Detect a slow raidz child during reads

[don-brady]

Motivation and Context

There is a concern, and has been observed in practice, that a slow disk can bring down the overall read performance of raidz. Currently in ZFS, a slow disk is detected by comparing the disk read latency to a custom threshold value, such as 30 seconds. This can be tuned to a lower threshold but that requires understanding the context in which it will be applied. And hybrid pools can have a wide range of expected disk latencies.

What might be a better approach, is to identify the presence of a slow disk outlier based on its latency distance from the latencies of its peers.  This would offer a more dynamic solution that can adapt to different type of media and workloads.

Description

The solution proposed here comes in two parts

1. Detecting a persistent read outlier – adapts over time under various factors (workload, environment, etc.)
2. Placing outlier drive in a sit out period -- the raidz group can use parity to reconstruct the data that was skipped.

Detecting Outliers
The most recent latency value for each child is saved in the vdev_t. Then periodically, the samples from all the children are sorted and a statistical outlier can be detected if present. The code uses a Tukey's fence, with K = 2, for detecting extreme outliers. This rule defines extreme outliers as data points outside the fence of the third quartile plus two times the Interquartile Range (IQR). This range is the distance between the first and third quartile.

Sitting Out
After a vdev has encounter multiple outlier detections (> 50), it is marked for being in a sit out period that by default lasts for 10 minutes.

Each time a slow disk is placed into a sit out period, its vdev_stat.vs_slow_ios count is incremented and a zevent class ereport.fs.zfs.delay is posted.  

The length of the sit out period can be changed using the raid_read_sit_out_secs module parameter.  Setting it to zero disables slow outlier detection.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.

How Has This Been Tested?

Tested with various configs, including dRAID.

For an extreme example, an HDD was used in an 8 wide SSD raidz2 and it was compared to taking the HDD offline. This was using a fio(1) streaming read workload across 4 threads to 20GB files. Both the record size and IO request size were 1MB.

Original	Offline	Detection
401 secs	54 secs	58 sec
204 IOPS	1521 IOPS	1415 IOPS
204 MiB/s	1521 MiB/s	1415 MiB/s

Also measured the cost over time of vdev_child_slow_outlier() where the statistical analysis occurs (every 20ms).

@vdev_child_slow_outlier_hist[ns]:  
[256, 512)           832 |@                                                   | 
[512, 1K)          14372 |@@@@@@@@@@@@@@@@@@@@@@@                             | 
[1K, 2K)           32231 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@| 
[2K, 4K)           23283 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@               | 
[4K, 8K)            7773 |@@@@@@@@@@@@                                        | 
[8K, 16K)           2790 |@@@@                                                | 
[16K, 32K)           411 |                                                    | 
[32K, 64K)           122 |                                                    | 
[64K, 128K)           51 |                                                    | 
[128K, 256K)          26 |                                                    | 
[256K, 512K)           5 |                                                    | 

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[tonyhutter]

Initial thoughts

This is going to be really useful on large pools. We often see SCSI disks "lose their mind" for a few minutes in our larger pools, tanking performance. It's nice that this feature will basically read and rebuild the data from parity first rather than attempt to read from the bad disk. I can see us using this in production.

Will disks that are "sitting out" still be read during a scrub?

If a disk is sitting out, and a read is unable to reconstruct from the other "healthy" disks, will ZFS read from the sitting out disk as a last-ditch effort?

Could this work for mirrors as well?

What happens if the algorithm wants to sit out more disks in a raid group than there is parity?

What happens if a pool has dissimilar disks in its raid group? Like, say, 7 NVMe and 1 HDD. Will it sit out the HDD since it will be so much slower than the other drives? That might actually be a nice side effect...

It would be good to see which disks are currently sitting out via a vdev property. I would expect users would want to see this for troubleshooting performance problems and locating sick disks. I can see users wanting to manually set it on sick disks as well:

# Check if /dev/sda is currently being sit out via the algorithm
$ zpool get sitting_out tank L0 sda
NAME                PROPERTY         VALUE   SOURCE
sda                 sitting_out      on      -

# Manually set /dev/sdb to sit out
$ zpool set sitting_out=on tank L0 sdb
$ zpool get sitting_out tank L0 sdb
NAME                PROPERTY       VALUE   SOURCE
sdb                 sitting_out    on      local

You could tell if sitting_out was manually or automatically set by looking at the SOURCE value. If the user manually set the vdev to sit out, we would probably want it to persist that way rather than periodically retesting the vdev's latency.

