedence over this
one. See the section "ZFS TRANSACTION DELAY".
.sp
Default value: 25
.RE
.sp
.ne 2
.na
\fBzfs_dirty_data_max_percent\fR (int)
.ad
.RS 12n
Determines the dirty space limit, expressed as a percentage of all
memory. Once this limit is exceeded, new writes are halted until space frees
up. The parameter \fBzfs_dirty_data_max\fR takes precedence over this
one. See the section "ZFS TRANSACTION DELAY".
.sp
Default value: 10%, subject to \fBzfs_dirty_data_max_max\fR.
.RE
.sp
.ne 2
.na
\fBzfs_dirty_data_sync\fR (int)
.ad
.RS 12n
Start syncing out a transaction group if there is at least this much dirty data.
.sp
Default value: \fB67,108,864\fR.
.RE
Illumos 5138 - add tunable for maximum number of blocks freed in one txg Reviewed by: Adam Leventhal <adam.leventhal@delphix.com> Reviewed by: Mattew Ahrens <mahrens@delphix.com> Reviewed by: Josef 'Jeff' Sipek <jeffpc@josefsipek.net> Reviewed by: Richard Elling <richard.elling@gmail.com> Reviewed by: George Wilson <george.wilson@delphix.com> Approved by: Dan McDonald <danmcd@omniti.com> References: https://www.illumos.org/issues/5138 https://github.com/illumos/illumos-gate/commit/af3465d Porting notes: Because support for exposing a uint64_t parameter wasn't added until v3.17-rc1 the zfs_free_max_blocks variable has been declared as a unsigned long. This is already far larger than required and it allows us to avoid additional autoconf compatibility code. The default value has been set to 100,000 on Linux instead of ULONG_MAX which is used on Illumos. This was done to limit the number of outstanding IOs in the system when snapshots are destroyed. This helps ensure individual TXG sync times are kept reasonable and memory isn't wasted managing a huge backlog of outstanding IOs. Ported by: Turbo Fredriksson <turbo@bayour.com> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #2675 Closes #2581
2014-09-07 15:06:08 +00:00
.sp
.ne 2
.na
\fBzfs_free_max_blocks\fR (ulong)
.ad
.RS 12n
Maximum number of blocks freed in a single txg.
.sp
Default value: \fB100,000\fR.
.RE
Illumos #4045 write throttle & i/o scheduler performance work 4045 zfs write throttle & i/o scheduler performance work 1. The ZFS i/o scheduler (vdev_queue.c) now divides i/os into 5 classes: sync read, sync write, async read, async write, and scrub/resilver. The scheduler issues a number of concurrent i/os from each class to the device. Once a class has been selected, an i/o is selected from this class using either an elevator algorithem (async, scrub classes) or FIFO (sync classes). The number of concurrent async write i/os is tuned dynamically based on i/o load, to achieve good sync i/o latency when there is not a high load of writes, and good write throughput when there is. See the block comment in vdev_queue.c (reproduced below) for more details. 2. The write throttle (dsl_pool_tempreserve_space() and txg_constrain_throughput()) is rewritten to produce much more consistent delays when under constant load. The new write throttle is based on the amount of dirty data, rather than guesses about future performance of the system. When there is a lot of dirty data, each transaction (e.g. write() syscall) will be delayed by the same small amount. This eliminates the "brick wall of wait" that the old write throttle could hit, causing all transactions to wait several seconds until the next txg opens. One of the keys to the new write throttle is decrementing the amount of dirty data as i/o completes, rather than at the end of spa_sync(). Note that the write throttle is only applied once the i/o scheduler is issuing the maximum number of outstanding async writes. See the block comments in dsl_pool.c and above dmu_tx_delay() (reproduced below) for more details. This diff has several other effects, including: * the commonly-tuned global variable zfs_vdev_max_pending has been removed; use per-class zfs_vdev_*_max_active values or zfs_vdev_max_active instead. * the size of each txg (meaning the amount of dirty data written, and thus the time it takes to write out) is now controlled differently. There is no longer an explicit time goal; the primary determinant is amount of dirty data. Systems that are under light or medium load will now often see that a txg is always syncing, but the impact to performance (e.g. read latency) is minimal. Tune zfs_dirty_data_max and zfs_dirty_data_sync to control this. * zio_taskq_batch_pct = 75 -- Only use 75% of all CPUs for compression, checksum, etc. This improves latency by not allowing these CPU-intensive tasks to consume all CPU (on machines with at least 4 CPU's; the percentage is rounded up). --matt APPENDIX: problems with the current i/o scheduler The current ZFS i/o scheduler (vdev_queue.c) is deadline based. The problem with this is that if there are always i/os pending, then certain classes of i/os can see very long delays. For example, if there are always synchronous reads outstanding, then no async writes will be serviced until they become "past due". One symptom of this situation is that each pass of the txg sync takes at least several seconds (typically 3 seconds). If many i/os become "past due" (their deadline is in the past), then we must service all of these overdue i/os before any new i/os. This happens when we enqueue a batch of async writes for the txg sync, with deadlines 2.5 seconds in the future. If we can't complete all the i/os in 2.5 seconds (e.g. because there were always reads pending), then these i/os will become past due. Now we must service all the "async" writes (which could be hundreds of megabytes) before we service any reads, introducing considerable latency to synchronous i/os (reads or ZIL writes). Notes on porting to ZFS on Linux: - zio_t gained new members io_physdone and io_phys_children. Because object caches in the Linux port call the constructor only once at allocation time, objects may contain residual data when retrieved from the cache. Therefore zio_create() was updated to zero out the two new fields. - vdev_mirror_pending() relied on the depth of the per-vdev pending queue (vq->vq_pending_tree) to select the least-busy leaf vdev to read from. This tree has been replaced by vq->vq_active_tree which is now used for the same purpose. - vdev_queue_init() used the value of zfs_vdev_max_pending to determine