Date: Fri, 16 Nov 2007 13:14:31 +0000 From: bruce@cran.org.uk To: Laszlo Nagy <gandalf@shopzeus.com> Cc: freebsd-questions@freebsd.org Subject: Re: How to set maximum disk cache size? Message-ID: <20071116131430.GA27885@muon.bluestop.org> In-Reply-To: <473D8940.3030501@shopzeus.com> References: <473C7C0A.4060708@shopzeus.com> <20071115182220.E60452@wojtek.tensor.gdynia.pl> <473CAF70.1090006@cran.org.uk> <473D8940.3030501@shopzeus.com>
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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 > >>>cache > >>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 > >http://lists.freebsd.org/pipermail/freebsd-performance/2004-April/000785.html > > > >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 > changed. > > Thanks, > > Laszlo > 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 http://lists.freebsd.org/pipermail/freebsd-current/2007-September/077250.html for a more recent discussion. -- Bruce
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