Alternatively you could name the property penalty_box rather than sitting_out, but the terminology is your call. I'll give you that it's less wordy to say "the disk is sitting out" rather than "the disk is in the penalty box" though. Other ideas: prefer_no_reads, avoid_reads, minimize_reads.

[don-brady]

Latest changes...

• Use weighted moving average for saved read latency values
• Refactored vdev_skip_latency_outlier() -- it now only checks state of sit out expiration. Moved sit out state change logic into vdev_child_slow_outlier().
• Removed overloading of vdev_stat_lock.
• Allow for only one vdev_child_slow_outlier() to run at a time (closed race window).

[don-brady]

Will disks that are "sitting out" still be read during a scrub?

Yes. both resilver and scrub reads will not sit out.

Could this work for mirrors as well?

Possibly but that would require a different outlier detection.

What happens if the algorithm wants to sit out more disks in a raid group than there is parity?

Currently one one outlier sit out at a time is allowed.

What happens if a pool has dissimilar disks in its raid group? Like, say, 7 NVMe and 1 HDD. Will it sit out the HDD since it will be so much slower than the other drives? That might actually be a nice side effect...

Yes, it would likely end up sitting out the HDD since it will be an outlier. I did test this exact scenario and could hear the sit out period. 😃

It would be good to see which disks are currently sitting out via a vdev property.

I was using zpool iostat -v to observe that a disk was sitting out. There is also a zevent each time we enter a sit out. It's a slow io event but we could also tag it to indicate that it is also entering a sit out. I could add a read-only vdev property for read sit out. Not sure I would want to make it writable. The design here is meant to be dynamic/adaptive. The admin can still offline a disk.

[behlendorf]

I could add a read-only vdev property for read sit out. Not sure I would want to make it writable. The design here is meant to be dynamic/adaptive. The admin can still offline a disk.

A read-only vdev property seems like the way to go. That would provide some additional visibility in to when a drive is sitting out. I don't think we'd want to make it writable for exactly the reasons you mentioned.

[don-brady]

Added read-only VDEV_PROP_SIT_OUT_READS property.

[don-brady]

Some recent feedback changes. Rebased to latest master and squash commits.

[tonyhutter]

I gave this a try today using the script below. I was able to sit out the disk I delayed, but I wasn't able to remove the sit-out state afterwards. Maybe I need non-ARC reads to trigger the sitting-out -> not-sitting-out logic?

#!/bin/bash
# Run this from a zfs git workspace
#

# shorten sit out period for testing
sudo bash -c 'echo 5 > /sys/module/zfs/parameters/raidz_read_sit_out_secs'

truncate -s 200M file{0,1,2,3,4,5,6,7,8,9}
sudo ./zpool create tank raidz2 `pwd`/file{0..9}
sudo dd if=/dev/urandom bs=1M count=100 of=/tank/bigfile
sudo ./zpool export tank
sudo ./zpool import -d . tank

echo "Initial state"
# We should be sitting out
sudo ./zpool get -Hp sit_out_reads tank `pwd`/file9

# Add 500ms delay on last disk
sudo ./zinject -d `pwd`/file9 -D500:1 tank

# Do some reads
sudo dd if=/tank/bigfile of=/dev/null bs=4k 

echo "Should be sitting out"
# We should be sitting out
sudo ./zpool get -Hp sit_out_reads tank `pwd`/file9

# Clear fault injection
sudo ./zinject -c all

echo "wait for us to stop sitting out part 1"

sleep 6

# Are we still sitting out?
sudo ./zpool get -Hp sit_out_reads tank `pwd`/file9

# Do some more reads to see if we can trigger the vdev to stop sitting out
sudo dd if=/tank/bigfile of=/dev/null bs=4k 

echo "wait for us to stop sitting out part 2"
sleep 6

# Are we still sitting out?
sudo ./zpool get -Hp sit_out_reads tank `pwd`/file9

[clhedrick]

A response said that resilver reads would not be affected. I'd like to suggest that this may be a bad idea. We currently have a RAIDZ2 that is rebuilding. One of the disks is very, very slow but still responds. It's really slowing down the rebuild. I'd like the resilver to avoid reading from it. For a scrub I can see that you want to read everything, but I'm not so sure about resilver.

