Still getting kmem exhausted panic

Jeremy Chadwick freebsd at
Tue Sep 28 11:50:50 UTC 2010

On Tue, Sep 28, 2010 at 01:24:28PM +0200, Willem Jan Withagen wrote:
> This is with stable as of yesterday,but with an un-tunned ZFS box I
> was still able to generate a kmem exhausted panic.
> Hard panic, just 3 lines.
> The box contains 12Gb memory, runs on a 6 core (with HT) xeon.
> 6* 2T WD black caviar in raidz2 with 2*512Mb mirrored log.
> The box died while rsyncing 5.8T from its partnering system.
> (that was the only activity on the box)

It would help if you could provide output from the following commands
(even after the box has rebooted):

$ sysctl -a | egrep ^vm.kmem
$ sysctl -a | egrep ^vfs.zfs.arc
$ sysctl kstat.zfs.misc.arcstats

> So the obvious would to conclude that auto-tuning voor ZFS on
> 8.1-Stable is not yet quite there.
> So I guess that we still need tuning advice even for 8.1.
> And thus prevent a hard panic.

Andriy Gapon provides this general recommendation:

The advice I've given for RELENG_8 (as of the time of this writing),
8.1-STABLE, and 8.1-RELEASE, is that for amd64 you'll need to tune:


An example machine: amd64, with 4GB physical RAM installed (3916MB
available for use (verified via dmesg)) uses values:


Another example machine: amd64, with 8GB physical RAM installed (7875MB
available for use) uses values:


I believe the trick -- Andriy, please correct me if I'm wrong -- is the
tuning of vfs.zfs.arc_max, which is now a hard limit rather than a "high

However, I believe there have been occasional reports of exhaustion
panics despite both of these being set[1].  Those reports are being
investigated on an individual basis.

I set some other ZFS-related parameters as well (disabling prefetch,
adjusting txg.timeout, etc.), but those shouldn't be necessary to gain
stability at this point in time.

I can't provide tuning advice for i386.


| Jeremy Chadwick                                   jdc at |
| Parodius Networking              |
| UNIX Systems Administrator                  Mountain View, CA, USA |
| Making life hard for others since 1977.              PGP: 4BD6C0CB |

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