the number of vdev I/O buffers to pre-allocate. That global no longer exists, so we instead use the sum of the *_max_active values for each of the five I/O classes described above. - The Illumos implementation of dmu_tx_delay() delays a transaction by sleeping in condition variable embedded in the thread (curthread->t_delay_cv). We do not have an equivalent CV to use in Linux, so this change replaced the delay logic with a wrapper called zfs_sleep_until(). This wrapper could be adopted upstream and in other downstream ports to abstract away operating system-specific delay logic. - These tunables are added as module parameters, and descriptions added to the zfs-module-parameters.5 man page. spa_asize_inflation zfs_deadman_synctime_ms zfs_vdev_max_active zfs_vdev_async_write_active_min_dirty_percent zfs_vdev_async_write_active_max_dirty_percent zfs_vdev_async_read_max_active zfs_vdev_async_read_min_active zfs_vdev_async_write_max_active zfs_vdev_async_write_min_active zfs_vdev_scrub_max_active zfs_vdev_scrub_min_active zfs_vdev_sync_read_max_active zfs_vdev_sync_read_min_active zfs_vdev_sync_write_max_active zfs_vdev_sync_write_min_active zfs_dirty_data_max_percent zfs_delay_min_dirty_percent zfs_dirty_data_max_max_percent zfs_dirty_data_max zfs_dirty_data_max_max zfs_dirty_data_sync zfs_delay_scale The latter four have type unsigned long, whereas they are uint64_t in Illumos. This accommodates Linux's module_param() supported types, but means they may overflow on 32-bit architectures. The values zfs_dirty_data_max and zfs_dirty_data_max_max are the most likely to overflow on 32-bit systems, since they express physical RAM sizes in bytes. In fact, Illumos initializes zfs_dirty_data_max_max to 2^32 which does overflow. To resolve that, this port instead initializes it in arc_init() to 25% of physical RAM, and adds the tunable zfs_dirty_data_max_max_percent to override that percentage. While this solution doesn't completely avoid the overflow issue, it should be a reasonable default for most systems, and the minority of affected systems can work around the issue by overriding the defaults. - Fixed reversed logic in comment above zfs_delay_scale declaration. - Clarified comments in vdev_queue.c regarding when per-queue minimums take effect. - Replaced dmu_tx_write_limit in the dmu_tx kstat file with dmu_tx_dirty_delay and dmu_tx_dirty_over_max. The first counts how many times a transaction has been delayed because the pool dirty data has exceeded zfs_delay_min_dirty_percent. The latter counts how many times the pool dirty data has exceeded zfs_dirty_data_max (which we expect to never happen). - The original patch would have regressed the bug fixed in zfsonlinux/zfs@c418410, which prevented users from setting the zfs_vdev_aggregation_limit tuning larger than SPA_MAXBLOCKSIZE. A similar fix is added to vdev_queue_aggregate(). - In vdev_queue_io_to_issue(), dynamically allocate 'zio_t search' on the heap instead of the stack. In Linux we can't afford such large structures on the stack. Reviewed by: George Wilson <george.wilson@delphix.com> Reviewed by: Adam Leventhal <ahl@delphix.com> Reviewed by: Christopher Siden <christopher.siden@delphix.com> Reviewed by: Ned Bass <bass6@llnl.gov> Reviewed by: Brendan Gregg <brendan.gregg@joyent.com> Approved by: Robert Mustacchi <rm@joyent.com> References: http://www.illumos.org/issues/4045 illumos/illumos-gate@69962b5647e4a8b9b14998733b765925381b727e Ported-by: Ned Bass <bass6@llnl.gov> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1913
2013-08-29 03:01:20 +00:00
.sp
.ne 2
.na
\fBzfs_vdev_async_read_max_active\fR (int)
.ad
.RS 12n
Maxium asynchronous read I/Os active to each device.
See the section "ZFS I/O SCHEDULER".
.sp
Default value: \fB3\fR.
.RE
.sp
.ne 2
.na
\fBzfs_vdev_async_read_min_active\fR (int)
.ad
.RS 12n
Minimum asynchronous read I/Os active to each device.
See the section "ZFS I/O SCHEDULER".
.sp
Default value: \fB1\fR.
.RE
.sp
.ne 2
.na
\fBzfs_vdev_async_write_active_max_dirty_percent\fR (int)
.ad
.RS 12n
When the pool has more than
\fBzfs_vdev_async_write_active_max_dirty_percent\fR dirty data, use
\fBzfs_vdev_async_write_max_active\fR to limit active async writes. If
the dirty data is between min and max, the active I/O limit is linearly
interpolated. See the section "ZFS I/O SCHEDULER".
.sp
Default value: \fB60\fR.
.RE
.sp
.ne 2
.na
\fBzfs_vdev_async_write_active_min_dirty_percent\fR (int)
.ad
.RS 12n
When the pool has less than
\fBzfs_vdev_async_write_active_min_dirty_percent\fR dirty data, use
\fBzfs_vdev_async_write_min_active\fR to limit active async writes. If
the dirty data is between min and max, the active I/O limit is linearly
interpolated. See the section "ZFS I/O SCHEDULER".
.sp
Default value: \fB30\fR.
.RE
.sp
.ne 2
.na
\fBzfs_vdev_async_write_max_active\fR (int)
.ad
.RS 12n
Maxium asynchronous write I/Os active to each device.
See the section "ZFS I/O SCHEDULER".
.sp
Default value: \fB10\fR.
.RE
.sp
.ne 2
.na
\fBzfs_vdev_async_write_min_active\fR (int)
.ad
.RS 12n
Minimum asynchronous write I/Os active to each device.
See the section "ZFS I/O SCHEDULER".
.sp
Default value: \fB1\fR.
.RE
.sp
.ne 2
.na
\fBzfs_vdev_max_active\fR (int)
.ad
.RS 12n
The maximum number of I/Os active to each device. Ideally, this will be >=
the sum of each queue's max_active. It must be at least the sum of each
queue's min_active. See the section "ZFS I/O SCHEDULER".
.sp
Default value: \fB1,000\fR.
.RE
.sp
.ne 2
.na
\fBzfs_vdev_scrub_max_active\fR (int)
.ad
.RS 12n
Maxium scrub I/Os active to each device.
See the section "ZFS I/O SCHEDULER".
.sp
Default value: \fB2\fR.
.RE
.sp
.ne 2
.na
\fBzfs_vdev_scrub_min_active\fR (int)
.ad
.RS 12n
Minimum scrub I/Os active to each device.
See the section "ZFS I/O SCHEDULER".
.sp
Default value: \fB1\fR.
.RE
.sp
.ne 2
.na
\fBzfs_vdev_sync_read_max_active\fR (int)
.ad
.RS 12n
Maxium synchronous read I/Os active to each device.
See the section "ZFS I/O SCHEDULER".
.sp
Default value: \fB10\fR.
.RE
.sp
.ne 2
.na
\fBzfs_vdev_sync_read_min_active\fR (int)
.ad
.RS 12n
Minimum synchronous read I/Os active to each device.
See the section "ZFS I/O SCHEDULER".
.sp
Default value: \fB10\fR.
.RE
.sp
.ne 2
.na
\fBzfs_vdev_sync_write_max_active\fR (int)
.ad
.RS 12n
Maxium synchronous write I/Os active to each device.
