Disk/ZFS activity crash on 11.2-STABLE [SOLVED]

Jim Long list at museum.rain.com
Fri Jul 13 19:11:41 UTC 2018

On Thu, Jul 12, 2018 at 02:42:48PM -0700, Jim Long wrote:
> On Thu, Jul 12, 2018 at 02:49:53PM -0400, Mike Tancsa wrote:
> --snip--
> > I would try and set a ceiling. On
> > RELENG_11 you dont need to reboot
> > 
> > Try
> > sysctl -w vfs.zfs.arc_max=77946198016
> > 
> > which shaves off 20G from what ARC can gobble up. Not sure if thats your
> > issue, but it is an issue for some users.
> > 
> > If you are still hurting for caching, an SSD drive or NVME and make it a
> > caching device for your pool.
> > 
> > and what does
> > zpool status
> > show ?
> I set the limit to the value you suggested, and the next test ran less
> than three minutes before the machine rebooted, with no crash dump produced.
> I further reduced the limit to 50G and it's been running for about 50 minutes
> so far.  Fingers crossed.  I do have L2ARC I can add if need be.
> I'll keep you posted on how this run goes.
> Thank you,
> Jim

It appears that limiting the ARC size did it.  The 'zfs send -R' was
able to complete with ARC limited to 50G, and a second run with a 60G
ARC limit also completed.

That is a very handy tunable to know about.  Being able to reduce cache
size on a running system when needed, to free up RAM, or whatever.  I
was curious to find the answer to your query about the average size of
files on the system, so I ran a 'zdb -b' on the pool.  That process
began to page out large amounts of RAM into swap, which was making the
system rather sluggish, especially once I decided to kill the zdb
process.  By dropping the ARC size limit, I was able to temporarily free
some RAM so that the process could succumb to the SIGKILL signal.

Thank you very much for your advice in guiding me to this resolution!


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