Patent ID: 7085902

Claim:
A method for managing a data storage system that includes first and second storage subsystems, the method comprising: copying data stored on the first storage subsystem to the second storage subsystem in an asynchronous mirroring process; maintaining a record on the second storage subsystem, indicative of locations at which the data have been updated on the first storage subsystem and have not yet been copied to the second storage subsystem; receiving at the second storage subsystem, from a host processor, a request to access the data stored at a specified location on the data storage system; if the specified location is included in the record, sending a message from the second storage subsystem to the first storage subsystem requesting a synchronous update of the data at the specified location; and providing the data stored at the specified location from the second storage subsystem to the host processor after receiving at the second storage subsystem a response to the message from the first storage subsystem.