FS-specified FSID for non-device based filesystems?

[Oleg Drokin <green@linuxhacker.ru>]

Hello!

   Right now NFSD depends on FS_REQUIRES_DEV to be set in sb->s_type->fs_flags
   or manually set fsid= in /etc/exports in order to export filesystem.
   But there are non-device based filesystems (Lustre for example ;) ) that
   would like to be exported via NFS without guiding users through this
   fsid= business. Of course it is easy to cheat through by setting
   FS_REQUIRES_DEV, but this does not solve entire problem. While allowing
   for single-node export of filesystem to work ok, more generic problem of
   exporting same filesystem from several NFS servers in clustered manner
   won't work because sb->s_dev might be different on different nodes.
   This is not only Lustre problem, I believe same problem will happen
   with GFS or OCFS2 or whatever other SAN-based fs if different nodes
   have different device names (/dev/sda | /dev/sdb) for same SAN device,
   in such a case fsid calculated would be different on different nodes too.

   I wonder if e.g. another export op could be introduced to ask filesystem
   for its unique id (if supported) and use that instead of sb->s_dev in
   fsid calculations. Does this look sane?
   I hope I am not missing anything?
   Or are there better ideas?

   Thanks.

Bye,
    Oleg