Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932549AbWARAVv (ORCPT ); Tue, 17 Jan 2006 19:21:51 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S932550AbWARAVv (ORCPT ); Tue, 17 Jan 2006 19:21:51 -0500 Received: from ms-smtp-03-smtplb.tampabay.rr.com ([65.32.5.133]:60365 "EHLO ms-smtp-03.tampabay.rr.com") by vger.kernel.org with ESMTP id S932549AbWARAVt (ORCPT ); Tue, 17 Jan 2006 19:21:49 -0500 Message-ID: <43CD8A19.3010100@cfl.rr.com> Date: Tue, 17 Jan 2006 19:21:45 -0500 From: Phillip Susi User-Agent: Mozilla Thunderbird 1.0.7 (X11/20051010) X-Accept-Language: en-us, en MIME-Version: 1.0 To: Michael Loftis CC: linux-kernel@vger.kernel.org Subject: Re: FYI: RAID5 unusably unstable through 2.6.14 References: In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1672 Lines: 37 Your understanding of statistics leaves something to be desired. As you add disks the probability of a single failure is grows linearly, but the probability of double failure grows much more slowly. For example: If 1 disk has a 1/1000 chance of failure, then 2 disks have a (1/1000)^2 chance of double failure, and 3 disks have a (1/1000)^2 * 3 chance of double failure 4 disks have a (1/1000)^2 * 7 chance of double failure Thus the probability of double failure on this 4 drive array is ~142 times less than the odds of a single drive failing. As the probably of a single drive failing becomes more remote, then the ratio of that probability to the probability of double fault in the array grows exponentially. ( I think I did that right in my head... will check on a real calculator later ) This is why raid-5 was created: because the array has a much lower probabiliy of double failure, and thus, data loss, than a single drive. Then of course, if you are really paranoid, you can go with raid-6 ;) Michael Loftis wrote: > Absolutely not. The more spindles the more chances of a double failure. > Simple statistics will mean that unless you have mirrors the more drives > you add the more chance of two of them (really) failing at once and > choking the whole system. > > That said, there very well could be (are?) cases where md needs to do a > better job of handling the world unravelling. > - - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/