Patent ID: 8694742

Claim:
A storage system comprising: a plurality of Redundant Array of Independent/Inexpensive Disks (RAID) groups created from a plurality of storage devices, the plurality of RAID groups including a first RAID group and a second RAID group; a controller providing a host virtual volume to which a host computer sends read/write requests, the controller configured to: manage a plurality of virtual volumes including a first virtual volume and a second virtual volume, the first virtual volume and the second virtual volume being a pair of mirror volumes related to the host virtual volume; allocate at least one of a plurality of storage regions provided from the first RAID group to a first virtual storage region of the first virtual volume according to a first write request to the host virtual volume from the host computer; allocate at least one of a plurality of storage regions provided from the second RAID group to a second virtual storage region of the second virtual volume according to the first write request, so that write data corresponding to the first virtual storage region and write data corresponding to the second virtual storage region are stored in the storage regions belongs to a different RAID group; and allocate at least one of the plurality of storage regions provided from the second RAID group to a third virtual storage region of the first virtual volume.