[tonyhutter]

For a scrub I can see that you want to read everything, but I'm not so sure about resilver.

I agree with this - we would want the resilver to finish as quickly as possible. That would mean sitting out the sick disk if we could reconstruct from parity.

[amotin]

Same as above, I think this could be switched to seconds.

[amotin]

While some limit on number of samples makes sense, I am not sure it makes sense to apply it to the number of children. 3-wide RAIDZ1 may also have the same problems, but may be there we could use wider safety interval. If anywhere, the limit should be applied to a number of samples we have averaged for each children before we can trust any statistics. Though averaging hides the dispersion, so it might need more thinking how to account it better.

[amotin]

I hope we have these 512 bytes of stack to waste.

[behlendorf]

It's more than I'd like, but my feeling is it should be alright. It does raise a few question though.

1. Does the kmem_alloc() really hurt performance given that we call this at most once per raid_outlier_check_interval_ms?
2. If it really is that expensive, it looks like we could allocate this buffer once and stash a pointer to it in the top-level raidz/draid vdev. The atomic_cas_64(&vd->vdev_last_latency_check) check on line 2923 should prevent concurrent access, and this would get right of the alloc in all cases. We'd definitely want to add a comment explaining this.

[don-brady]

64 was chosen for benefit of draid path. Stack function depth is bounded and not that deep (below). But still, 512 is big and perhaps outside the spirt of being a good stack usage citizen.

	vdev_child_slow_outlier
	vdev_raidz_io_done
	zio_vdev_io_done
	zio_execute
	taskq_thread

[tonyhutter]

@don-brady I added a test case in my https://github.com/tonyhutter/zfs/tree/slow_vdev_sit_out branch. Commit is here: tonyhutter@8a58493

Would you mind pulling it into your patch stack? I have run it locally in a VM but have not tested in on the qemu runners yet.

[behlendorf]

To skip vdevs which are sitting out during a resilver we need to modify this check slightly. Each resilvering drive will add one to rr->rr_missingdata and if we're going to sit out one more drive there needs to be sufficient parity available. This updated code should do the trick. You'll need to update the raidz similarly.

Can you also add versions of tests/functional/replacement/attach_resilver.ksh, tests/functional/replacement/replace_resilver.ksh which sit out a drive during the resilver. We'll want to test raidz[1-3] and draid[1-3].

        /*
         * When the row contains a latency outlier and sufficient parity
         * exists to reconstruct the column data, then skip reading the
         * known slow child vdev as a performance optimization.
         */
        if (rr->rr_outlier_cnt > 0) {
                int available_parity = rr->rr_firstdatacol - rr->rr_missingparity;
                int required_parity = rr->rr_missingdata + 1;

                if (available_parity >= required_parity) {
                        for (int c = rr->rr_cols - 1;
                             c >= rr->rr_firstdatacol; c--) {
                                raidz_col_t *rc = &rr->rr_col[c];

                                if (rc->rc_error == 0 &&
                                    rc->rc_latency_outlier) {
                                        rr->rr_missingdata++;
                                        rc->rc_error = SET_ERROR(EAGAIN);
                                        rc->rc_skipped = 1;
                                        break;
                                }
                        }
                }
        }

[tonyhutter]

"raidz" should be removed from the parameter name in Linux to be consistent with the FreeBSD name. Right now it is:

FreeBSD
$ sysctl -ad | grep sit_out
vfs.zfs.vdev.read_sit_out_secs: Raidz/draid slow disk sit out time period in seconds

Linux
$ ls -l /sys/module/zfs/parameters/ | grep sit_out
-rw-r--r--. 1 root root 4096 Jan  9 17:41 raidz_read_sit_out_secs

[tonyhutter]

@don-brady this will resolve the test case changes and get it passing on FreeBSD:

DIFF

diff --git a/tests/zfs-tests/include/libtest.shlib b/tests/zfs-tests/include/libtest.shlib
index fbad1747b..975a50141 100644
--- a/tests/zfs-tests/include/libtest.shlib
+++ b/tests/zfs-tests/include/libtest.shlib
@@ -3090,6 +3090,26 @@ function wait_scrubbed #pool timeout
        done
 }
 
