Patent ID: 7949828

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 a plurality of storage extents in the storage device; and a virtual volume accessed by the host computer and including a plurality of virtual extents, at least one of the plurality of storage extents being allocated to at least one of the plurality of virtual extents, and at least one storage extent of the plurality of storage extents not being allocated to at least one of the plurality of virtual extents, wherein in response to an access request to a first virtual volume from the host computer, the controller allocates a first storage extent of the plurality of storage extents to the first virtual extent of the plurality of the virtual extents and allocates a second storage extent of the plurality of storage extents to a second virtual extent of the plurality of the virtual extents, if no storage extent in the storage device has been allocated to the first virtual extent and the second virtual extent, wherein the second virtual extent is expected to be accessed by a subsequent access request after said access request to a first virtual volume.