Re: What to do about subvolumes?

[Trond Myklebust <Trond.Myklebust@netapp.com>]

On Tue, 2010-12-07 at 17:51 +0100, Christoph Hellwig wrote:
> On Sat, Dec 04, 2010 at 09:27:56AM +1100, Dave Chinner wrote:
> > A property of NFS fileshandles is that they must be stable across
> > server reboots. Is this anon dev_t used as part of the NFS
> > filehandle and if so how can you guarantee that it is stable?
> 
> It's just as stable as a real dev_t in the times of hotplug and udev.
> As long as you don't touch anything including not upgrading the kernel
> it's remain stable, otherwise it will break.  That's why modern
> nfs-utils default to using the uuid-based filehandle schemes instead of
> the dev_t based ones.  At least that's what I told - I really hope it's
> using the real UUIDs from the filesystem and not the horrible fsid hack
> that was once added - for some filesystems like XFS that field does not
> actually have any relation to the UUID historically.  And while we could
> have changed that it's too late now that nfs was hacked into abusing
> that field.

IIRC, NFS uses the full true uuid for NFSv3 and NFSv4 filehandles, but
they won't fit into the NFSv2 32-byte filehandles, so there is an
'8-byte fsid' and '4-byte fsid + inode number' workaround for that...

See the mk_fsid() helper in fs/nfsd/nfsfh.h

Cheers
  Trond
-- 
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@netapp.com
www.netapp.com