From mboxrd@z Thu Jan 1 00:00:00 1970 From: Keld =?iso-8859-1?Q?J=F8rn?= Simonsen Subject: Re: mdadm raid1 read performance Date: Thu, 5 May 2011 13:10:06 +0200 Message-ID: <20110505111006.GB11441@www2.open-std.org> References: <20110504105822.21e23bc3@notabene.brown> <4DC0F2B6.9050708@fnarfbargle.com> <20110505094538.0cef02cc@notabene.brown> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20110505094538.0cef02cc@notabene.brown> Sender: linux-raid-owner@vger.kernel.org To: NeilBrown Cc: Liam Kurmos , Roberto Spadim , Brad Campbell , Drew , linux-raid@vger.kernel.org List-Id: linux-raid.ids On Thu, May 05, 2011 at 09:45:38AM +1000, NeilBrown wrote: > On Thu, 5 May 2011 00:08:59 +0100 Liam Kurmos wrote: > > > as a separate question, what should be the theoretical performance of raid5? > > x(N-1) > > So a 4 drive RAID5 should read at 3 time the speed of a single drive. Actually, theoretically, it should be more than that for reading, more like N minus some overhead. In a raid5 stripe of 4 disks, when reading you do not read the checksum block, and thus you should be able to have all 4 drives occupied with reading real data. Some benchmarks back this up, http://home.comcast.net/~jpiszcz/20080329-raid/ http://blog.jamponi.net/2008/07/raid56-and-10-benchmarks-on-26255_10.html The latter reports a 3.44 times performance for raid5 reads with 4 disks, significantly over the N-1 = 3.0 mark. For writing, you are correct with the N-1 formular. best regards keld