Using the RBD backend driver (Ceph Storage) for OpenStack Compute (nova) ephemeral disks applies two additional settings to libvirt:
hw_disk_discard : unmap
disk_cachemodes : network=writeback
This allows reclaiming of unused blocks on the Ceph pool and caching of network writes, which improves the performance for OpenStack Compute ephemeral disks using the RBD driver.
Also see http://docs.ceph.com/docs/master/rbd/rbd-openstack/