[OmniOS-discuss] Slow scrub on SSD-only pool

wuffers moo at wuffers.net
Tue Apr 19 21:31:30 UTC 2016

You might want to check this old thread:


Richard Elling had some interesting insights on how the scrub works:

"So I think the pool is not scheduling scrub I/Os very well. You can
increase the number of
scrub I/Os in the scheduler by adjusting the zfs_vdev_scrub_max_active
tunable. The
default is 2, but you'll have to consider that a share (in the stock market
sense) where
the active sync reads and writes are getting 10 each. You can try bumping
up the value
and see what happens over some time, perhaps 10 minutes or so -- too short
of a time
and you won't get a good feeling for the impact (try this in off-peak time).
echo zfs_vdev_scrub_max_active/W0t5 | mdb -kw
will change the value from 2 to 5, increasing its share of the total I/O

You can see the progress of scan (scrubs do scan) workload by looking at
the ZFS
debug messages.
echo ::zfs_dbgmsg | mdb -k
These will look mysterious... they are. But the interesting bits are about
how many blocks
are visited in some amount of time (txg sync interval). Ideally, this will
change as you
adjust zfs_vdev_scrub_max_active."

I had to increase my zfs_vdev_scrub_max_active parameter higher than 5, but
it sounds like the default setting for that tunable is no longer
satisfactory for today's high performance systems.

On Sun, Apr 17, 2016 at 4:07 PM, Stephan Budach <stephan.budach at jvm.de>

> Am 17.04.16 um 20:42 schrieb Dale Ghent:
> On Apr 17, 2016, at 9:07 AM, Stephan Budach <stephan.budach at JVM.DE> wrote:
>>> Well… searching the net somewhat more thoroughfully, I came across an
>>> archived discussion which deals also with a similar issue. Somewhere down
>>> the conversation, this parameter got suggested:
>>> echo "zfs_scrub_delay/W0" | mdb -kw
>>> I just tried that as well and although the caculated speed climbs rathet
>>> slowly up, iostat now shows  approx. 380 MB/s read from the devices, which
>>> rates  at 24 MB/s per single device * 8 *2.
>>> Being curious, I issued a echo "zfs_scrub_delay/W1" | mdb -kw to see
>>> what would happen and that command immediately drowned the rate on each
>>> device down to 1.4 MB/s…
>>> What is the rational behind that? Who wants to wait for weeks for a
>>> scrub to finish? Usually, I am having znapzend run as well, creating
>>> snapshots on a regular basis. Wouldn't that hurt scrub performance even
>>> more?
>> zfs_scrub_delay is described here:
>> http://src.illumos.org/source/xref/illumos-gate/usr/src/uts/common/fs/zfs/dsl_scan.c#63
>> How busy are your disks if you subtract the IO caused by a scrub? Are you
>> doing these scrubs with your VMs causing normal IO as well?
>> Scrubbing, overall, is treated as a background maintenance process. As
>> such, it is designed to not interfere with "production IO" requests. It
>> used to be that scrubs ran as fast as disk IO and bus bandwidth would
>> allow, which in turn severely impacted the IO performance of running
>> applications, and in some cases this would cause problems for production or
>> user services.  The scrub delay setting which you've discovered is the main
>> governor of this scrub throttle code[1], and by setting it to 0, you are
>> effectively removing the delay it imposes on itself to allow
>> non-scrub/resilvering IO requests to finish.
>> The solution in your case is specific to yourself and how you operate
>> your servers and services. Can you accept degraded application IO while a
>> scrub or resilver is running? Can you not? Maybe only during certain times?
>> /dale
>> [1]
>> http://src.illumos.org/source/xref/illumos-gate/usr/src/uts/common/fs/zfs/dsl_scan.c#1841
> I do get the notion if this, but if the increase from 0 to 1 reduces the
> throughput from 24Mb/s to 1MB/s, this seems way overboard to me. Having to
> wait for a couple of hours when running with 0 as opposed to days (up to
> 10) when running at 1  - on a 1.3 TB zpool - doesn't seem to be the right
> choice. If this tunable offered some more room for choice,  that would be
> great, but it obviously doesn't.
> It's the weekend and my VMs aren't excatly hogging their disks, so there
> was plenty of I/O available… I'd wish for a more granular setting regarding
> this setting.
> Anyway, the scrub finished a couple of hours later and of course, I can
> always set this tunable to 0, should I need it,
> Thanks,
> Stephan
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