From: Pavel Machek Subject: Re: [patch] document flash/RAID dangers Date: Wed, 26 Aug 2009 02:39:26 +0200 Message-ID: <20090826003926.GQ4300@elf.ucw.cz> 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> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii 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: david@lang.hm Return-path: Received: from atrey.karlin.mff.cuni.cz ([195.113.26.193]:54210 "EHLO atrey.karlin.mff.cuni.cz" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932253AbZHZAje (ORCPT ); Tue, 25 Aug 2009 20:39:34 -0400 Content-Disposition: inline In-Reply-To: Sender: linux-ext4-owner@vger.kernel.org List-ID: 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. >>>> THESE devices have the property of potentially corrupting blocks being >>>> written at the time of the power failure, >>> >>> this is true of all devices >> >> Actually I don't think so. I believe SATA disks do not corrupt even >> the sector they are writing to -- they just have big enough >> capacitors. And yes I believe ext3 depends on that. > > you are incorrect on this. > > ext3 (like every other filesystem) just accepts the risk (zfs makes some > attempt to detect such corruption) I'd like Ted to comment on this. He wrote the original document, and I'd prefer not to introduce mistakes. Pavel -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html