[RFC PATCH 1/2] Union Mount: glibc readdir support

[Erez Zadok]

In message <20080506042117.GA29298@in.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.