Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759459AbXEaHIK (ORCPT ); Thu, 31 May 2007 03:08:10 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1756167AbXEaHH5 (ORCPT ); Thu, 31 May 2007 03:07:57 -0400 Received: from brick.kernel.dk ([80.160.20.94]:19279 "EHLO kernel.dk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755840AbXEaHH4 (ORCPT ); Thu, 31 May 2007 03:07:56 -0400 Date: Thu, 31 May 2007 09:06:57 +0200 From: Jens Axboe To: David Chinner Cc: david@lang.hm, Phillip Susi , Neil Brown , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, dm-devel@redhat.com, linux-raid@vger.kernel.org, Stefan Bader , Andreas Dilger , Tejun Heo Subject: Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md. Message-ID: <20070531070656.GK32105@kernel.dk> References: <18010.12472.209452.148229@notabene.brown> <20070528024559.GA85884050@sgi.com> <465C871F.708@cfl.rr.com> <20070529234832.GT85884050@sgi.com> <20070530061723.GY85884050@sgi.com> <20070531002011.GC85884050@sgi.com> <20070531062644.GI32105@kernel.dk> <20070531070307.GK85884050@sgi.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20070531070307.GK85884050@sgi.com> Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2312 Lines: 51 On Thu, May 31 2007, David Chinner wrote: > On Thu, May 31, 2007 at 08:26:45AM +0200, Jens Axboe wrote: > > On Thu, May 31 2007, David Chinner wrote: > > > IOWs, there are two parts to the problem: > > > > > > 1 - guaranteeing I/O ordering > > > 2 - guaranteeing blocks are on persistent storage. > > > > > > Right now, a single barrier I/O is used to provide both of these > > > guarantees. In most cases, all we really need to provide is 1); the > > > need for 2) is a much rarer condition but still needs to be > > > provided. > > > > > > > if I am understanding it correctly, the big win for barriers is that you > > > > do NOT have to stop and wait until the data is on persistant media before > > > > you can continue. > > > > > > Yes, if we define a barrier to only guarantee 1), then yes this > > > would be a big win (esp. for XFS). But that requires all filesystems > > > to handle sync writes differently, and sync_blockdev() needs to > > > call blkdev_issue_flush() as well.... > > > > > > So, what do we do here? Do we define a barrier I/O to only provide > > > ordering, or do we define it to also provide persistent storage > > > writeback? Whatever we decide, it needs to be documented.... > > > > The block layer already has a notion of the two types of barriers, with > > a very small amount of tweaking we could expose that. There's absolutely > > zero reason we can't easily support both types of barriers. > > That sounds like a good idea - we can leave the existing > WRITE_BARRIER behaviour unchanged and introduce a new WRITE_ORDERED > behaviour that only guarantees ordering. The filesystem can then > choose which to use where appropriate.... Precisely. The current definition of barriers are what Chris and I came up with many years ago, when solving the problem for reiserfs originally. It is by no means the only feasible approach. I'll add a WRITE_ORDERED command to the #barrier branch, it already contains the empty-bio barrier support I posted yesterday (well a slightly modified and cleaned up version). -- Jens Axboe - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/