See the section "ZFS I/O SCHEDULER".
.sp
Default value: \fB10\fR.
.RE
.sp
.ne 2
.na
\fBzfs_vdev_sync_write_min_active\fR (int)
.ad
.RS 12n
Minimum synchronous write I/Os active to each device.
See the section "ZFS I/O SCHEDULER".
.sp
Default value: \fB10\fR.
.RE
Document ZFS module parameters. This is a first draft of a zfs-module-parameters(5) man page. I have just extracted the parameter name and its description with modinfo, then checked the source what type it is and its default value. This will need more work, preferably someone that actually know these values and what to use them for. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1856
2013-11-16 06:52:54 +00:00
.sp
.ne 2
.na
\fBzfs_disable_dup_eviction\fR (int)
.ad
.RS 12n
Disable duplicate buffer eviction
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE
.sp
.ne 2
.na
\fBzfs_expire_snapshot\fR (int)
.ad
.RS 12n
Seconds to expire .zfs/snapshot
.sp
Default value: \fB300\fR.
.RE
.sp
.ne 2
.na
\fBzfs_flags\fR (int)
.ad
.RS 12n
Document zfs_flags module parameter Add a table describing the debugging flags that can be set in the zfs_flags module parameter. Also change the module_param type to 'uint' so users aren't shown a negative value. The updated man page text is reproduced below for convenience. zfs_flags (int) Set additional debugging flags. The following flags may be bitwise-or'd together. +-------------------------------------------------------+ |Value Symbolic Name | | Description | +-------------------------------------------------------+ | 1 ZFS_DEBUG_DPRINTF | | Enable dprintf entries in the debug log. | +-------------------------------------------------------+ | 2 ZFS_DEBUG_DBUF_VERIFY * | | Enable extra dbuf verifications. | +-------------------------------------------------------+ | 4 ZFS_DEBUG_DNODE_VERIFY * | | Enable extra dnode verifications. | +-------------------------------------------------------+ | 8 ZFS_DEBUG_SNAPNAMES | | Enable snapshot name verification. | +-------------------------------------------------------+ | 16 ZFS_DEBUG_MODIFY | | Check for illegally modified ARC buffers. | +-------------------------------------------------------+ | 32 ZFS_DEBUG_SPA | | Enable spa_dbgmsg entries in the debug log. | +-------------------------------------------------------+ | 64 ZFS_DEBUG_ZIO_FREE | | Enable verification of block frees. | +-------------------------------------------------------+ | 128 ZFS_DEBUG_HISTOGRAM_VERIFY | | Enable extra spacemap histogram verifications. | +-------------------------------------------------------+ * Requires debug build. Default value: 0. Signed-off-by: Ned Bass <bass6@llnl.gov> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #2988
2014-12-23 00:54:43 +00:00
Set additional debugging flags. The following flags may be bitwise-or'd
together.
.sp
.TS
box;
rB lB
lB lB
r l.
Value Symbolic Name
Description
_
1 ZFS_DEBUG_DPRINTF
Enable dprintf entries in the debug log.
_
2 ZFS_DEBUG_DBUF_VERIFY *
Enable extra dbuf verifications.
_
4 ZFS_DEBUG_DNODE_VERIFY *
Enable extra dnode verifications.
_
8 ZFS_DEBUG_SNAPNAMES
Enable snapshot name verification.
_
16 ZFS_DEBUG_MODIFY
Check for illegally modified ARC buffers.
_
32 ZFS_DEBUG_SPA
Enable spa_dbgmsg entries in the debug log.
_
64 ZFS_DEBUG_ZIO_FREE
Enable verification of block frees.
_
128 ZFS_DEBUG_HISTOGRAM_VERIFY
Enable extra spacemap histogram verifications.
.TE
.sp
* Requires debug build.
Document ZFS module parameters. This is a first draft of a zfs-module-parameters(5) man page. I have just extracted the parameter name and its description with modinfo, then checked the source what type it is and its default value. This will need more work, preferably someone that actually know these values and what to use them for. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1856
2013-11-16 06:52:54 +00:00
.sp
Document zfs_flags module parameter Add a table describing the debugging flags that can be set in the zfs_flags module parameter. Also change the module_param type to 'uint' so users aren't shown a negative value. The updated man page text is reproduced below for convenience. zfs_flags (int) Set additional debugging flags. The following flags may be bitwise-or'd together. +-------------------------------------------------------+ |Value Symbolic Name | | Description | +-------------------------------------------------------+ | 1 ZFS_DEBUG_DPRINTF | | Enable dprintf entries in the debug log. | +-------------------------------------------------------+ | 2 ZFS_DEBUG_DBUF_VERIFY * | | Enable extra dbuf verifications. | +-------------------------------------------------------+ | 4 ZFS_DEBUG_DNODE_VERIFY * | | Enable extra dnode verifications. | +-------------------------------------------------------+ | 8 ZFS_DEBUG_SNAPNAMES | | Enable snapshot name verification. | +-------------------------------------------------------+ | 16 ZFS_DEBUG_MODIFY | | Check for illegally modified ARC buffers. | +-------------------------------------------------------+ | 32 ZFS_DEBUG_SPA | | Enable spa_dbgmsg entries in the debug log. | +-------------------------------------------------------+ | 64 ZFS_DEBUG_ZIO_FREE | | Enable verification of block frees. | +-------------------------------------------------------+ | 128 ZFS_DEBUG_HISTOGRAM_VERIFY | | Enable extra spacemap histogram verifications. | +-------------------------------------------------------+ * Requires debug build. Default value: 0. Signed-off-by: Ned Bass <bass6@llnl.gov> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #2988
2014-12-23 00:54:43 +00:00
Default value: \fB0\fR.
