Am 19.04.16 um 23:31 schrieb wuffers:
You might want to check this old thread:
http://lists.omniti.com/pipermail/omnios-discuss/2014-July/002927.html
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 workload.
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.bud...@jvm.de
<mailto:stephan.bud...@jvm.de>> wrote:
Am 17.04.16 um 20:42 schrieb Dale Ghent:
On Apr 17, 2016, at 9:07 AM, Stephan Budach
<stephan.bud...@jvm.de <mailto:stephan.bud...@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
Interesting read - and it surely works. If you set the tunable before
you start the scrub you can immediately see the thoughput being much
higher than with the standard setting. Now, what would be the tunable to
use zfs_vdev_scrub_max_active or zfs_scrub_delay?
Cheers,
Stephan
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