8.0-R-p2 ZFS: unixbench causing kmem exhaustion panic

Ivan Voras ivoras at freebsd.org
Wed Jan 13 18:35:34 UTC 2010

2010/1/13 Doug Poland <doug at polands.org>:
> On Wed, January 13, 2010 11:55, Ivan Voras wrote:
>> Doug Poland wrote:
>>> So the question is, can ZFS be tuned to not panic or hang no matter
>>> what I throw at it?
>> Apparently not.
>>  > I began with a system with no tunables in /boot/loader.conf
>>  > (vm.kmem_size and vm.kmem_size_max).  Then I tried increasing
>>  > vm.kmem_size and vm.kmem_size_max a GB at a time, until I was at
>> 4GB.
>> Try adding vfs.zfs.arc_max=512M to /boot/loader.conf.
> Would you suggest tweaking the vm.kmem_size tunables in addition to
> arc_max?

No, unless they auto-tune to something lesser than approximately arc_max*3.

I try to set arc_max to be a third (or a quarter) the kmem_size, and
tune kmem_size ad_hoc to suit the machine and its purpose.

The reason for this is that "arc_max" is just a guideline, not a hard
limit... the ZFS ARC usage can and will spike to much larger values,
usually in the most inopportune moment.

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