A method and storage system for increasing an amount of memory in a queuing area on. The storage system includes first and second storage subsystems connected to each other via a path. A primary volume in the first storage subsystem and a remote secondary volume in the second storage subsystem are mirrored and operated in the asynchronous mode. A queuing area having memory is provided in the second storage subsystem for temporarily storing data transferred to the second storage subsystem from the first storage subsystem in response to a write input/output (I/O) issued by a host to write data in the primary volume. Data temporarily stored in the memory is retrieved and stored in the remote secondary volume. An unused area of the queuing area is monitored and the memory increased if the unused area becomes less than a predetermined amount.