Patent ID: 7398364

Claim:
A storage system which is connectable to another storage system and a computer via a network, comprising: a controller; a plurality of interfaces connected to the controller and the network; and a plurality of storage devices connected to the controller, wherein the controller executes a transferring of data, which is to be stored in the plurality of storage devices based on an I/O request issued by the computer, to the another storage system via the network using one of a plurality of copy modes, wherein the controller switches the one of the plurality of copy modes to another one of the plurality of copy modes based on an access rate of I/O requests from the computer to the storage system and executes transferring data using said another one of the plurality of copy modes, wherein the plurality of copy modes comprise first and second copy modes, respectively, wherein the controller indicates completion of the I/O request after receiving notification of completion of storing the data from the another storage system when the first copy mode is used, wherein the controller indicates completion of the I/O request before receiving notification of completion of storing the data from the another storage system when the second copy mode is used, wherein the controller switches from the first copy mode to the second copy mode when the access rate exceeds a threshold, wherein the controller switches from the second copy mode to the first copy mode when the access rate drops below the threshold, and wherein the threshold is determined based on a formula.