From mboxrd@z Thu Jan 1 00:00:00 1970 From: Asdo Subject: Re: Why does one get mismatches? Date: Wed, 03 Mar 2010 00:27:25 +0100 Message-ID: <4B8D9EDD.2080300@shiftmail.org> References: <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> <4B8D5581.8070509@shiftmail.org> <20100302185216.GB2243@lazy.lzy> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-reply-to: <20100302185216.GB2243@lazy.lzy> Sender: linux-raid-owner@vger.kernel.org To: Piergiorgio Sartor Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids Piergiorgio Sartor wrote: > Hi, > >> Now it would be interesting to understand why the mismatches don't >> happen when LVM is above MD-raid!? >> > > well, maybe LVM copies the buffer, and after it > plays it nice with MD, i.e. no changes on the fly. > LVM copies the buffer!? I don't think so... LVM is near-zero overhead, so I would be surprised if it copied the buffer. Also I don't think it was needed in their case, except maybe if there is an I/O at the boundary of a logical volume or LVM stripe, which would certainly be a mistake at requestor side. LVM also does not merge requests AFAIR. (visible with mdstat -x 1) > Or maybe, it is just one system that behaves > properly with LVM over MD. > hmm maybe... But me also I have never seen mismatches and the only raid-1's I have are above LVM. (except /boot but that's almost never modified)