Patent ID: 8219774

Claim:
A storage controller comprising: a plurality of storage devices; and a control circuit providing a plurality of virtual volumes, wherein a storage area in a plurality of pool volumes provided with the plurality of storage devices can be allocated to one of the plurality of virtual volumes on a predetermined unit basis in response to a write access request to a logical area in the one of the plurality of virtual volumes, wherein the storage area is not allocated to the logical area before the write access request, wherein the control circuit carries out a search in the plurality of pool volumes for a certain storage area storing a certain data pattern on the predetermined unit basis, and releases the certain storage area identified during the search in the plurality of pool volumes from allocation to a certain logical area in the plurality of virtual volumes, after which the control circuit can use the released certain storage area in the plurality of pool volumes for allocation to a virtual volume of the plurality of virtual volumes as a destination of another write access request.