linux-kernel - Re: [RFC PATCH 1/2] Union Mount: glibc readdir support

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Message-Id: <200805092005.m49K5s4N014510@agora.fsl.cs.sunysb.edu>
Date:	Fri, 9 May 2008 16:05:54 -0400
From:	Erez Zadok <ezk@...sunysb.edu>
To:	bharata@...ux.vnet.ibm.com
Cc:	Ulrich Drepper <drepper@...hat.com>, bsn.0007@...il.com,
	libc-alpha@...rceware.org, Jan Blunck <jblunck@...e.de>,
	Erez Zadok <ezk@...sunysb.edu>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, viro@...iv.linux.org.uk,
	Christoph Hellwig <hch@....de>, Mingming Cao <cmm@...ibm.com>,
	Dave Hansen <haveblue@...ibm.com>,
	Trond Myklebust <trond.myklebust@....uio.no>,
	David Woodhouse <dwmw2@...radead.org>
Subject: Re: [RFC PATCH 1/2] Union Mount: glibc readdir support 

In message <20080506042117.GA29298@...ibm.com>, Bharata B Rao writes:
> On Wed, Apr 30, 2008 at 11:08:41PM -0700, Ulrich Drepper wrote:
> > 
> > Before anything further is discussed I need to know how you expect to
> > handle NFS?  I don't see how, with a userlevel readdir implementation,
> > you can support NFS.
> 
> At the client side, I don't see a problem with unioning NFS with other
> FS. Kernel would still return all dirents of the union including those
> from NFS and glibc readdir would be able to handle them appropriately.
> Or am I missing something ?
> 
> At the server side, I can't see how NFS server could export a union.
> I don't see how this could be sanely done with Union Mount. Erez, how
> does Unionfs handle this ?
[...]

The ODF version of unionfs stores, among other things, persistent inode
numbers in a small/special /odf partition.  If the inums aren't persistent,
they could get flushed out of memory, making it very difficult to
reconstruct the inode w/ the same inum later on.

Erez.
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