From mboxrd@z Thu Jan 1 00:00:00 1970 From: Asdo Subject: Re: Why does one get mismatches? Date: Tue, 02 Mar 2010 19:14:25 +0100 Message-ID: <4B8D5581.8070509@shiftmail.org> 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> <20100302110232.GA14131@maude.comedia.it> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-reply-to: <20100302110232.GA14131@maude.comedia.it> Sender: linux-raid-owner@vger.kernel.org To: linux-raid@vger.kernel.org List-Id: linux-raid.ids Luca Berra wrote: >>> >>> 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. > I think so, that it would not happen: the filesystem would stay consistent. (while the mismatches could still happen) The problem is that the barriers were introduced in all md raids in the 2.6.33 (just released), and also I have read XFS has a major performance drop with barriers activated. People will be tempted to disable barriers. AFAIR the performance drop was visible with 1 disk alone, imagine now with RAID. And I expect similar performance drops with other filesystems, correct me if I am wrong. Now it would be interesting to understand why the mismatches don't happen when LVM is above MD-raid!? The mechanisms presented up to now on this ML for mismatches don't explain why on LVM the same issue doesn't show up. I think. So you might want to use raid-1 and raid-10 under LVM, just in case....