Subject: Re: Networked filesystems vs backing_dev_info
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Jan Harkes <jaharkes@cs.cmu.edu>
Cc: linux-kernel <linux-kernel@vger.kernel.org>,
       linux-fsdevel <linux-fsdevel@vger.kernel.org>,
       David Howells <dhowells@redhat.com>, sfrench@samba.org,
       Andrew Morton <akpm@linux-foundation.org>, vandrove@vc.cvut.cz
In-Reply-To: <20071027152227.GE3200@cs.cmu.edu>
References: <1193477666.5648.61.camel@lappy>
	 <20071027152227.GE3200@cs.cmu.edu>
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Date: Sat, 27 Oct 2007 17:32:16 +0200
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On Sat, 2007-10-27 at 11:22 -0400, Jan Harkes wrote:
> On Sat, Oct 27, 2007 at 11:34:26AM +0200, Peter Zijlstra wrote:
> > I had me a little look at bdi usage in networked filesystems.
> > 
> >  NFS, CIFS, (smbfs), AFS, CODA and NCP
> > 
> > And of those, NFS is the only one that I could find that creates
> > backing_dev_info structures. The rest seems to fall back to
> > default_backing_dev_info.
> 
> While a file is opened in Coda we associate the open file handle with a
> local cache file. All read and write operations are redirected to this
> local file and we even redirect inode->i_mapping. Actual reads and
> writes are completely handled by the underlying file system. We send the
> new file contents back to the servers only after all local references
> have been released (last-close semantics).
> 
> As a result, there is no need for backing_dev_info structures in Coda,
> if any congestion control is needed it will be handled by the underlying
> file system where our locally cached copies are stored.

Ok, that works. Thanks for this explanation!

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