From: david@lang.hm Subject: Re: [patch] document flash/RAID dangers Date: Tue, 25 Aug 2009 18:17:02 -0700 (PDT) Message-ID: References: <20090825094244.GC15563@elf.ucw.cz> <20090825161110.GP17684@mit.edu> <20090825222112.GB4300@elf.ucw.cz> <20090825224004.GD4300@elf.ucw.cz> <20090825233701.GH4300@elf.ucw.cz> <20090826001206.GL4300@elf.ucw.cz> <20090826003926.GQ4300@elf.ucw.cz> Mime-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed Cc: Theodore Tso , Ric Wheeler , 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 mail.lang.hm ([64.81.33.126]:33138 "EHLO bifrost.lang.hm" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932650AbZHZBSB (ORCPT ); Tue, 25 Aug 2009 21:18:01 -0400 In-Reply-To: <20090826003926.GQ4300@elf.ucw.cz> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Wed, 26 Aug 2009, Pavel Machek wrote: > On Tue 2009-08-25 17:20:13, david@lang.hm wrote: >> On Wed, 26 Aug 2009, Pavel Machek wrote: >> >>> On Tue 2009-08-25 16:56:40, david@lang.hm wrote: >>>> On Wed, 26 Aug 2009, Pavel Machek wrote: >>>> >>>>> There are storage devices that high highly undesirable properties >>>>> when they are disconnected or suffer power failures while writes are >>>>> in progress; such devices include flash devices and MD RAID 4/5/6 >>>>> arrays. >>>> >>>> change this to say 'degraded MD RAID 4/5/6 arrays' >>>> >>>> also find out if DM RAID 4/5/6 arrays suffer the same problem (I strongly >>>> suspect that they do) >>> >>> I changed it to say MD/DM. >>> >>>> then you need to add a note that if the array becomes degraded before a >>>> scrub cycle happens previously hidden damage (that would have been >>>> repaired by the scrub) can surface. >>> >>> I'd prefer not to talk about scrubing and such details here. Better >>> leave warning here and point to MD documentation. >> >> I disagree with that, the way you are wording this makes it sound as if >> raid isn't worth it. if you are going to say that raid is risky you need >> to properly specify when it is risky > > Ok, would this help? I don't really want to go to scrubbing details. > > (*) Degraded array or single disk failure "near" the powerfail is > neccessary for this property of RAID arrays to bite. that sounds reasonable David Lang