Patent ID: 8606993

Claim:
A storage system being coupled to a host computer, comprising: a plurality of storage devices configuring a plurality of device groups, each of the plurality of device groups comprising a plurality of storage regions, each of the plurality of storage regions constituted by multiple storage areas, a size of each of the storage regions being a first fixed size, a size of each of the storage areas being a second fixed size; and a controller being adapted to: provide a virtual volume to the host computer; allocate a storage region belonging to the plurality of storage regions exclusively to the virtual volume in response to receiving a write command to the virtual volume from the host computer; write data of the write command to any storage area of the allocated storage region on a storage area basis irrespective of a portion in the virtual volume to which the write command is directed; and wherein the controller is adapted to: select a first storage region among a plurality of first storage regions comprised by a first device group based on an access frequency of each of the plurality of first storage regions; and migrate data of the selected first storage region from the first device group to a second device group on a storage region basis.