Here's the user guide for drbd:
"Protocol B. Memory synchronous (semi-synchronous) replication protocol. Local write operations on the primary node are considered completed as soon as the local disk write has occurred, and the replication packet has reached the peer node. Normally, no writes are lost in case of forced fail-over. However, in the event of simultaneous power failure on both nodes and concurrent, irreversible destruction of the primary's data store, the most recent writes completed on the primary may be lost."
This would be good for where you have the two boxes on separate UPSes and need high performance (lower latency than "fully synchronous"). It sounds like you wouldn't have a single point of failure even with the memory-synchronous mode.
I understand that open-e's implementation of DRBD is not always completely up to the latest, bleeding-edge drbd release because it needs to be stress-tested and integrated into the GUI. But, it sounds like this is something that should be available sometime from open-e.
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