Patent ID: 7680984

Claim:
A storage system having a storage device and a controller that controls data input and output to/from the storage device in response to an access request from a host computer connected to the storage system via a communication path, the storage system comprising: an actual volume including at least one storage extent in the storage device, and a virtual volume accessed by the host computer and including a plurality of virtual extents, at least one storage extent in the storage device being allocated to at least one of the plurality of virtual extents, and at least one storage extent in the storage device not being allocated to at least one of the plurality of virtual extents, wherein in response to an access request to a first virtual extent in the virtual volume from the host computer, the controller allocates a first storage extent in the storage device to the first virtual extent if no storage extent in the storage device has been allocated to the first virtual extent, and the controller detects any decrease in necessity for maintaining an allocation between the first virtual extent and the first storage extent, and cancels the allocation based on the detection result.