Re: [NFS] -EXDEV between mounts that are same fs

["J. Bruce Fields" <bfields@fieldses.org>]

On Sun, Feb 10, 2008 at 10:33:40AM -0500, Christoph Hellwig wrote:
> On Sun, Feb 10, 2008 at 10:26:18AM -0500, J. Bruce Fields wrote:
> > > The point of the setup is of course to avoid exposing more information
> > > than is necessary, while still having the benefit of having a single
> > > pool of disk space for all exported resources.
> > 
> > I notice that btrfs claims support for "subvolumes"--multiple filesystem
> > namespaces sharing the same storage.  Seems like a useful feature for
> > this situation.  I don't know whether it'd be possible to add to other
> > filesystems.
> 
> That stuff is done using a code sniplet from me, which does nothing but
> allowing to mount into a subdirectory of a filesystem.  It will look
> exactly the same as bind mounts.

To get the equivalent of subtree checking, we also need a reference from
each inode back to the subvolume it belongs to, so we can determine the
subvolume from a filehandle.  I assume that's a more intrusive change.

--b.

> I have hacked up implementations for
> ext2 and xfs aswell and in the form btfs does it currently it would
> be doable for any filesystem.  But to actually make it useful you'd
> need quotas for each subvolume which is something currently only
> xfs could provide (if I could be arsed into finally implementing
> my multiple subvolumes proposal fully).
> 

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