+# Wait for vdev 'sit_out' property to be cleared.
+#
+# $1 pool name
+# $2 vdev name
+# $3 timeout
+#
+function wait_sit_out #pool vdev timeout
+{
+       typeset timeout=${3:-300}
+       typeset vdev=$2
+       typeset pool=$1
+       for (( timer = 0; timer < $timeout; timer++ )); do
+               if [[ "$(get_vdev_prop sit_out $pool $vdev)" == "off" ]] ; then
+                   return 0
+               fi
+               sleep 1
+       done
+       false
+}
+
 # Backup the zed.rc in our test directory so that we can edit it for our test.
 #
 # Returns: Backup file name.  You will need to pass this to zed_rc_restore().
diff --git a/tests/zfs-tests/include/tunables.cfg b/tests/zfs-tests/include/tunables.cfg
index 7d667c553..1e82f6606 100644
--- a/tests/zfs-tests/include/tunables.cfg
+++ b/tests/zfs-tests/include/tunables.cfg
@@ -69,7 +69,7 @@ MULTIHOST_INTERVAL            multihost.interval              zfs_multihost_interval
 OVERRIDE_ESTIMATE_RECORDSIZE   send.override_estimate_recordsize       zfs_override_estimate_recordsize
 PREFETCH_DISABLE               prefetch.disable                zfs_prefetch_disable
 RAIDZ_EXPAND_MAX_REFLOW_BYTES  vdev.expand_max_reflow_bytes    raidz_expand_max_reflow_bytes
-RAIDZ_READ_SIT_OUT_SECS        vdev.raidz_read_sit_out_secs    raidz_read_sit_out_secs
+READ_SIT_OUT_SECS              vdev.read_sit_out_secs          raidz_read_sit_out_secs
 REBUILD_SCRUB_ENABLED          rebuild_scrub_enabled           zfs_rebuild_scrub_enabled
 REMOVAL_SUSPEND_PROGRESS       removal_suspend_progress        zfs_removal_suspend_progress
 REMOVE_MAX_SEGMENT             remove_max_segment              zfs_remove_max_segment
diff --git a/tests/zfs-tests/tests/functional/events/slow_vdev_sit_out.ksh b/tests/zfs-tests/tests/functional/events/slow_vdev_sit_out.ksh
index 27c3a8f5f..d859d1681 100755
--- a/tests/zfs-tests/tests/functional/events/slow_vdev_sit_out.ksh
+++ b/tests/zfs-tests/tests/functional/events/slow_vdev_sit_out.ksh
@@ -29,17 +29,14 @@
 #      1. Create various raidz/draid pools
 #      2. Inject delays into one of the disks
 #      3. Verify disk is set to 'sit out' for awhile.
-#      4. Wait for RAIDZ_READ_SIT_OUT_SECS and verify sit out state is lifted.
+#      4. Wait for READ_SIT_OUT_SECS and verify sit out state is lifted.
 #
 
 . $STF_SUITE/include/libtest.shlib
 
-# Avoid name conflicts with the default pool
-TESTPOOL2=testpool2
-
 function cleanup
 {
-       restore_tunable RAIDZ_READ_SIT_OUT_SECS
+       restore_tunable READ_SIT_OUT_SECS
        log_must zinject -c all
        destroy_pool $TESTPOOL2
        log_must rm -f $TEST_BASE_DIR/vdev.$$.*
@@ -50,8 +47,8 @@ log_assert "Verify sit_out works"
 log_onexit cleanup
 
 # shorten sit out period for testing
-save_tunable RAIDZ_READ_SIT_OUT_SECS
-set_tunable32 RAIDZ_READ_SIT_OUT_SECS 5
+save_tunable READ_SIT_OUT_SECS
+set_tunable32 READ_SIT_OUT_SECS 5
 
 log_must truncate -s 150M $TEST_BASE_DIR/vdev.$$.{0..9}
 
@@ -71,7 +68,7 @@ for raidtype in raidz raidz2 raidz3 draid1 draid2 draid3 ; do
 
        # Do some reads and wait for us to sit out
        for i in {1..100} ; do
-               dd if=/$TESTPOOL2/bigfile skip=$i bs=1M count=1 of=/dev/null &> /dev/null
+               dd if=/$TESTPOOL2/bigfile skip=$i bs=1M count=1 of=/dev/null
 
                sit_out=$(get_vdev_prop sit_out $TESTPOOL2 $BAD_VDEV)
                if [[ "$sit_out" == "on" ]] ; then
@@ -85,8 +82,7 @@ for raidtype in raidz raidz2 raidz3 draid1 draid2 draid3 ; do
        log_must zinject -c all
 
        # Wait for us to exit our sit out period
-       sleep 6
-       log_must test "$(get_vdev_prop sit_out $TESTPOOL2 $BAD_VDEV)" == "off"
+       log_must wait_sit_out $TESTPOOL2 $BAD_VDEV 10
        destroy_pool $TESTPOOL2
 done

[don-brady]

"raidz" should be removed from the parameter name in Linux to be consistent with the FreeBSD name.

