=================
Volume Federation
=================

RBD
===
When using RBD (Ceph) as the driver for Cinder, volumes are stored in a Ceph
pool. Access to the pool is granted through a CephX authentication key and
this key is shared among all Nova compute nodes.

Please follow Ceph's documentation_ on setting up OpenStack with RBD.

The ``libvirt`` secret key should match the credentials for accessing the pool
in both deployments. So if you have two OpenStack deployments and they
respectively access the RBD pool through secret keys
``bab2fea0-a834-48ad-8b27-6b7ce6aeab13`` and
``dfb36a63-484c-48b2-8a47-4de8915dacfb`` that map to CephX authentications,
then compute nodes in both deployments should have both of these secret keys
configured.

.. NOTE::

   Due to `bug 1635008`_ it is not possible to use RBD for ephemeral storage
   and also for federating volumes in RBD prior to the Ocata release. This is
   because Nova will trust the RBD information that it uses to store ephemeral
   disks over the information coming from Cinder, which should be the correct
   information.


.. _`bug 1635008`: https://bugs.launchpad.net/nova/+bug/1635008
.. _documentation: http://docs.ceph.com/docs/master/rbd/rbd-openstack/


LVM
===
LVM uses an iSCSI server to share the Cinder volume with Nova compute nodes.
The credentials to access each volume are randomly generated by Cinder and
passed to Nova during volume attachment. No special setup needs to be done
in this scenario, other than to ensure compute nodes are able to access the
iSCSI server through the network.