Patent ID: 7464235

Claim:
A storage system comprising a main server, a backup server, a first storage controller, a second storage controller, a third storage controller, and a network connecting among the servers and the controllers, wherein said first storage controller has first and second primary logical volumes each formed from an actual volume therein, and first and second secondary logical volumes each formed from a virtual volume selectively mapped to an external volume thereby forming a copy pair with one of said primary logical volumes, wherein said second storage controller has an actual volume therein which constitutes one of said external volumes selectively mapped to one of said secondary logical volumes, wherein said third storage controller has an actual volume therein which constitutes one of said external volume selectively mapped to one of said secondary logical volumes, wherein said backup server has device data showing whether a mapping relationship exists between said external volumes and said secondary logical volumes and whether said external volumes are controlled by the backup server, wherein a pair split of a copy pair, which consists of one of said primary logical volumes and one of said secondary logical volumes, is determined as adequate if a mapping relationship exists between said one secondary logical volume and said one external volume, and if said one external volume is controlled by the backup server, and wherein only when the pair split of the copy pair is determined as adequate, the copy pair is spitted thereby performing backup or restoration of said one external volume.