Document ZFS module parameters. This is a first draft of a zfs-module-parameters(5) man page. I have just extracted the parameter name and its description with modinfo, then checked the source what type it is and its default value. This will need more work, preferably someone that actually know these values and what to use them for. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1856
2013-11-16 06:52:54 +00:00
.RE
Illumos 4390 - I/O errors can corrupt space map when deleting fs/vol 4390 i/o errors when deleting filesystem/zvol can lead to space map corruption Reviewed by: George Wilson <george.wilson@delphix.com> Reviewed by: Christopher Siden <christopher.siden@delphix.com> Reviewed by: Adam Leventhal <ahl@delphix.com> Reviewed by: Dan McDonald <danmcd@omniti.com> Reviewed by: Saso Kiselkov <saso.kiselkov@nexenta.com> Approved by: Dan McDonald <danmcd@omniti.com> References: https://www.illumos.org/issues/4390 https://github.com/illumos/illumos-gate/commit/7fd05ac Porting notes: Previous stack-reduction efforts in traverse_visitb() caused a fair number of un-mergable pieces of code. This patch should reduce its stack footprint a bit more. The new local bptree_entry_phys_t in bptree_add() is dynamically-allocated using kmem_zalloc() for the purpose of stack reduction. The new global zfs_free_leak_on_eio has been defined as an integer rather than a boolean_t as was the case with the related zfs_recover global. Also, zfs_free_leak_on_eio's definition has been inserted into zfs_debug.c for consistency with the existing definition of zfs_recover. Illumos placed it in spa_misc.c. Ported by: Tim Chase <tim@chase2k.com> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #2545
2014-06-05 21:20:08 +00:00
.sp
.ne 2
.na
\fBzfs_free_leak_on_eio\fR (int)
.ad
.RS 12n
If destroy encounters an EIO while reading metadata (e.g. indirect
blocks), space referenced by the missing metadata can not be freed.
Normally this causes the background destroy to become "stalled", as
it is unable to make forward progress. While in this stalled state,
all remaining space to free from the error-encountering filesystem is
"temporarily leaked". Set this flag to cause it to ignore the EIO,
permanently leak the space from indirect blocks that can not be read,
and continue to free everything else that it can.
The default, "stalling" behavior is useful if the storage partially
fails (i.e. some but not all i/os fail), and then later recovers. In
this case, we will be able to continue pool operations while it is
partially failed, and when it recovers, we can continue to free the
space, with no leaks. However, note that this case is actually
fairly rare.
Typically pools either (a) fail completely (but perhaps temporarily,
e.g. a top-level vdev going offline), or (b) have localized,
permanent errors (e.g. disk returns the wrong data due to bit flip or
firmware bug). In case (a), this setting does not matter because the
pool will be suspended and the sync thread will not be able to make
forward progress regardless. In case (b), because the error is
permanent, the best we can do is leak the minimum amount of space,
which is what setting this flag will do. Therefore, it is reasonable
for this flag to normally be set, but we chose the more conservative
approach of not setting it, so that there is no possibility of
leaking space in the "partial temporary" failure case.
.sp
Default value: \fB0\fR.
.RE
Document ZFS module parameters. This is a first draft of a zfs-module-parameters(5) man page. I have just extracted the parameter name and its description with modinfo, then checked the source what type it is and its default value. This will need more work, preferably someone that actually know these values and what to use them for. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1856
2013-11-16 06:52:54 +00:00
.sp
.ne 2
.na
\fBzfs_free_min_time_ms\fR (int)
.ad
.RS 12n
Min millisecs to free per txg
.sp
Default value: \fB1,000\fR.
.RE
.sp
.ne 2
.na
\fBzfs_immediate_write_sz\fR (long)
.ad
.RS 12n
Largest data block to write to zil
.sp
Default value: \fB32,768\fR.
.RE
.sp
.ne 2
.na
\fBzfs_mdcomp_disable\fR (int)
.ad
.RS 12n
Disable meta data compression
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE
Illumos 4976-4984 - metaslab improvements 4976 zfs should only avoid writing to a failing non-redundant top-level vdev 4978 ztest fails in get_metaslab_refcount() 4979 extend free space histogram to device and pool 4980 metaslabs should have a fragmentation metric 4981 remove fragmented ops vector from block allocator 4982 space_map object should proactively upgrade when feature is enabled 4983 need to collect metaslab information via mdb 4984 device selection should use fragmentation metric Reviewed by: Matthew Ahrens <mahrens@delphix.com> Reviewed by: Adam Leventhal <adam.leventhal@delphix.com> Reviewed by: Christopher Siden <christopher.siden@delphix.com> Approved by: Garrett D'Amore <garrett@damore.org> References: https://www.illumos.org/issues/4976 https://www.illumos.org/issues/4978 https://www.illumos.org/issues/4979 https://www.illumos.org/issues/4980 https://www.illumos.org/issues/4981 https://www.illumos.org/issues/4982 https://www.illumos.org/issues/4983 https://www.illumos.org/issues/4984 https://github.com/illumos/illumos-gate/commit/2e4c998 Notes: The "zdb -M" option has been re-tasked to display the new metaslab fragmentation metric and the new "zdb -I" option is used to control the maximum number of in-flight I/Os. The new fragmentation metric is derived from the space map histogram which has been rolled up to the vdev and pool level and is presented to the user via "zpool list". Add a number of module parameters related to the new metaslab weighting logic. Ported by: Tim Chase <tim@chase2k.com> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #2595
2014-07-19 20:19:24 +00:00
.sp
.ne 2
.na
\fBzfs_metaslab_fragmentation_threshold\fR (int)
.ad
.RS 12n
Allow metaslabs to keep their active state as long as their fragmentation
percentage is less than or equal to this value. An active metaslab that
exceeds this threshold will no longer keep its active status allowing
better metaslabs to be selected.
.sp
Default value: \fB70\fR.
.RE
.sp
.ne 2
.na
\fBzfs_mg_fragmentation_threshold\fR (int)
.ad
.RS 12n
Metaslab groups are considered eligible for allocations if their
fragmenation metric (measured as a percentage) is less than or equal to
this value. If a metaslab group exceeds this threshold then it will be
skipped unless all metaslab groups within the metaslab class have also
crossed this threshold.
.sp
Default value: \fB85\fR.
.RE
Convert zfs_mg_noalloc_threshold to a module parameter and document The parameter was added as illumos issue 4081 which was committed to zfsonlinux in ac72fac3eaa569902cad88053167f7d74e7fe7e4. This patch documents the parameter and allows for it to be set as a module parameter. Signed-off-by: Tim Chase <tim@chase2k.com> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #2483
2014-07-10 03:36:03 +00:00
.sp
.ne 2
.na
\fBzfs_mg_noalloc_threshold\fR (int)
.ad
.RS 12n
Defines a threshold at which metaslab groups should be eligible for
allocations. The value is expressed as a percentage of free space
beyond which a metaslab group is always eligible for allocations.
If a metaslab group's free space is less than or equal to the
the threshold, the allocator will avoid allocating to that group
unless all groups in the pool have reached the threshold. Once all
groups have reached the threshold, all groups are allowed to accept
allocations. The default value of 0 disables the feature and causes
all metaslab groups to be eligible for allocations.
This parameter allows to deal with pools having heavily imbalanced
vdevs such as would be the case when a new vdev has been added.
