CEPH is a popular open source storage software. However its write latencies are high. VirtuCache caches CEPH volumes to in-host SSDs and by doing so reduces VM level latencies considerably.
CEPH storage from Virtunet has all the features of traditional iSCSI SAN with the exception that it is reasonably priced because it uses commodity servers with all off-the-shelf hardware. And so it is ideally suited for backup and DR storage which needs to be cheap above all else.