FreeBSD 7.2-RC1 - ZFS related kernel panic "kmem_map too small"

Gary Jennejohn gary.jennejohn at
Wed Apr 22 10:30:23 UTC 2009

On Wed, 22 Apr 2009 00:34:00 +0200
Kai Gallasch <gallasch at> wrote:

[snip a lot of stuff]
> In /usr/src/UPDATING I read:
> [..]
> 20090207:
>         ZFS users on amd64 machines with 4GB or more of RAM should
>         reevaluate their need for setting vm.kmem_size_max and
>         vm.kmem_size manually.  In fact, after recent changes to the
>         kernel, the default value of vm.kmem_size is larger than the
>         suggested manual setting in most ZFS/FreeBSD tuning guides.
> So I understood this as "vm.kmem_size is set unnecessary large by
> default. You should think about decreasing it to save some RAM"
> On my amd64 server the default values of kmem_size are
> vm.kmem_size_scale: 3
> vm.kmem_size_max: 3865468109
> vm.kmem_size_min: 0
> vm.kmem_size: 1201446912
> Can someone give me a hint how to debug this problem further, or how to
> find some reasonable values for setting vm.kmem_size_max and
> vm.kmem_size with 16G of RAM?

Hmm, I wonder whether this applies to 7.2-RC1.  I don't know whether
the kernel changes have been committed to 7.2 or whether they were
already present when we started work on 7.2 because I haven't been
paying much attention.

On my 8-current amd64 machine with only 4GB of RAM I see larger values
than you see with 16GB:

sysctl vm.kmem_size_max
vm.kmem_size_max: 4509713203
sysctl vm.kmem_size
vm.kmem_size: 1335824384

Gary Jennejohn

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