Setting the threshold to a non-zero percentage will stop allocations
from being made to vdevs that aren't filled to the specified percentage
and allow lesser filled vdevs to acquire more allocations than they
otherwise would under the old \fBzfs_mg_alloc_failures\fR facility.
.sp
Default value: \fB0\fR.
.RE
Document ZFS module parameters. This is a first draft of a zfs-module-parameters(5) man page. I have just extracted the parameter name and its description with modinfo, then checked the source what type it is and its default value. This will need more work, preferably someone that actually know these values and what to use them for. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1856
2013-11-16 06:52:54 +00:00
.sp
.ne 2
.na
\fBzfs_no_scrub_io\fR (int)
.ad
.RS 12n
Set for no scrub I/O
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE
.sp
.ne 2
.na
\fBzfs_no_scrub_prefetch\fR (int)
.ad
.RS 12n
Set for no scrub prefetching
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE
.sp
.ne 2
.na
\fBzfs_nocacheflush\fR (int)
.ad
.RS 12n
Disable cache flushes
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE
.sp
.ne 2
.na
\fBzfs_nopwrite_enabled\fR (int)
.ad
.RS 12n
Enable NOP writes
.sp
Use \fB1\fR for yes (default) and \fB0\fR to disable.
.RE
.sp
.ne 2
.na
\fBzfs_pd_blks_max\fR (int)
.ad
.RS 12n
Max number of blocks to prefetch
.sp
Default value: \fB100\fR.
.RE
.sp
.ne 2
.na
\fBzfs_prefetch_disable\fR (int)
.ad
.RS 12n
Disable all ZFS prefetching
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE
.sp
.ne 2
.na
\fBzfs_read_chunk_size\fR (long)
.ad
.RS 12n
Bytes to read per chunk
.sp
Default value: \fB1,048,576\fR.
.RE
.sp
.ne 2
.na
\fBzfs_read_history\fR (int)
.ad
.RS 12n
Historic statistics for the last N reads
.sp
Default value: \fB0\fR.
.RE
.sp
.ne 2
.na
\fBzfs_read_history_hits\fR (int)
.ad
.RS 12n
Include cache hits in read history
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE
.sp
.ne 2
.na
\fBzfs_recover\fR (int)
.ad
.RS 12n
Set to attempt to recover from fatal errors. This should only be used as a
last resort, as it typically results in leaked space, or worse.
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE
.sp
.ne 2
.na
\fBzfs_resilver_delay\fR (int)
.ad
.RS 12n
Expand the description of scan-related and other parameters. Document that the scan-related parameters are, in fact, applicable only to scrub and/or resilver operations as appropriate. Expand a few of the prefetch-related descriptions. Add clarification to other module parameters. Signed-off-by: Tim Chase <tim@chase2k.com> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #2361
2014-06-04 12:23:31 +00:00
Number of ticks to delay prior to issuing a resilver I/O operation when
a non-resilver or non-scrub I/O operation has occurred within the past
\fBzfs_scan_idle\fR ticks.
Document ZFS module parameters. This is a first draft of a zfs-module-parameters(5) man page. I have just extracted the parameter name and its description with modinfo, then checked the source what type it is and its default value. This will need more work, preferably someone that actually know these values and what to use them for. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1856
2013-11-16 06:52:54 +00:00
.sp
Default value: \fB2\fR.
.RE
.sp
.ne 2
.na
\fBzfs_resilver_min_time_ms\fR (int)
.ad
.RS 12n
Min millisecs to resilver per txg
.sp
Default value: \fB3,000\fR.
.RE
.sp
.ne 2
.na
\fBzfs_scan_idle\fR (int)
.ad
.RS 12n
Expand the description of scan-related and other parameters. Document that the scan-related parameters are, in fact, applicable only to scrub and/or resilver operations as appropriate. Expand a few of the prefetch-related descriptions. Add clarification to other module parameters. Signed-off-by: Tim Chase <tim@chase2k.com> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #2361
2014-06-04 12:23:31 +00:00
Idle window in clock ticks. During a scrub or a resilver, if
a non-scrub or non-resilver I/O operation has occurred during this
window, the next scrub or resilver operation is delayed by, respectively
\fBzfs_scrub_delay\fR or \fBzfs_resilver_delay\fR ticks.
Document ZFS module parameters. This is a first draft of a zfs-module-parameters(5) man page. I have just extracted the parameter name and its description with modinfo, then checked the source what type it is and its default value. This will need more work, preferably someone that actually know these values and what to use them for. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1856
2013-11-16 06:52:54 +00:00
.sp
Default value: \fB50\fR.
.RE
.sp
.ne 2
.na
\fBzfs_scan_min_time_ms\fR (int)
.ad
.RS 12n
Min millisecs to scrub per txg
.sp
Default value: \fB1,000\fR.
.RE
.sp
.ne 2
.na
\fBzfs_scrub_delay\fR (int)
.ad
.RS 12n
Expand the description of scan-related and other parameters. Document that the scan-related parameters are, in fact, applicable only to scrub and/or resilver operations as appropriate. Expand a few of the prefetch-related descriptions. Add clarification to other module parameters. Signed-off-by: Tim Chase <tim@chase2k.com> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #2361
2014-06-04 12:23:31 +00:00
Number of ticks to delay prior to issuing a scrub I/O operation when
a non-scrub or non-resilver I/O operation has occurred within the past
\fBzfs_scan_idle\fR ticks.
Document ZFS module parameters. This is a first draft of a zfs-module-parameters(5) man page. I have just extracted the parameter name and its description with modinfo, then checked the source what type it is and its default value. This will need more work, preferably someone that actually know these values and what to use them for. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1856
2013-11-16 06:52:54 +00:00
.sp
Default value: \fB4\fR.
.RE
Add zfs_send_corrupt_data module option Tuning setting to ignore read/checksum errors when sending data. Signed-off-by: Turbo Fredriksson <turbo@bayour.com> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1982 Issue #1897
2013-12-17 21:53:52 +00:00
.sp
.ne 2
.na
\fBzfs_send_corrupt_data\fR (int)
.ad
.RS 12n
Allow to send corrupt data (ignore read/checksum errors when sending data)
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE
Document ZFS module parameters. This is a first draft of a zfs-module-parameters(5) man page. I have just extracted the parameter name and its description with modinfo, then checked the source what type it is and its default value. This will need more work, preferably someone that actually know these values and what to use them for. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1856
2013-11-16 06:52:54 +00:00
.sp
.ne 2
.na
\fBzfs_sync_pass_deferred_free\fR (int)
.ad
.RS 12n
Defer frees starting in this pass
.sp
Default value: \fB2\fR.
