linux-kernel - Re: [PATCH 16/20] ceph: nfs re-export support

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Message-Id: <1247779311.12292.162.camel@heimdal.trondhjem.org>
Date:	Thu, 16 Jul 2009 17:21:51 -0400
From:	Trond Myklebust <trond.myklebust@....uio.no>
To:	Sage Weil <sage@...dream.net>
Cc:	"J. Bruce Fields" <bfields@...ldses.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 16/20] ceph: nfs re-export support

On Thu, 2009-07-16 at 12:50 -0700, Sage Weil wrote:
> On Thu, 16 Jul 2009, J. Bruce Fields wrote:
> > On Wed, Jul 15, 2009 at 02:24:46PM -0700, Sage Weil wrote:
> > > Basic NFS re-export support is included.  This mostly works.  However,
> > > Ceph's MDS design precludes the ability to generate a (small)
> > > filehandle that will be valid forever, so this is of limited utility.
> > 
> > Is there any hope of fixing that?
> 
> Yes, but it requires some additional ondisk metadata the MDS isn't 
> maintaining yet (a parent directory backpointer on file objects).  
> 
> The MDS changes will mean more random IO for rename intensive workloads, 
> but the backpointers would also be useful for rebuilding the directory 
> tree in the event of some catastrophic metadata loss or corruption.  
> (Currently they're only there for directories, not all files.)

Note that a filehandle that contains parent directory information is
still not one that is valid forever. It will change in the case of a
cross-directory rename, and so isn't a filehandle in the NFSv2/v3 sense.
Even in the NFSv4 case, it would have to be labelled as 'volatile'.

Cheers
    Trond

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