linux-kernel - Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md.

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Date:	Tue, 29 May 2007 17:01:24 -0700 (PDT)
From:	david@...g.hm
To:	David Chinner <dgc@....com>
cc:	Phillip Susi <psusi@....rr.com>, Neil Brown <neilb@...e.de>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	dm-devel@...hat.com, linux-raid@...r.kernel.org,
	Jens Axboe <jens.axboe@...cle.com>,
	Stefan Bader <Stefan.Bader@...ibm.com>,
	Andreas Dilger <adilger@...sterfs.com>,
	Tejun Heo <htejun@...il.com>
Subject: Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems,
 and dm/md.

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
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