From owner-freebsd-current@FreeBSD.ORG Wed Apr 23 14:36:42 2008 Return-Path: Delivered-To: freebsd-current@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id 69CF91065673 for ; Wed, 23 Apr 2008 14:36:42 +0000 (UTC) (envelope-from kris@FreeBSD.org) Received: from weak.local (freefall.freebsd.org [IPv6:2001:4f8:fff6::28]) by mx1.freebsd.org (Postfix) with ESMTP id 95EB38FC28; Wed, 23 Apr 2008 14:36:41 +0000 (UTC) (envelope-from kris@FreeBSD.org) Message-ID: <480F4972.20609@FreeBSD.org> Date: Wed, 23 Apr 2008 16:36:34 +0200 From: Kris Kennaway User-Agent: Thunderbird 2.0.0.12 (Macintosh/20080213) MIME-Version: 1.0 To: Andrew Gallatin References: <18447.17658.759349.720175@grasshopper.cs.duke.edu> In-Reply-To: <18447.17658.759349.720175@grasshopper.cs.duke.edu> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: freebsd-current@freebsd.org Subject: Re: ZFS file caching question X-BeenThere: freebsd-current@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Discussions about the use of FreeBSD-current List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 23 Apr 2008 14:36:42 -0000 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