[PC-BSD Testing] vfs.zfs.arch_max on 9.0 AMD64 (jan.13, 2011 snapshot)

Bruce Cran bruce at cran.org.uk
Thu Feb 3 14:38:31 PST 2011

On Thu, Feb 03, 2011 at 10:02:07AM -0500, Kris Moore wrote:
> I guess the question would be what is the downside if we drop that max
> size? They have it set to that by default already, so there must have
> been a reason to pick it originally.

As I understand it the only downside would be less memory available for
filesystem caching/buffering. However we need to be careful not to increase
the default, for example on machines with only 4GB RAM. There have been lots
of discussions about arc_max and the famous "kmem_map too small" panics. The
current default seems to be good for preventing that but I think it might be
a bit too high for desktop systems: on a 6GB machine it's set to 5GB and 72MB
got swapped out when running a buildworld. When using UFS I'd normally see
just a couple of MB ever get swapped out after the machine's been running for
a few days.

Bruce Cran

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