.RE
.sp
.ne 2
.na
\fBzfs_sync_pass_dont_compress\fR (int)
.ad
.RS 12n
Don't compress starting in this pass
.sp
Default value: \fB5\fR.
.RE
.sp
.ne 2
.na
\fBzfs_sync_pass_rewrite\fR (int)
.ad
.RS 12n
Rewrite new bps starting in this pass
.sp
Default value: \fB2\fR.
.RE
.sp
.ne 2
.na
\fBzfs_top_maxinflight\fR (int)
.ad
.RS 12n
Expand the description of scan-related and other parameters. Document that the scan-related parameters are, in fact, applicable only to scrub and/or resilver operations as appropriate. Expand a few of the prefetch-related descriptions. Add clarification to other module parameters. Signed-off-by: Tim Chase <tim@chase2k.com> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #2361
2014-06-04 12:23:31 +00:00
Max I/Os per top-level vdev during scrub or resilver operations.
Document ZFS module parameters. This is a first draft of a zfs-module-parameters(5) man page. I have just extracted the parameter name and its description with modinfo, then checked the source what type it is and its default value. This will need more work, preferably someone that actually know these values and what to use them for. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1856
2013-11-16 06:52:54 +00:00
.sp
Default value: \fB32\fR.
.RE
.sp
.ne 2
.na
\fBzfs_txg_history\fR (int)
.ad
.RS 12n
Historic statistics for the last N txgs
.sp
Default value: \fB0\fR.
.RE
.sp
.ne 2
.na
\fBzfs_txg_timeout\fR (int)
.ad
.RS 12n
Max seconds worth of delta per txg
.sp
Default value: \fB5\fR.
.RE
.sp
.ne 2
.na
\fBzfs_vdev_aggregation_limit\fR (int)
.ad
.RS 12n
Max vdev I/O aggregation size
.sp
Default value: \fB131,072\fR.
.RE
.sp
.ne 2
.na
\fBzfs_vdev_cache_bshift\fR (int)
.ad
.RS 12n
Shift size to inflate reads too
.sp
Default value: \fB16\fR.
.RE
.sp
.ne 2
.na
\fBzfs_vdev_cache_max\fR (int)
.ad
.RS 12n
Inflate reads small than max
.RE
.sp
.ne 2
.na
\fBzfs_vdev_cache_size\fR (int)
.ad
.RS 12n
Total size of the per-disk cache
.sp
Default value: \fB0\fR.
.RE
.sp
.ne 2
.na
\fBzfs_vdev_mirror_switch_us\fR (int)
.ad
.RS 12n
Switch mirrors every N usecs
.sp
Default value: \fB10,000\fR.
.RE
.sp
.ne 2
.na
\fBzfs_vdev_read_gap_limit\fR (int)
.ad
.RS 12n
Aggregate read I/O over gap
.sp
Default value: \fB32,768\fR.
.RE
.sp
.ne 2
.na
\fBzfs_vdev_scheduler\fR (charp)
.ad
.RS 12n
I/O scheduler
.sp
Default value: \fBnoop\fR.
.RE
.sp
.ne 2
.na
\fBzfs_vdev_write_gap_limit\fR (int)
.ad
.RS 12n
Aggregate write I/O over gap
.sp
Default value: \fB4,096\fR.
.RE
.sp
.ne 2
.na
\fBzfs_zevent_cols\fR (int)
.ad
.RS 12n
Max event column width
.sp
Default value: \fB80\fR.
.RE
.sp
.ne 2
.na
\fBzfs_zevent_console\fR (int)
.ad
.RS 12n
Log events to the console
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE
.sp
.ne 2
.na
\fBzfs_zevent_len_max\fR (int)
.ad
.RS 12n
Max event queue length
.sp
Default value: \fB0\fR.
.RE
.sp
.ne 2
.na
\fBzil_replay_disable\fR (int)
.ad
.RS 12n
Disable intent logging replay
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE
.sp
.ne 2
.na
\fBzil_slog_limit\fR (ulong)
.ad
.RS 12n
Max commit bytes to separate log device
.sp
Default value: \fB1,048,576\fR.
.RE
.sp
.ne 2
.na
\fBzio_delay_max\fR (int)
.ad
.RS 12n
Max zio millisec delay before posting event
.sp
Default value: \fB30,000\fR.
.RE
.sp
.ne 2
.na
\fBzio_injection_enabled\fR (int)
.ad
.RS 12n
Enable fault injection
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE
.sp
.ne 2
.na
\fBzio_requeue_io_start_cut_in_line\fR (int)
.ad
.RS 12n
Prioritize requeued I/O
.sp
Default value: \fB0\fR.
.RE
.sp
.ne 2
.na
\fBzvol_inhibit_dev\fR (uint)
.ad
.RS 12n
Do not create zvol device nodes
.sp
Use \fB1\fR for yes and \fB0\fR for no (default).
.RE
.sp
.ne 2
.na
\fBzvol_major\fR (uint)
.ad
.RS 12n
Major number for zvol device
.sp
Default value: \fB230\fR.
.RE
.sp
.ne 2
.na
\fBzvol_max_discard_blocks\fR (ulong)
.ad
.RS 12n
Max number of blocks to discard at once
.sp
Default value: \fB16,384\fR.
.RE
.sp
.ne 2
.na
\fBzvol_threads\fR (uint)
.ad
.RS 12n
Number of threads for zvol device
.sp
Default value: \fB32\fR.
