From: david@lang.hm Subject: Re: [patch] document flash/RAID dangers Date: Tue, 25 Aug 2009 17:26:11 -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> <4A947839.4010601@redhat.com> <20090826000657.GK4300@elf.ucw.cz> <4A947E05.8070406@redhat.com> <20090826002045.GO4300@elf.ucw.cz> Mime-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed Cc: Ric Wheeler , 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: In-Reply-To: <20090826002045.GO4300@elf.ucw.cz> Sender: linux-doc-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org 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. These devices have the property of potentially >>>>> corrupting blocks being written at the time of the power failure, and >>>>> worse yet, amplifying the region where blocks are corrupted such that >>>>> additional sectors are also damaged during the power failure. >>>> >>>> I would strike the entire mention of MD devices since it is your >>>> assertion, not a proven fact. You will cause more data loss from common >>> >>> That actually is a fact. That's how MD RAID 5 is designed. And btw >>> those are originaly Ted's words. >> >> Ted did not design MD RAID5. > > So what? He clearly knows how it works. > > Instead of arguing he's wrong, will you simply label everything as > unproven? > >>>> events (single sector errors, complete drive failure) by steering people >>>> away from more reliable storage configurations because of a really rare >>>> edge case (power failure during split write to two raid members while >>>> doing a RAID rebuild). >>> >>> I'm not sure what's rare about power failures. Unlike single sector >>> errors, my machine actually has a button that produces exactly that >>> event. Running degraded raid5 arrays for extended periods may be >>> slightly unusual configuration, but I suspect people should just do >>> that for testing. (And from the discussion, people seem to think that >>> degraded raid5 is equivalent to raid0). >> >> Power failures after a full drive failure with a split write during a rebuild? > > Look, I don't need full drive failure for this to happen. I can just > remove one disk from array. I don't need power failure, I can just > press the power button. I don't even need to rebuild anything, I can > just write to degraded array. > > Given that all events are under my control, statistics make little > sense here. if you are intentionally causing several low-probability things to happen at once you increase the risk of corruption note that you also need a write to take place, and be interrupted in just the right way. David Lang