From owner-freebsd-geom@FreeBSD.ORG Wed Jul 5 21:14:54 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 B73F216A4DD for ; Wed, 5 Jul 2006 21:14:54 +0000 (UTC) (envelope-from arne_woerner@yahoo.com) Received: from web30308.mail.mud.yahoo.com (web30308.mail.mud.yahoo.com [68.142.200.101]) by mx1.FreeBSD.org (Postfix) with SMTP id 5108D43D45 for ; Wed, 5 Jul 2006 21:14:54 +0000 (GMT) (envelope-from arne_woerner@yahoo.com) Received: (qmail 47045 invoked by uid 60001); 5 Jul 2006 21:14:53 -0000 DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws; s=s1024; d=yahoo.com; h=Message-ID:Received:Date:From:Subject:To:Cc:In-Reply-To:MIME-Version:Content-Type:Content-Transfer-Encoding; b=kK61fO7LxeL7LvhqEWcyTvisPjOItpXk/Udu1tGXTkTf6Ma1fhVrcTU7+KEhcM0EYC2xYylOWFFJRwMVN7txOPoc8pvueSyhhdAjgXzSAvi+tNVIAPpwRx8trZpmftpH3gD6ZOcVDj6VwuNKX07ZkqGEk1IvPQrcLQ9PBSzAVJg= ; Message-ID: <20060705211453.47043.qmail@web30308.mail.mud.yahoo.com> Received: from [213.54.92.253] by web30308.mail.mud.yahoo.com via HTTP; Wed, 05 Jul 2006 14:14:53 PDT Date: Wed, 5 Jul 2006 14:14:53 -0700 (PDT) From: "R. B. Riddick" To: Eric Anderson In-Reply-To: <44AC23FA.8040606@centtech.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: 8bit 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: Wed, 05 Jul 2006 21:14:54 -0000 --- 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. > > 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... -Arne __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com