From owner-freebsd-geom@FreeBSD.ORG Thu Jul 6 12:14:33 2006 Return-Path: X-Original-To: freebsd-geom@freebsd.org Delivered-To: freebsd-geom@freebsd.org Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id C7D9D16A4DE for ; Thu, 6 Jul 2006 12:14:33 +0000 (UTC) (envelope-from anderson@centtech.com) Received: from mh1.centtech.com (moat3.centtech.com [207.200.51.50]) by mx1.FreeBSD.org (Postfix) with ESMTP id 4F6AB43D46 for ; Thu, 6 Jul 2006 12:14:33 +0000 (GMT) (envelope-from anderson@centtech.com) Received: from [10.177.171.220] (neutrino.centtech.com [10.177.171.220]) by mh1.centtech.com (8.13.1/8.13.1) with ESMTP id k66CEW9B085853; Thu, 6 Jul 2006 07:14:32 -0500 (CDT) (envelope-from anderson@centtech.com) Message-ID: <44ACFEAB.1020306@centtech.com> Date: Thu, 06 Jul 2006 07:14:35 -0500 From: Eric Anderson User-Agent: Thunderbird 1.5.0.4 (X11/20060612) MIME-Version: 1.0 To: "R. B. Riddick" References: <20060705211453.47043.qmail@web30308.mail.mud.yahoo.com> In-Reply-To: <20060705211453.47043.qmail@web30308.mail.mud.yahoo.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit X-Virus-Scanned: ClamAV 0.87.1/1586/Wed Jul 5 14:22:07 2006 on mh1.centtech.com X-Virus-Status: Clean Cc: freebsd-geom@freebsd.org Subject: Re: new class / geom_cache / request for comments X-BeenThere: freebsd-geom@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: GEOM-specific discussions and implementations List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 06 Jul 2006 12:14:33 -0000 R. B. Riddick wrote: > --- Eric Anderson wrote: >> R. B. Riddick wrote: >>> --- Eric Anderson wrote: >>>> Just curious, how is this different/better than the regular buffer >>>> cache? >>> Do you mean this cache of those file systems? >>> If yes: The answer should be: "It just makes everything worse." >>> (there is not even some read ahead...) >> Not sure what I'm missing here, please clue me in. I guess I am indeed >> talking about the cache normally used for filesystems. If I read a >> 500MB file in, first time it comes from disk, second time it comes from >> memory cache, correct? How is geom_cache different? >> > Yes, I think so... > But geom_cache is just useful, when file system's buffer cache cannot help. > > E. g.: > A degraded RAID5 on 4 consumers (3 good plus 1 failed). > When we want to get a data block, that resides on the failed consumer, we have > to read all corresponding blocks (2+1) in order to rebuild the missing block. > When we do a sequential read, we would have to read the consumers, that hold > the data blocks twice (2 x 2). > So the geom_cache could help here (2+1 real reads plus 2 from the cache), if > the provider is not too busy. Ok, I understand now. I could see this being useful for fsck's also. I wonder if it's possible to have the cache distributed across multiple machines? (just out of curiosity) >>> It is just useful, if you dont have any other caches (e. g. a ufs on a >>> geom_raid5 (I think I should have it tomorrow... :-) ) on some geom_cache >>> providers)... >> I suppose I just need to play with it to completely understand.. >> > :-) > Maybe it is useless... and I dont see it... and nobody dares to say it... > I saw, that the CVS tree does not have a sys/geom/cache directory... Not useless! I think it's a good addition.. I've even thought about a geom_cache before, but then wrote it off to 'oh wait, the filesystem caches would handle all that', but you've thought further and realized a good use for it.. Thanks for writing it! Eric -- ------------------------------------------------------------------------ Eric Anderson Sr. Systems Administrator Centaur Technology Anything that works is better than anything that doesn't. ------------------------------------------------------------------------