I was matching the existing name_prefix convention for the other raidz tunables (which may be wrong).

On Linux tunables should have some prefix scope since they all end up in parameters. Maybe use a name_prefix of `vdev' if you want consistency across platforms.

[tonyhutter]

@don-brady I need to move slow_vdev_sit_out.ksh out of the test/functional/events dir to somewhere else. That should fix the failures on FreeBSD. Let me get back to you with a patch for that fix..

[tonyhutter]

@don-brady this gets my test case working again on FreeBSD:

DIFF

diff --git a/tests/runfiles/common.run b/tests/runfiles/common.run
index 1ed9478c4..917650739 100644
--- a/tests/runfiles/common.run
+++ b/tests/runfiles/common.run
@@ -702,9 +702,11 @@ tests = ['dio_aligned_block', 'dio_async_always', 'dio_async_fio_ioengines',
     'dio_unaligned_block', 'dio_unaligned_filesize']
 tags = ['functional', 'direct']
 
-[tests/functional/events]
+[tests/functional/sit_out]
 tests = ['slow_vdev_sit_out']
-tags = ['functional', 'events']
+pre =
+post =
+tags = ['functional', 'sit_out']
 
 [tests/functional/exec]
 tests = ['exec_001_pos', 'exec_002_neg']
@@ -904,7 +906,7 @@ tags = ['functional', 'rename_dirs']
 tests = ['attach_import', 'attach_multiple', 'attach_rebuild',
     'attach_resilver', 'detach', 'rebuild_disabled_feature',
     'rebuild_multiple', 'rebuild_raidz', 'replace_import', 'replace_rebuild',
-    'replace_resilver', 'replace_resilver_sit_out' 'resilver_restart_001',
+    'replace_resilver', 'replace_resilver_sit_out', 'resilver_restart_001',
     'resilver_restart_002', 'scrub_cancel']
 tags = ['functional', 'replacement']
 
diff --git a/tests/zfs-tests/tests/Makefile.am b/tests/zfs-tests/tests/Makefile.am
index 42e7bce85..a24d17bc5 100644
--- a/tests/zfs-tests/tests/Makefile.am
+++ b/tests/zfs-tests/tests/Makefile.am
@@ -1497,7 +1497,6 @@ nobase_dist_datadir_zfs_tests_tests_SCRIPTS += \
        functional/events/events_001_pos.ksh \
        functional/events/events_002_pos.ksh \
        functional/events/setup.ksh \
-       functional/events/slow_vdev_sit_out.ksh \
        functional/events/zed_cksum_config.ksh \
        functional/events/zed_cksum_reported.ksh \
        functional/events/zed_diagnose_multiple.ksh \
@@ -1994,6 +1993,7 @@ nobase_dist_datadir_zfs_tests_tests_SCRIPTS += \
        functional/scrub_mirror/scrub_mirror_003_pos.ksh \
        functional/scrub_mirror/scrub_mirror_004_pos.ksh \
        functional/scrub_mirror/setup.ksh \
+       functional/sit_out/slow_vdev_sit_out.ksh \
        functional/slog/cleanup.ksh \
        functional/slog/setup.ksh \
        functional/slog/slog_001_pos.ksh \
diff --git a/tests/zfs-tests/tests/functional/events/slow_vdev_sit_out.ksh b/tests/zfs-tests/tests/functional/sit_out/slow_vdev_sit_out.ksh
similarity index 100%
rename from tests/zfs-tests/tests/functional/events/slow_vdev_sit_out.ksh
rename to tests/zfs-tests/tests/functional/sit_out/slow_vdev_sit_out.ksh

[amotin]

Suggested change

	if (n % 2 == 0)
	q3 = latency_median_value(&data[n>>1], n>>1);
	else
	q3 = latency_median_value(&data[(n+1) >> 1], n>>1);
	q3 = latency_median_value(&data[(n + 1) / 2], n / 2);

In case of n being even the + 1 makes no difference.