How to set maximum disk cache size?
bruce at cran.org.uk
bruce at cran.org.uk
Fri Nov 16 05:14:34 PST 2007
On Fri, Nov 16, 2007 at 01:12:48PM +0100, Laszlo Nagy wrote:
> Bruce Cran ?rta:
> >Wojciech Puchar wrote:
> >> Laszlo wrote:
> >>>Hi All,
> >>>Is there a way (sysctl?) to tell FreeBSD (6.2 RELEASE) how many
> >>>memory can it use for caching file data from disk?
> >>>It might be that FreeBSD will use all available RAM, and reduce the
> >>it already does
> >It may seem strange since it's generally accepted that you can never
> >have enough disk cache, but FreeBSD apparently doesn't actually use
> >all the free memory for caching. By default it uses up to 256MB for
> >buffering/caching and there's no way it can use all available memory
> >on i386 in machines with more than 1GB installed since the
> >buffer/cache is allocated from KVM and the default maximum is 1GB.
> >You can increase the amount of memory used, but it might not help -
> >there's a thread on performance@ from 2004 which describes how it all
> >works; see
> >The information there is quite old now though so I don't know if
> >things are done differently in 6.x.
> OK, and how about amd64 arch? The reason I ask this is that we have a
> big postresql database (over 3GB) and PostgreSQL rely on the OS for
> caching files in memory. This database is mostly read-only, so it would
> be nice to use all free memory for caching. Especially that this machine
> is the database server, it does nothing else. Now, it is an i386 but we
> are about to migrate to AMD X2, then we can put in 8GB of memory. But
> only if the OS can use if for caching. Otherwise it would be useless.
> Thank you for the link. That thread is quite old - things might have
I'm just going by what I've read on current@ but it seems things are
still the same, both on i386 and amd64. I don't know if it's happened
yet, but I think there was a plan to dramatically increase the kernel
address space for 7.0 on amd64, mainly triggered I think by ZFS which
likes to allocate loads of kernel memory. See
for a more recent discussion.
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