Poor ZFS+NFSv3 read/write performance and panic

[David Adam]

On Mon, 8 Feb 2016, Tom Curry wrote:
> On Sun, Feb 7, 2016 at 11:58 AM, David Adam <zanchey at ucc.gu.uwa.edu.au>
> wrote:
> 
> > Just wondering if anyone has any idea how to identify which devices are
> > implicated in ZFS' vdev_deadman(). I have updated the firmware on the
> > mps(4) card that has our disks attached but that hasn't helped.
> 
> I too ran into this problem and spent quite some time troubleshooting
> hardware. For me it turns out it was not hardware at all, but software.
> Specifically the ZFS ARC. Looking at your stack I see some arc reclaim up
> top, it's possible you're running into the same issue. There is a monster
> of a PR that details this here
> https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=187594
> 
> If you would like to test this theory out, the fastest way is to limit the
> ARC by adding the following to /boot/loader.conf and rebooting
> vfs.zfs.arc_max="24G"
> 
> Replacing 24G with what makes sense for your system, aim for 3/4 of total
> memory for starters. If this solves the problem there are more scientific
> methods to a permanent fix, one would be applying the patch in the PR
> above, another would be a more finely tuned arc_max value.

Thanks Tom - this certainly did sound promising, but setting the ARC to 
11G of our 16G of RAM didn't help. `zfs-stats` confirmed that the ARC was 
the expected size and that there was still 461 MB of RAM free.

We'll keep looking!

David Adam
zanchey at ucc.gu.uwa.edu.au