.RE
Illumos #4045 write throttle & i/o scheduler performance work 4045 zfs write throttle & i/o scheduler performance work 1. The ZFS i/o scheduler (vdev_queue.c) now divides i/os into 5 classes: sync read, sync write, async read, async write, and scrub/resilver. The scheduler issues a number of concurrent i/os from each class to the device. Once a class has been selected, an i/o is selected from this class using either an elevator algorithem (async, scrub classes) or FIFO (sync classes). The number of concurrent async write i/os is tuned dynamically based on i/o load, to achieve good sync i/o latency when there is not a high load of writes, and good write throughput when there is. See the block comment in vdev_queue.c (reproduced below) for more details. 2. The write throttle (dsl_pool_tempreserve_space() and txg_constrain_throughput()) is rewritten to produce much more consistent delays when under constant load. The new write throttle is based on the amount of dirty data, rather than guesses about future performance of the system. When there is a lot of dirty data, each transaction (e.g. write() syscall) will be delayed by the same small amount. This eliminates the "brick wall of wait" that the old write throttle could hit, causing all transactions to wait several seconds until the next txg opens. One of the keys to the new write throttle is decrementing the amount of dirty data as i/o completes, rather than at the end of spa_sync(). Note that the write throttle is only applied once the i/o scheduler is issuing the maximum number of outstanding async writes. See the block comments in dsl_pool.c and above dmu_tx_delay() (reproduced below) for more details. This diff has several other effects, including: * the commonly-tuned global variable zfs_vdev_max_pending has been removed; use per-class zfs_vdev_*_max_active values or zfs_vdev_max_active instead. * the size of each txg (meaning the amount of dirty data written, and thus the time it takes to write out) is now controlled differently. There is no longer an explicit time goal; the primary determinant is amount of dirty data. Systems that are under light or medium load will now often see that a txg is always syncing, but the impact to performance (e.g. read latency) is minimal. Tune zfs_dirty_data_max and zfs_dirty_data_sync to control this. * zio_taskq_batch_pct = 75 -- Only use 75% of all CPUs for compression, checksum, etc. This improves latency by not allowing these CPU-intensive tasks to consume all CPU (on machines with at least 4 CPU's; the percentage is rounded up). --matt APPENDIX: problems with the current i/o scheduler The current ZFS i/o scheduler (vdev_queue.c) is deadline based. The problem with this is that if there are always i/os pending, then certain classes of i/os can see very long delays. For example, if there are always synchronous reads outstanding, then no async writes will be serviced until they become "past due". One symptom of this situation is that each pass of the txg sync takes at least several seconds (typically 3 seconds). If many i/os become "past due" (their deadline is in the past), then we must service all of these overdue i/os before any new i/os. This happens when we enqueue a batch of async writes for the txg sync, with deadlines 2.5 seconds in the future. If we can't complete all the i/os in 2.5 seconds (e.g. because there were always reads pending), then these i/os will become past due. Now we must service all the "async" writes (which could be hundreds of megabytes) before we service any reads, introducing considerable latency to synchronous i/os (reads or ZIL writes). Notes on porting to ZFS on Linux: - zio_t gained new members io_physdone and io_phys_children. Because object caches in the Linux port call the constructor only once at allocation time, objects may contain residual data when retrieved from the cache. Therefore zio_create() was updated to zero out the two new fields. - vdev_mirror_pending() relied on the depth of the per-vdev pending queue (vq->vq_pending_tree) to select the least-busy leaf vdev to read from. This tree has been replaced by vq->vq_active_tree which is now used for the same purpose. - vdev_queue_init() used the value of zfs_vdev_max_pending to determine the number of vdev I/O buffers to pre-allocate. That global no longer exists, so we instead use the sum of the *_max_active values for each of the five I/O classes described above. - The Illumos implementation of dmu_tx_delay() delays a transaction by sleeping in condition variable embedded in the thread (curthread->t_delay_cv). We do not have an equivalent CV to use in Linux, so this change replaced the delay logic with a wrapper called zfs_sleep_until(). This wrapper could be adopted upstream and in other downstream ports to abstract away operating system-specific delay logic. - These tunables are added as module parameters, and descriptions added to the zfs-module-parameters.5 man page. spa_asize_inflation zfs_deadman_synctime_ms zfs_vdev_max_active zfs_vdev_async_write_active_min_dirty_percent zfs_vdev_async_write_active_max_dirty_percent zfs_vdev_async_read_max_active zfs_vdev_async_read_min_active zfs_vdev_async_write_max_active zfs_vdev_async_write_min_active zfs_vdev_scrub_max_active zfs_vdev_scrub_min_active zfs_vdev_sync_read_max_active zfs_vdev_sync_read_min_active zfs_vdev_sync_write_max_active zfs_vdev_sync_write_min_active zfs_dirty_data_max_percent zfs_delay_min_dirty_percent zfs_dirty_data_max_max_percent zfs_dirty_data_max zfs_dirty_data_max_max zfs_dirty_data_sync zfs_delay_scale The latter four have type unsigned long, whereas they are uint64_t in Illumos. This accommodates Linux's module_param() supported types, but means they may overflow on 32-bit architectures. The values zfs_dirty_data_max and zfs_dirty_data_max_max are the most likely to overflow on 32-bit systems, since they express physical RAM sizes in bytes. In fact, Illumos initializes zfs_dirty_data_max_max to 2^32 which does overflow. To resolve that, this port instead initializes it in arc_init() to 25% of physical RAM, and adds the tunable zfs_dirty_data_max_max_percent to override that percentage. While this solution doesn't completely avoid the overflow issue, it should be a reasonable default for most systems, and the minority of affected systems can work around the issue by overriding the defaults. - Fixed reversed logic in comment above zfs_delay_scale declaration. - Clarified comments in vdev_queue.c regarding when per-queue minimums take effect. - Replaced dmu_tx_write_limit in the dmu_tx kstat file with dmu_tx_dirty_delay and dmu_tx_dirty_over_max. The first counts how many times a transaction has been delayed because the pool dirty data has exceeded zfs_delay_min_dirty_percent. The latter counts how many times the pool dirty data has exceeded zfs_dirty_data_max (which we expect to never happen). - The original patch would have regressed the bug fixed in zfsonlinux/zfs@c418410, which prevented users from setting the zfs_vdev_aggregation_limit tuning larger than SPA_MAXBLOCKSIZE. A similar fix is added to vdev_queue_aggregate(). - In vdev_queue_io_to_issue(), dynamically allocate 'zio_t search' on the heap instead of the stack. In Linux we can't afford such large structures on the stack. Reviewed by: George Wilson <george.wilson@delphix.com> Reviewed by: Adam Leventhal <ahl@delphix.com> Reviewed by: Christopher Siden <christopher.siden@delphix.com> Reviewed by: Ned Bass <bass6@llnl.gov> Reviewed by: Brendan Gregg <brendan.gregg@joyent.com> Approved by: Robert Mustacchi <rm@joyent.com> References: http://www.illumos.org/issues/4045 illumos/illumos-gate@69962b5647e4a8b9b14998733b765925381b727e Ported-by: Ned Bass <bass6@llnl.gov> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1913
2013-08-29 03:01:20 +00:00
.SH ZFS I/O SCHEDULER
ZFS issues I/O operations to leaf vdevs to satisfy and complete I/Os.
