A disk array connected to a storage area network via a fiber channel has one or more ports each controlled by a processor. Even the disk array with one port and one processor executes online processing and backup processing at the same time while considering an online processing load. A port controller not only accepts a request from a host computer but issues a request to other storage controllers to allow online processing and backup processing to be executed at the same time. In addition, the disk array, if provided with a plurality of ports, selects ports or schedules processing depending upon the load to prevent backup processing from affecting online processing performance.