From mboxrd@z Thu Jan 1 00:00:00 1970 From: Luca Berra Subject: Re: Why does one get mismatches? Date: Tue, 2 Mar 2010 12:02:32 +0100 Message-ID: <20100302110232.GA14131@maude.comedia.it> References: <4B7B0D45.7040801@tmr.com> <6db64f7872286165ac1fd3436e9d6476@localhost> <20100218100547.7aecdc34@notabene.brown> <4B853BBF.7000607@tmr.com> <20100225083936.07cd48ad@notabene.brown> <20100228080949.GA30574@maude.comedia.it> <20100302160100.621f9811@notabene.brown> <20100302073624.GA28827@maude.comedia.it> <4877c76c1003020204r477e942fo8ada66e1e9426295@mail.gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1; format=flowed Return-path: Content-Disposition: inline In-Reply-To: <4877c76c1003020204r477e942fo8ada66e1e9426295@mail.gmail.com> Sender: linux-raid-owner@vger.kernel.org To: linux-raid@vger.kernel.org List-Id: linux-raid.ids On Tue, Mar 02, 2010 at 02:04:47AM -0800, Michael Evans wrote: >On Mon, Mar 1, 2010 at 11:36 PM, Luca Berra wrote: >> On Tue, Mar 02, 2010 at 04:01:00PM +1100, Neil Brown wrote: >>>> Disk 1 contains A and B in the correct order, it is missing C and B' but >>>> we >>>> dont care, app should be able to recover from a crash >>>> >>>> Disk 2 contains A and B', but they are wrongly ordered because C is >>>> missing >>>> >>>> If in the above case A and C are data blocks and B contains a journal >>>> related to A and C, booting from disk 2 could result in inconsistent >>>> data. >>>> >>>> can the above really happen? >>>> would using barriers remove the above concern? >>>> am i missing something else? >>> >>> These is no inconsistency here that a filesystem would not equally expect >>> from a single device. >>> After the crash-while-writing B', it should expect to see either B or B', >>> and it does, depending on which device is primary. >>> >>> Nothing to see here. >> >> I will try to explain better, >> the problem is not related to the confusion between B or B' >> >> the problem is that on one disk we have B' _without_ C. >> >You're demanding full atomic commits; this is precisely what journals >and /barriers/ are for. > >Are you are bypassing them in a quest for performance and paying for >it on crashes? >Or is this a hardware bug? >Or is it some glitch in the block device layering leading to barrier >requests not being honored? I just asked for confirmation that with /barriers/ the scenario above would not happen. L. -- Luca Berra -- bluca@comedia.it Communication Media & Services S.r.l. /"\ \ / ASCII RIBBON CAMPAIGN X AGAINST HTML MAIL / \