From: Ric Wheeler Subject: Re: [patch] ext2/3: document conditions when reliable operation is possible Date: Wed, 26 Aug 2009 08:01:53 -0400 Message-ID: <4A952431.1030509@redhat.com> References: <20090825232601.GF4300@elf.ucw.cz> <4A947682.2010204@redhat.com> <20090825235359.GJ4300@elf.ucw.cz> <4A947DA9.2080906@redhat.com> <20090826001645.GN4300@elf.ucw.cz> <4A948259.40007@redhat.com> <20090826010018.GA17684@mit.edu> <4A948C94.7040103@redhat.com> <20090826025849.GF32712@mit.edu> <4A9510D2.1090704@redhat.com> <20090826111208.GA26595@elf.ucw.cz> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Theodore Tso , Florian Weimer , Goswin von Brederlow , Rob Landley , kernel list , Andrew Morton , mtk.manpages@gmail.com, rdunlap@xenotime.net, linux-doc@vger.kernel.org, linux-ext4@vger.kernel.org, corbet@lwn.net To: Pavel Machek Return-path: Received: from mx1.redhat.com ([209.132.183.28]:16172 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756902AbZHZMBu (ORCPT ); Wed, 26 Aug 2009 08:01:50 -0400 In-Reply-To: <20090826111208.GA26595@elf.ucw.cz> Sender: linux-ext4-owner@vger.kernel.org List-ID: On 08/26/2009 07:12 AM, Pavel Machek wrote: > On Wed 2009-08-26 06:39:14, Ric Wheeler wrote: > >> On 08/25/2009 10:58 PM, Theodore Tso wrote: >> >>> On Tue, Aug 25, 2009 at 09:15:00PM -0400, Ric Wheeler wrote: >>> >>> >>>> I agree with the whole write up outside of the above - degraded RAID >>>> does meet this requirement unless you have a second (or third, counting >>>> the split write) failure during the rebuild. >>>> >>>> >>> The argument is that if the degraded RAID array is running in this >>> state for a long time, and the power fails while the software RAID is >>> in the middle of writing out a stripe, such that the stripe isn't >>> completely written out, we could lose all of the data in that stripe. >>> >>> In other words, a power failure in the middle of writing out a stripe >>> in a degraded RAID array counts as a second failure. >>> To me, this isn't a particularly interesting or newsworthy point, >>> since a competent system administrator who cares about his data and/or >>> his hardware will (a) have a UPS, and (b) be running with a hot spare >>> and/or will imediately replace a failed drive in a RAID array. >>> >> I agree that this is not an interesting (or likely) scenario, certainly >> when compared to the much more frequent failures that RAID will protect >> against which is why I object to the document as Pavel suggested. It >> will steer people away from using RAID and directly increase their >> chances of losing their data if they use just a single disk. >> > So instead of fixing or at least documenting known software deficiency > in Linux MD stack, you'll try to surpress that information so that > people use more of raid5 setups? > > Perhaps the better documentation will push them to RAID1, or maybe > make them buy an UPS? > Pavel > I am against documenting unlikely scenarios out of context that will lead people to do the wrong thing. ric