The I/O scheduler determines when and in what order those operations are
issued. The I/O scheduler divides operations into five I/O classes
prioritized in the following order: sync read, sync write, async read,
async write, and scrub/resilver. Each queue defines the minimum and
maximum number of concurrent operations that may be issued to the
device. In addition, the device has an aggregate maximum,
\fBzfs_vdev_max_active\fR. Note that the sum of the per-queue minimums
must not exceed the aggregate maximum. If the sum of the per-queue
maximums exceeds the aggregate maximum, then the number of active I/Os
may reach \fBzfs_vdev_max_active\fR, in which case no further I/Os will
be issued regardless of whether all per-queue minimums have been met.
.sp
For many physical devices, throughput increases with the number of
concurrent operations, but latency typically suffers. Further, physical
devices typically have a limit at which more concurrent operations have no
effect on throughput or can actually cause it to decrease.
.sp
The scheduler selects the next operation to issue by first looking for an
I/O class whose minimum has not been satisfied. Once all are satisfied and
the aggregate maximum has not been hit, the scheduler looks for classes
whose maximum has not been satisfied. Iteration through the I/O classes is
done in the order specified above. No further operations are issued if the
aggregate maximum number of concurrent operations has been hit or if there
are no operations queued for an I/O class that has not hit its maximum.
Every time an I/O is queued or an operation completes, the I/O scheduler
looks for new operations to issue.
.sp
In general, smaller max_active's will lead to lower latency of synchronous
operations. Larger max_active's may lead to higher overall throughput,
depending on underlying storage.
.sp
The ratio of the queues' max_actives determines the balance of performance
between reads, writes, and scrubs. E.g., increasing
\fBzfs_vdev_scrub_max_active\fR will cause the scrub or resilver to complete
more quickly, but reads and writes to have higher latency and lower throughput.
.sp
All I/O classes have a fixed maximum number of outstanding operations
except for the async write class. Asynchronous writes represent the data
that is committed to stable storage during the syncing stage for
transaction groups. Transaction groups enter the syncing state
periodically so the number of queued async writes will quickly burst up
and then bleed down to zero. Rather than servicing them as quickly as
possible, the I/O scheduler changes the maximum number of active async
write I/Os according to the amount of dirty data in the pool. Since
both throughput and latency typically increase with the number of
concurrent operations issued to physical devices, reducing the
burstiness in the number of concurrent operations also stabilizes the
response time of operations from other -- and in particular synchronous
-- queues. In broad strokes, the I/O scheduler will issue more
concurrent operations from the async write queue as there's more dirty
data in the pool.
.sp
Async Writes
.sp
The number of concurrent operations issued for the async write I/O class
follows a piece-wise linear function defined by a few adjustable points.
.nf
| o---------| <-- zfs_vdev_async_write_max_active
^ | /^ |
| | / | |
active | / | |
I/O | / | |
count | / | |
| / | |
|-------o | | <-- zfs_vdev_async_write_min_active
0|_______^______|_________|
0% | | 100% of zfs_dirty_data_max
| |
| `-- zfs_vdev_async_write_active_max_dirty_percent
`--------- zfs_vdev_async_write_active_min_dirty_percent
.fi
Until the amount of dirty data exceeds a minimum percentage of the dirty
data allowed in the pool, the I/O scheduler will limit the number of
concurrent operations to the minimum. As that threshold is crossed, the
number of concurrent operations issued increases linearly to the maximum at
the specified maximum percentage of the dirty data allowed in the pool.
.sp
Ideally, the amount of dirty data on a busy pool will stay in the sloped
part of the function between \fBzfs_vdev_async_write_active_min_dirty_percent\fR
and \fBzfs_vdev_async_write_active_max_dirty_percent\fR. If it exceeds the
maximum percentage, this indicates that the rate of incoming data is
greater than the rate that the backend storage can handle. In this case, we
must further throttle incoming writes, as described in the next section.
.SH ZFS TRANSACTION DELAY
We delay transactions when we've determined that the backend storage
isn't able to accommodate the rate of incoming writes.
.sp
If there is already a transaction waiting, we delay relative to when
that transaction will finish waiting. This way the calculated delay time
is independent of the number of threads concurrently executing
transactions.
.sp
If we are the only waiter, wait relative to when the transaction
started, rather than the current time. This credits the transaction for
"time already served", e.g. reading indirect blocks.
.sp
The minimum time for a transaction to take is calculated as:
.nf
min_time = zfs_delay_scale * (dirty - min) / (max - dirty)
min_time is then capped at 100 milliseconds.
.fi
.sp
The delay has two degrees of freedom that can be adjusted via tunables. The
percentage of dirty data at which we start to delay is defined by
\fBzfs_delay_min_dirty_percent\fR. This should typically be at or above
\fBzfs_vdev_async_write_active_max_dirty_percent\fR so that we only start to
delay after writing at full speed has failed to keep up with the incoming write
rate. The scale of the curve is defined by \fBzfs_delay_scale\fR. Roughly speaking,
this variable determines the amount of delay at the midpoint of the curve.
.sp
.nf
delay
10ms +-------------------------------------------------------------*+
| *|
9ms + *+
| *|
8ms + *+
| * |
7ms + * +
| * |
6ms + * +
| * |
5ms + * +
| * |
4ms + * +
| * |
3ms + * +
| * |
2ms + (midpoint) * +
| | ** |
1ms + v *** +
| zfs_delay_scale ----------> ******** |
0 +-------------------------------------*********----------------+
0% <- zfs_dirty_data_max -> 100%
.fi
.sp
Note that since the delay is added to the outstanding time remaining on the
most recent transaction, the delay is effectively the inverse of IOPS.
Here the midpoint of 500us translates to 2000 IOPS. The shape of the curve
was chosen such that small changes in the amount of accumulated dirty data
in the first 3/4 of the curve yield relatively small differences in the
amount of delay.
.sp
The effects can be easier to understand when the amount of delay is
represented on a log scale:
.sp
.nf
delay
100ms +-------------------------------------------------------------++
+ +
| |
+ *+
10ms + *+
+ ** +
| (midpoint) ** |
+ | ** +
1ms + v **** +
+ zfs_delay_scale ----------> ***** +
| **** |
+ **** +
100us + ** +
+ * +
| * |
+ * +
10us + * +
+ +
| |
+ +
+--------------------------------------------------------------+
0% <- zfs_dirty_data_max -> 100%
.fi
.sp
Note here that only as the amount of dirty data approaches its limit does
the delay start to increase rapidly. The goal of a properly tuned system
should be to keep the amount of dirty data out of that range by first
ensuring that the appropriate limits are set for the I/O scheduler to reach
optimal throughput on the backend storage, and then by changing the value
of \fBzfs_delay_scale\fR to increase the steepness of the curve.