A method for managing a data storage system includes copying data stored on a first storage subsystem to a second storage subsystem in an asynchronous mirroring process. A record is maintained 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. Upon 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, the second storage subsystem initiates a synchronous transfer of the data at the specified location from the first storage subsystem.