From owner-freebsd-performance@FreeBSD.ORG Mon May 2 13:52:52 2005 Return-Path: Delivered-To: freebsd-performance@freebsd.org Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id 5882916A4D0 for ; Mon, 2 May 2005 13:52:52 +0000 (GMT) Received: from mail.rfnj.org (ns1.rfnj.org [66.180.172.156]) by mx1.FreeBSD.org (Postfix) with ESMTP id C43A343D58 for ; Mon, 2 May 2005 13:52:51 +0000 (GMT) (envelope-from asym@rfnj.org) Received: from megalomaniac.rfnj.org (ool-45736df1.dyn.optonline.net [69.115.109.241]) by mail.rfnj.org (Postfix) with ESMTP id 0AEDD2A3; Mon, 2 May 2005 09:52:51 -0400 (EDT) Message-Id: <6.2.1.2.2.20050502094757.037077f0@mail.rfnj.org> X-Mailer: QUALCOMM Windows Eudora Version 6.2.1.2 Date: Mon, 02 May 2005 09:58:49 -0400 To: "Steven Hartland" , "Eric Anderson" , "Poul-Henning Kamp" From: Allen In-Reply-To: <002701c54f1a$dde7b0e0$b3db87d4@multiplay.co.uk> References: <17479.1115040178@critter.freebsd.dk> <002701c54f1a$dde7b0e0$b3db87d4@multiplay.co.uk> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; format=flowed cc: freebsd-performance@freebsd.org Subject: Re: Very low disk performance on 5.x X-BeenThere: freebsd-performance@freebsd.org X-Mailman-Version: 2.1.1 Precedence: list List-Id: Performance/tuning List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 02 May 2005 13:52:52 -0000 At 09:28 5/2/2005, Steven Hartland wrote: >----- Original Message ----- From: "Poul-Henning Kamp" >>>Wouldn't this be a problem for writes then too? >>I presume you would only compare read to write performance on a RAID5 >>device which has battery backed cache. >>Without a battery backed cache (or pretending to have one) RAID5 >>write performance is abysmall no matter which alignment you use. > >This not what's been reported we are seeing writes that are 2x the >speed of reads. Give the additional overhead that writes encure >on RAID5 this should never be the case. The results I go in my tests >where: FWIW I have exactly this situation on a Mylex controller I have, if I enable the write cache on the card. For some reason or another, doing so results in marginally improved write speeds, but sustained read speeds that drop well below 1/4 of the values they get when the *write* cache is disabled. I have never figured out nor discovered a satisfactory explaination for why this is the case, and unlike Erics situation, this is consistent for me on that card across all OSes and tests. May be some more fuel for the fire however, to someone in the know. Also you should keep in mind, there could simply be some really goofy controller option enabled, that forces the RAID5 to behave in a "degraded" state for reads -- forcing it to read up all the other disks in the stripe and calculate the XOR again, to make sure the data it read off the disk matches the checksum. It's rare, but I've seen it before, and it will cause exactly this sort of RAID5 performance inversion. Since the XOR is recalculated on every write and requires only reading up one sector on a different disk, options that do the above will result in read scores drastically lower than writes to the same array.