Re-exporting NFS

From Ceph wiki

The kernel client can re-export a ceph mount via NFS. The only real caveat here is that you need to manually specify an fsid when exporting the file system. (note: using fsid has benefits if the nfs server is run in a HA scenario and might failover, otherwise the mounts might go stale - so this is a GOOD thing)

$ mount -t ceph 1.2.3.4:/ /mnt/ceph
$ exportfs client:/mnt/ceph -o fsid=1234,rw,no_squash_root

That's about it. You'll probably need to remove the export before you can unmount, e.g.

$ exportfs -u client:/mnt/ceph
$ umount /mnt/ceph

Below is a note about reliability of exporting ceph via NFS from the source to the kernel module. Basically, ceph's design makes it difficult to provide a single, universally consistent identifier to the NFS export interface. This causes stale filehandles, presumably when a cache object is released on the NFS server (ceph client).

/*
* NFS export support
*
* NFS re-export of a ceph mount is, at present, only semireliable.
* The basic issue is that the Ceph architectures doesn't lend itself
* well to generating filehandles that will remain valid forever.
*
* So, we do our best.  If you're lucky, your inode will be in the
* client's cache.  If it's not, and you have a connectable fh, then
* the MDS server may be able to find it for you.  Otherwise, you get
* ESTALE.
*
* There are ways to this more reliable, but in the non-connectable fh
* case, we won't every work perfectly, and in the connectable case,
* some changes are needed on the MDS side to work better.
*/