Date: Sun, 11 May 2008 00:59:39 -0500 From: Barry Pederson <bp@barryp.org> To: Kris Kennaway <kris@FreeBSD.org> Cc: freebsd-current@freebsd.org Subject: Re: Re: ZFS file caching question Message-ID: <48268B4B.2040606@barryp.org> 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 wrote: > 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 I noticed in the release notes for 8.0-CURRENT under "What's New": http://www.freebsd.org/relnotes/CURRENT/relnotes/new.html it says: "A problem with using mmap(2) on ZFS filesystems has been fixed." Can anyone say which commits fixed this, and if they have been or will be MFCed to 7-STABLE? I'm curious because I've been having lots of trouble with Cyrus IMAP on 7.0-RELEASE corrupting cyrus.cache files, and was somewhat suspicious it could be mmap related. Barry
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