From owner-freebsd-performance@FreeBSD.ORG Mon May 2 13:28:54 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 AAFB416A4CE for ; Mon, 2 May 2005 13:28:54 +0000 (GMT) Received: from mh2.centtech.com (moat3.centtech.com [207.200.51.50]) by mx1.FreeBSD.org (Postfix) with ESMTP id 0BEFB43D45 for ; Mon, 2 May 2005 13:28:54 +0000 (GMT) (envelope-from anderson@centtech.com) Received: from [10.177.171.220] (neutrino.centtech.com [10.177.171.220]) by mh2.centtech.com (8.13.1/8.13.1) with ESMTP id j42DSrFU074818; Mon, 2 May 2005 08:28:53 -0500 (CDT) (envelope-from anderson@centtech.com) Message-ID: <42762AD8.1020607@centtech.com> Date: Mon, 02 May 2005 08:27:52 -0500 From: Eric Anderson User-Agent: Mozilla/5.0 (X11; U; FreeBSD i386; en-US; rv:1.7.5) Gecko/20050325 X-Accept-Language: en-us, en MIME-Version: 1.0 To: Poul-Henning Kamp References: <17479.1115040178@critter.freebsd.dk> In-Reply-To: <17479.1115040178@critter.freebsd.dk> Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit 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 13:28:54 -0000 Poul-Henning Kamp wrote: > In message <4276281C.6060209@centtech.com>, Eric Anderson writes: > >>Poul-Henning Kamp wrote: >> >>>In message <427626DC.5030702@centtech.com>, Eric Anderson writes: >>> >>> >>> >>>>Don't mean to be terse here, but I'm talking about the same test done an >>>>two different RAID5 configurations, with different disks, and not just >>>>me - other users in this very thread see the same issue.. >>> >>> >>>Uhm, if you are using RAID5 and your requests are not aligned and >>>sized after the RAID5 you should *expect* read performance to be poor. >>> >>>If you your request ends up accessing two different blocks even just >>>once per stripe, this totally kills performance. >> >>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. If I write a 10GB file to disk (RAID array has 1GB cache, system has 1GB memory), then I should definitely see better read performance reading that same file back to /dev/null than writing it, right? How about this - you tell me what test to run, and I'll do it (as long as it doesn't destory my data). Eric -- ------------------------------------------------------------------------ Eric Anderson Sr. Systems Administrator Centaur Technology A lost ounce of gold may be found, a lost moment of time never. ------------------------------------------------------------------------