From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751471AbZHXJ0d (ORCPT ); Mon, 24 Aug 2009 05:26:33 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751441AbZHXJ0d (ORCPT ); Mon, 24 Aug 2009 05:26:33 -0400 Received: from atrey.karlin.mff.cuni.cz ([195.113.26.193]:57852 "EHLO atrey.karlin.mff.cuni.cz" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751244AbZHXJ0c (ORCPT ); Mon, 24 Aug 2009 05:26:32 -0400 Date: Mon, 24 Aug 2009 11:26:22 +0200 From: Pavel Machek To: Goswin von Brederlow Cc: Rob Landley , kernel list , Andrew Morton , mtk.manpages@gmail.com, tytso@mit.edu, rdunlap@xenotime.net, linux-doc@vger.kernel.org, linux-ext4@vger.kernel.org Subject: Re: ext2/3: document conditions when reliable operation is possible Message-ID: <20090824092622.GC25591@elf.ucw.cz> References: <20090312092114.GC6949@elf.ucw.cz> <200903121413.04434.rob@landley.net> <20090316122847.GI2405@elf.ucw.cz> <200903161426.24904.rob@landley.net> <20090323104525.GA17969@elf.ucw.cz> <87ljqn82zc.fsf@frosties.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87ljqn82zc.fsf@frosties.localdomain> X-Warning: Reading this can be dangerous to your mental health. User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi! > >> > This is not about barriers (that should be different topic). Atomic > >> > write means that either whole sector is written, or nothing at all is > >> > written. Because raid5 needs to update both master data and parity at > >> > the same time, I don't think it can guarantee this during powerfail. > > Actualy raid5 should have no problem with a power failure during > normal operations of the raid. The parity block should get marked out > of sync, then the new data block should be written, then the new > parity block and then the parity block should be flaged in sync. > > >> Good point, but I thought that's what journaling was for? > > > > I believe journaling operates on assumption that "either whole sector > > is written, or nothing at all is written". > > The real problem comes in degraded mode. In that case the data block > (if present) and parity block must be written at the same time > atomically. If the system crashes after writing one but before writing > the other then the data block on the missng drive changes its > contents. And for example with a chunk size of 1MB and 16 disks that > could be 15MB away from the block you actualy do change. And you can > not recover that after a crash as you need both the original and > changed contents of the block. > > So writing one sector has the risk of corrupting another (for the FS) > totally unconnected sector. No amount of journaling will help > there. The raid5 would need to do journaling or use battery backed > cache. Thanks, I updated my notes. Pavel -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html