Patent ID: 7921273

Claim:
A method, comprising: receiving a write request at a first storage control unit from a host application coupled to a host system; writing data corresponding to the write request at a cache and a non-volatile storage coupled to the first storage control unit; sending, by the first storage control unit, the data corresponding to the write request synchronously to a second storage control unit; asynchronously sending the synchronously sent data from the second storage control unit to a third storage control unit, wherein input/output (I/O) operations from the host system are temporarily quiesced at the first storage control unit while the third storage control unit completes receiving of selected data updates from the second storage control unit to maintain an equivalent copy of data at the third storage control unit in case both the first storage control unit and the second storage control units fail; and in response to an outage at the first storage control, copying, by a background asynchronous copy process in the second storage control unit to the third storage control unit, all remaining modified data stored in the second storage control unit that has not been copied to the third storage control unit to maintain an equivalent copy of data at the third storage control unit in case both the first storage control unit and the second storage control units fail.