Miroslav Lachman wrote:
Alan Somers wrote on 2019/05/09 14:50:
[...]
On 11.3 and even much older releases, you can greatly speed up scrub
and resilver by tweaking some sysctls. If you have spinning rust,
raise vfs.zfs.top_maxinflight so they'll do fewer seeks. I used to
set it to 8192 on machines with 32GB of RAM. Raising
vfs.zfs.resilver_min_time_ms to 5000 helps a little, too.
I have this in sysctl.conf
vfs.zfs.scrub_delay=0
vfs.zfs.top_maxinflight=128
vfs.zfs.resilver_min_time_ms=5000
vfs.zfs.resilver_delay=0
I found it somewhere in the mailinglist discussing this issue in the
past.
Isn't yours 8192 too much? The machine in question has 4x SATA drives
on very dump and slow controller and only 5GB of RAM.
Even if I read this
vfs.zfs.top_maxinflight: Maximum I/Os per top-level vdev
I am still not sure what it really means and how I can "calculate"
optimal value.
I calculated it by looking at the iops using gstat and then multiplied
it by the spindles in use. That seemed to give the optimum. Much lower
and the drives had idle time.... to much and it causes 'pauses' whilst
the writes happen. "Tuning" for me had no fine tuning, it seems very
sledgehammerish ... big changes are noticeable for better or worse..
small changes you cannot tell by eye, and I guess measuring known
operations (such as a controlled environment scrub) might show differing
results but I suspect with other things going on these negligible
changes are likely to be useless.
As Michelle pointed there is drawback when sysctls are optimized for
quick scrub, but this machines is only running nightly backup script
fetching data from other 20 machines so this scrip sets sysctl back to
sane defaults during backup
not really.. optimize for scrub should only affect the system whilst the
scrub and resilvers are actually happening,.. the rest of the time my
systems were not affected (noticeably) my problem was a scrub would kick
off and last a couple of weeks (1 week heavily preferrencing the scrub)
causing the video streaming to become stuttery .. which watching a movie
whilst this was happening was really not good... especially as it wasn't
for just a few seconds/minutes/hours.
sysctl vfs.zfs.scrub_delay=4 > /dev/null
sysctl vfs.zfs.top_maxinflight=32 > /dev/null
sysctl vfs.zfs.resilver_min_time_ms=3000 > /dev/null
sysctl vfs.zfs.resilver_delay=2 > /dev/null
At the and it reloads back optimized settings from sysctel.conf
Miroslav Lachman
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Michelle Sullivan
http://www.mhix.org/
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