From mboxrd@z Thu Jan 1 00:00:00 1970 From: Pavel Machek Subject: Re: raid is dangerous but that's secret (was Re: [patch] ext2/3: Date: Tue, 1 Sep 2009 10:46:50 +0200 Message-ID: <20090901084650.GF9942@elf.ucw.cz> References: <20090901005629.3932.qmail@science.horizon.com> <4a2c5faeb04cab59af9ba6ab512c9916.squirrel@neil.brown.name> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: George Spelvin , david@lang.hm, linux-doc@vger.kernel.org, linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org To: NeilBrown Return-path: Received: from atrey.karlin.mff.cuni.cz ([195.113.26.193]:37252 "EHLO atrey.karlin.mff.cuni.cz" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753072AbZIAIqz (ORCPT ); Tue, 1 Sep 2009 04:46:55 -0400 Content-Disposition: inline In-Reply-To: <4a2c5faeb04cab59af9ba6ab512c9916.squirrel@neil.brown.name> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Tue 2009-09-01 18:36:22, NeilBrown wrote: > On Tue, September 1, 2009 10:56 am, George Spelvin wrote: > > The fact that the ZFS decelopers observed drives writing the data to the > > wrong location emphasizes the importance of keeping the checksum with > > the pointer. An embedded checksum, no matter how good, can't tell you if > > the data is stale; you need a way to distinguish versions in the pointer. > > I would disagree with that. > If the embedded checksum is a function of both the data and the address > of the data (in whatever address space seems most appropriate) then it can > still verify that the data found with the checksum is the data that was > expected. > And storing the checksum with the data (where it is practical) means > index blocks can be more dense so on average fewer accesses to storage > are needed. Well, storing checksum with the pointer means that you catch dropped writes, too. Imagine the disk drive just fails to write block A. Adding checksum of address will not catch that... Pavel -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html