Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758357AbXEaHEe (ORCPT ); Thu, 31 May 2007 03:04:34 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754487AbXEaHEX (ORCPT ); Thu, 31 May 2007 03:04:23 -0400 Received: from netops-testserver-3-out.sgi.com ([192.48.171.28]:44352 "EHLO relay.sgi.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1754146AbXEaHEV (ORCPT ); Thu, 31 May 2007 03:04:21 -0400 Date: Thu, 31 May 2007 17:03:08 +1000 From: David Chinner To: Jens Axboe Cc: David Chinner , 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: <20070531070307.GK85884050@sgi.com> References: <18006.38689.818186.221707@notabene.brown> <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> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20070531062644.GI32105@kernel.dk> User-Agent: Mutt/1.4.2.1i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1898 Lines: 46 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.... Cheers, Dave. -- Dave Chinner Principal Engineer SGI Australian Software Group - 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/