storage control unit 302 the sub storage control unit 302 then performs the i o processing for the subvolume 602 b on the other hand if the sub host computer 102 performs an i o processing for the subvolume 602 the sub storage control unit 302 receives the i o processing from the host computer 102 and transfers the i o processing to the main storage control unit 301 the main storage control unit 301 then performs the i o processing for the main volume 601 c furthermore the main storage control unit 301 returns the received i o processing to the sub storage control unit 302 then the sub storage control unit 302 performs the i o processing for the subvolume 602 d each of the main volume 601 the subvolume 602 and the communication volume 603 is actually configured by one or more recording media the recording media may be magnetic disks optical disks etc in particular when the raid redundant array for inexpensive disks method is employed for such recording media the main sub storage control unit performs an i o processing sent to a logical volume from a host computer for a plurality of disks corresponding to the logical volume 0040 fig1 shows a block diagram of a storage system in the first example of the present invention the principles of the present invention shown in fig1 are illustrated in detail in fig1 in the storage system shown in fig1.