[Bug 238229] scrub exceeds I/O qlen scrub_max_active limit, causes latency on 11.3-BETA1

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=238229

            Bug ID: 238229
           Summary: scrub exceeds I/O qlen scrub_max_active limit, causes
                    latency on 11.3-BETA1
           Product: Base System
           Version: 11.2-STABLE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: Sceiemu at md5hashing.net

I freebsd-update'd an 11.2 pool+system to 11.3-BETA1 for the purpose of seeing
if sequential scrub was implemented and to test if so.

During the scrub, I watched the output of iostat -t da -x 1.  Usually, the qlen
field remains at a low value related to what was configured in
/etc/sysctl.conf.  In this case, I started with the default of 2 for
vfs.zfs.vdev.scrub_max_active, and tweaked it to 3, and 4.  I noticed that
while the qlen would often report that same number, if would rise up to as high
as 24 and fluctuate around higher values for seconds at a time.  Interactive
performance that accessed the disk was noticeably impacted.  I noticed this
during the actual scrub phase, I did not make note of what was happening during
the earlier scan phase.

Other items in the sysctl.conf were:
vfs.zfs.vdev.async_read_max_active=4
vfs.zfs.vdev.async_read_min_active=2
vfs.zfs.vdev.async_write_max_active=4
vfs.zfs.vdev.async_write_min_active=1
vfs.zfs.vdev.sync_write_max_active=4
vfs.zfs.vdev.sync_write_min_active=2
vfs.zfs.vdev.sync_read_min_active=2
vfs.zfs.vdev.sync_read_max_active=5
vfs.zfs.top_maxinflight=15

The only vdev was a single hard disk, with GELI on the pool partition.  The
disk controller was standard intel AHCI on supermicro x9 board.

It scrubbed 100G in about 45 minutes, which seems reasonable for sequential
scrub on this old disk drive.

Was this qlen behavior correct, perhaps a side-effect of how sequential scrub
works?

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