Date: Wed, 23 Apr 2008 10:49:23 -0400 (EDT) From: Andrew Gallatin <gallatin@cs.duke.edu> To: Kris Kennaway <kris@FreeBSD.org> Cc: freebsd-current@FreeBSD.org Subject: Re: ZFS file caching question Message-ID: <18447.19594.892539.640373@grasshopper.cs.duke.edu> In-Reply-To: <480F4972.20609@FreeBSD.org> References: <18447.17658.759349.720175@grasshopper.cs.duke.edu> <480F4972.20609@FreeBSD.org>
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Kris Kennaway writes: > Andrew Gallatin wrote: > > If I have, say, 512MB RAM and a 1GB file which is written or read > > sequentially on an otherwise idle system, I'd expect the last 512MB (- > > epsilon) of the file's pages to be cached in RAM. This is true for > > UFS, but it does not appear to be the case with ZFS (see example > > below). > > > > Can somebody explain how the arc cache in ZFS relates to the normal > > page cache used by traditional filesystems? Are ZFS filesystems > > cached exclusively in the arc cache, and not in the page cache? Is > > the arc cache per-filesystem, per-pool, or global for ZFS as a whole? > > The ZFS arc cache is completely independent from the normal buffer cache > on FreeBSD. This is inefficient in a number of ways. I have also seen > things that make me suspicious that it is not caching properly even when > you tune it to be "large enough" (if possible given memory constraints), > but I haven't confirmed this. In some ways this is kind of cool. I'm want to use FreeBSD+ZFS for a new desktop which will also host a media server. If I put the media on ZFS, and my home directory on UFS, then the gigantic HD media files recorded overnight won't push my desktop applications out of RAM overnight. That's a feature :) > > Hmm.. Could this be the cause of the problems with ZFS and mmap'ed files? > > What problems do you mean? There were coherency problems but I think > they were fixed. The last I saw about this on -current was: http://lists.freebsd.org/pipermail/freebsd-current/2008-February/083396.html This seemed to terminate with people disabling mmap in the applications, not with a fix to ZFS. Maybe I missed a commit.. Thanks, Drew
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