A method for storing data received from a host processor at a primary storage subsystem in a data storage system includes writing the data to a first volatile cache memory in the primary storage subsystem and copying the data from the primary storage subsystem to a secondary storage subsystem. The second subsystem writes the copied data to a second volatile cache memory and returns an acknowledgment to the primary storage subsystem responsively to writing the copied data to the second volatile cache memory and prior to saving the data in the second non-volatile storage media. The primary storage subsystem signals the host processor that the data have been stored in the data storage system responsively to the acknowledgment from the secondary storage subsystem.