From mboxrd@z Thu Jan 1 00:00:00 1970 From: david@lang.hm Subject: Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md. Date: Tue, 29 May 2007 17:01:24 -0700 (PDT) Message-ID: 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> Mime-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed Cc: Phillip Susi , Neil Brown , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, dm-devel@redhat.com, linux-raid@vger.kernel.org, Jens Axboe , Stefan Bader , Andreas Dilger , Tejun Heo To: David Chinner Return-path: In-Reply-To: <20070529234832.GT85884050@sgi.com> Sender: linux-raid-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org On Wed, 30 May 2007, David Chinner wrote: > On Tue, May 29, 2007 at 04:03:43PM -0400, Phillip Susi wrote: >> David Chinner wrote: >>> The use of barriers in XFS assumes the commit write to be on stable >>> storage before it returns. One of the ordering guarantees that we >>> need is that the transaction (commit write) is on disk before the >>> metadata block containing the change in the transaction is written >>> to disk and the current barrier behaviour gives us that. >> >> Barrier != synchronous write, > > Of course. FYI, XFS only issues barriers on *async* writes. > > But barrier semantics - as far as they've been described by everyone > but you indicate that the barrier write is guaranteed to be on stable > storage when it returns. this doesn't match what I have seen wtih barriers it's perfectly legal to have the following sequence of events 1. app writes block 10 to OS 2. app writes block 4 to OS 3. app writes barrier to OS 4. app writes block 5 to OS 5. app writes block 20 to OS 6. OS writes block 4 to disk drive 7. OS writes block 10 to disk drive 8. OS writes barrier to disk drive 9. OS writes block 5 to disk drive 10. OS writes block 20 to disk drive 11. disk drive writes block 10 to platter 12. disk drive writes block 4 to platter 13. disk drive writes block 20 to platter 14. disk drive writes block 5 to platter there is nothing that says that when the app finishes step #3 that the OS has even sent the data to the drive, let alone that the drive has flushed it to a platter if the disk drive doesn't support barriers then step #8 becomes 'issue flush' and steps 11 and 12 take place before step #9, 13, 14 David Lang