Cormac Hogan on Atlantis ILIO 4.0:
Atlantis ILIO Persistent VDI 4.0 introduces the concept of a session host, a replication host and a standby host. The session host is where your VDI desktops are running. Using ILIO Fast Replication, changes made to the desktop are sent from the session host over to the replication host. ILIO Fast Replication ensures that the desktop state maintains consistency on persistent back-end storage for fast recovery scenarios. The replication host is responsible for making sure that any changes made to the desktop are saved to a persistent storage device, either SAN or NAS, as shown here.
More on this can be found here: http://cormachogan.com/2013/02/26/atlantis-ilio-4-0-persistent-desktops-in-memory/