Patent ID: 7392357

Claim:
A system connected to a client computer via a network, comprising: a storage subsystem; and a server subsystem connected to the storage subsystem via a communication line, wherein the storage subsystem comprises a disk drive for storing data, and a disk controller for controlling input and output of data to and from the disk drive, wherein the server subsystem comprises an interface connected to the storage subsystem, a processor connected to the interface, and a memory connected to the processor, wherein the disk controller provides storage areas of the disk drive as a plurality of logical volumes including a primary volume for storing data requested by the client computer to be written, a differential volume for storing differential data as a difference between the primary volume and a differential snapshot of the primary volume, and a full-copy snapshot volume for storing all data in the primary volume at the time a full-copy snapshot is taken, wherein the disk controller stores a differential block bitmap indicating a block in which data stored in the primary volume differs from data stored in the full-copy snapshot volume, and wherein the processor obtains the differential block bitmap from the disk controller, identifies, based on the obtained differential block bitmap, a block where data stored in the primary volume differs from data stored in the full-copy snapshot volume, obtains, from the full-copy snapshot volume, differential data that is stored in the identified block, stores the obtained differential data in the differential volume, stores, in differential block arrangement information, a location of the differential data that is stored in the differential volume, and provides a differential snapshot that corresponds to the full-copy snapshot based on the primary volume, the differential volume, and the differential block arrangement information.