From mboxrd@z Thu Jan 1 00:00:00 1970 From: Phillip Susi Subject: Re: Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md. Date: Wed, 30 May 2007 12:55:14 -0400 Message-ID: <465DAC72.1010201@cfl.rr.com> References: <18006.38689.818186.221707@notabene.brown> <18010.12472.209452.148229@notabene.brown> <20070528094358.GM25091@agk.fab.redhat.com> <5201e28f0705290225v14fdac44hb0382a4137a84d01@mail.gmail.com> <20070529220500.GA6513@agk.fab.redhat.com> <5201e28f0705300212g3be16464u5ee1a4c80db27a11@mail.gmail.com> Reply-To: device-mapper development Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <5201e28f0705300212g3be16464u5ee1a4c80db27a11@mail.gmail.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: dm-devel-bounces@redhat.com Errors-To: dm-devel-bounces@redhat.com To: Stefan Bader Cc: Tejun Heo , Stefan Bader , David Chinner , linux-kernel@vger.kernel.org, linux-raid@vger.kernel.org, device-mapper development , Jens Axboe , linux-fsdevel@vger.kernel.org, Andreas Dilger List-Id: linux-raid.ids Stefan Bader wrote: > You got a linear target that consists of two disks. One drive (a) > supports barriers and the other one (b) doesn't. Device-mapper just > maps the requests to the appropriate disk. Now the following sequence > happens: > > 1. block x gets mapped to drive b > 2. block y (with barrier) gets mapped to drive a > > Since drive a supports barrier request we don't get -EOPNOTSUPP but > the request with block y might get written before block x since the > disk are independent. I guess the chances of this are quite low since > at some point a barrier request will also hit drive b but for the time > being it might be better to indicate -EOPNOTSUPP right from > device-mapper. The device mapper needs to ensure that ALL underlying devices get a barrier request when one comes down from above, even if it has to construct zero length barriers to send to most of them.