Patent ID: 8195913

Claim:
A storage system having a plurality of storage devices and a controller that controls data input and output to/from the plurality of storage devices in response to an access request from an application mounted on a host computer being coupled to the storage system via a communication path, the storage system comprising: a pool having a plurality of pool volumes each including a plurality of storage extents in the storage devices, and a virtual volume accessed by the application of 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 a write access request from the application, to a first virtual extent of the virtual volume, the controller is configured to allocate a first storage extent of the plurality of storage extents to the first 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 controller is configured to write data related to the write access to the first storage extent, wherein in response to a release request, which specifies a first address of at least one virtual extent in the virtual volume, issued by the host computer, the controller is configured to release the at least one storage extent allocated to the at least one virtual extent specified by the first address from the allocation and to write a specified data pattern in the released at least one storage extent.