Date: Wed, 23 Apr 2008 16:36:34 +0200 From: Kris Kennaway <kris@FreeBSD.org> To: Andrew Gallatin <gallatin@cs.duke.edu> Cc: freebsd-current@freebsd.org Subject: Re: ZFS file caching question Message-ID: <480F4972.20609@FreeBSD.org> In-Reply-To: <18447.17658.759349.720175@grasshopper.cs.duke.edu> References: <18447.17658.759349.720175@grasshopper.cs.duke.edu>
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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. > 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. Kris
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