From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S264455AbTLQRNm (ORCPT ); Wed, 17 Dec 2003 12:13:42 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S264469AbTLQRNm (ORCPT ); Wed, 17 Dec 2003 12:13:42 -0500 Received: from tmr-02.dsl.thebiz.net ([216.238.38.204]:4109 "EHLO gatekeeper.tmr.com") by vger.kernel.org with ESMTP id S264455AbTLQRNl (ORCPT ); Wed, 17 Dec 2003 12:13:41 -0500 To: linux-kernel@vger.kernel.org Path: gatekeeper.tmr.com!davidsen From: davidsen@tmr.com (bill davidsen) Newsgroups: mail.linux-kernel Subject: Re: raid0 slower than devices it is assembled of? Date: 17 Dec 2003 17:02:09 GMT Organization: TMR Associates, Schenectady NY Message-ID: References: <1071657159.2155.76.camel@abyss.local> X-Trace: gatekeeper.tmr.com 1071680529 6610 192.168.12.62 (17 Dec 2003 17:02:09 GMT) X-Complaints-To: abuse@tmr.com Originator: davidsen@gatekeeper.tmr.com Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org In article <1071657159.2155.76.camel@abyss.local>, Peter Zaitsev wrote: | One more issue with smaller stripes both for RAID5 and RAID0 (at least | for DBMS workloads) is - you normally want multi-block IO (ie fetching | many sequentially located pages) to be close in cost to reading single | page, which is true for single hard drive. However with small stripe | size you will hit many of underlying devices putting excessive not | necessary load. All this depends on what you're trying to optimize and the speed of the drives. I spent several years running on software raid and got to look harder than I wanted at the tuning. If the read size is large enough for transfer time to matter, not hidden in the latency, adjusting the stripe size so that you use many drives is a win. You want to avoid having a user i/o generate more than one i/o per drive if you can, which can lead to large stripe sizes. Also, the read to write ratio is important. RAID-5 does poorly with write, since the CRC needs to be recalculated and written each time. On read, unless you are in fallback mode, you just read the data and the performance is similar to RAID-0. If you have (a) a high read to write load, and (b) a very heavy read load, then RAID-1 works better, possibly with more than two copies of the data to reduce head motion contention. -- bill davidsen CTO, TMR Associates, Inc Doing interesting things with little computers since 1979.