Patent ID: 7032091

Claim:
A storage device, comprising: first, second and third storage volumes for storing data; a virtual storage volume which is virtually possessed by the storage device; and a control unit, wherein the control unit includes: means for receiving, from an information processing unit, a request for update of the data as the data exists at a prescribed point in time, the updated data to be stored in the first storage volume after the prescribed point in time; means for storing a duplicate of the data as the data exists at the prescribed point in time before the update, onto the second storage volume; means for storing, in an unused storage area of the third storage volume, a duplicate of the data as the data exists at the prescribed point in time before the update, in response to a determination that the second storage volume has a predetermined state; and a memory control unit which manages or controls a correspondence between the first, second and third storage volumes and the virtual storage volume, wherein the memory control unit includes information concerning to which one of the first, second and third storage volumes an access is to be made in response to a data I/O request from the information processing unit by reference to the correspondence.