From: Ric Wheeler Subject: Re: [patch] ext2/3: document conditions when reliable operation is possible Date: Tue, 25 Aug 2009 19:10:49 -0400 Message-ID: <4A946F79.3020103@redhat.com> References: <20090323104525.GA17969@elf.ucw.cz> <87ljqn82zc.fsf@frosties.localdomain> <20090824093143.GD25591@elf.ucw.cz> <82k50tjw7u.fsf@mid.bfk.de> <20090824130125.GG23677@mit.edu> <20090824195159.GD29763@elf.ucw.cz> <4A92F6FC.4060907@redhat.com> <20090824205209.GE29763@elf.ucw.cz> <4A930160.8060508@redhat.com> <20090824212518.GF29763@elf.ucw.cz> <20090824223915.GI17684@mit.edu> <19092.27809.480243.979274@notabene.brown> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Theodore Tso , Pavel Machek , 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 To: Neil Brown Return-path: In-Reply-To: <19092.27809.480243.979274@notabene.brown> Sender: linux-doc-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org On 08/25/2009 06:58 PM, Neil Brown wrote: > On Monday August 24, tytso@mit.edu wrote: >> On Mon, Aug 24, 2009 at 11:25:19PM +0200, Pavel Machek wrote: >>>> I have to admit that I have not paid enough attention to this specifics >>>> of your ext3 + flash card issue - is it the ftl stuff doing out of order >>>> IO's? >>> >>> The problem is that flash cards destroy whole erase block on unplug, >>> and ext3 can't cope with that. >> >> Sure --- but name **any** filesystem that can deal with the fact that >> 128k or 256k worth of data might disappear when you pull out the flash >> card while it is writing a single sector? > > A Log structured filesystem could certainly be written to deal with > such a situation, providing by 'deal with' you mean 'only loses data > that has not yet been acknowledged to the application'. Of course the > filesystem would need clear visibility into exactly how these blocks > are positioned. > > I've been playing with just such a filesystem for some time (never > really finding enough time) with the goal of making it work over RAID5 > with no data risk due to power loss. One day it will be functional > enough for others to try.... > > It is entirely possible that NILFS could be made to meet that > requirement, but I haven't made time to explore NILFS so I cannot be > sure. > > NeilBrown > I am not sure that log structure will protect you from this scenario since once you clean the log, the non-logged data is assumed to be correct. If your cheap flash storage device can nuke random regions of that clean storage, you will lose data.... ric