From: Trond Myklebust <trond.myklebust@fys.uio.no>
To: Benny Halevy <bhalevy@panasas.com>
In-Reply-To: <4D21DB53.9050104@panasas.com>
References: <978693366.32.1292516428080.JavaMail.root@thunderbeast.private.linuxbox.com>
	<1740153586.34.1292516481789.JavaMail.root@thunderbeast.private.linuxbox.com>
	<20101216230707.GB16760@infradead.org> <4D21DB53.9050104@panasas.com>
Date: Mon, 03 Jan 2011 09:40:11 -0500
Message-ID: <1294065611.16812.8.camel@heimdal.trondhjem.org>
Cc: Christoph Hellwig <hch@infradead.org>, linux-nfs@vger.kernel.org,
        nfsv4@ietf.org
Subject: Re: [nfsv4] layoutcommits and file layout
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On Mon, 2011-01-03 at 16:21 +0200, Benny Halevy wrote: 
> On 2010-12-17 01:07, Christoph Hellwig wrote:
> > On Thu, Dec 16, 2010 at 11:21:21AM -0500, Matt W. Benjamin wrote:
> >> Hi,
> >>
> >> We have a files implementation which wants to receive LAYOUTCOMMIT when a client is finished with a layout.  It was my clear understanding from rfc5661 that we could expect this behavior.
> > 
> > Care to post it to the list?
> > 
> 
> I don't know what Matt's server is doing but the fundamental problem is
> manifested with extending a file with parallel DS writes.
> Assuming that the DS writes are executed in arbitrary order,
> exposing the file length before LAYOUTCOMMIT can cause
> a concurrent reader to read a hole.  Although locking can
> solve this case, day-to-day applications that work well over
> local filesystem and legacy NFS may break because of this.

...and this differs from ordinary NFS writes exactly how?

Both cached and uncached (i.e. O_DIRECT) writes can and will be flushed
to disk in entirely random order when writing to the MDS. If you have a
parallel reader on another client (or even on the same client in the
case of O_DIRECT), and want it to see accurate data, then use locking.
If not, you will see holes and other strangeness.

IOW: There are no 'day-to-day applications that work well over legacy
NFS' that rely on this behaviour.

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