From: david@lang.hm Subject: Re: [patch] document flash/RAID dangers Date: Tue, 25 Aug 2009 18:19:40 -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> <4A94812C.5010803@redhat.com> <20090826004430.GR4300@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: <20090826004430.GR4300@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: >>>>> 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. >> >> Pavel, no S-ATA drive has capacitors to hold up during a power failure >> (or even enough power to destage their write cache). I know this from >> direct, personal knowledge having built RAID boxes at EMC for years. In >> fact, almost all RAID boxes require that the write cache be hardwired to >> off when used in their arrays. > > I never claimed they have enough power to flush entire cache -- read > the paragraph again. I do believe the disks have enough capacitors to > finish writing single sector, and I do believe ext3 depends on that. keep in mind that in a powerfail situation the data being sent to the drive may be corrupt (the ram gets flaky while a DMA to the drive copies the bad data to the drive, which writes it before the power loss gets bad enough for the drive to decide there is a problem and shutdown) you just plain cannot count on writes that are in flight when a powerfail happens to do predictable things, let alone what you consider sane or proper. David Lang