Patent ID: 7631156

Claim:
A storage system coupled to a network, comprising: an interface coupled to the network; a processor coupled to the interface; a first memory coupled to the processor; and a disk drive for storing data whose physical storage area is allocated to at least one of a plurality of blocks included in one or more volumes; wherein the plurality of blocks include a first block included in an operation volume, a second block included in a differential volume; wherein the processor performs a snapshot process to judge whether the physical storage area has been allocated or not to the first block when the interface receives a data writing request targeting the first block through the network; wherein when the physical area is judged to have been allocated, the processor copies data stored in the first block to the second block before writing data to be written by the writing request in the first block; and wherein when the physical storage area is judged not to have been allocated, the processor issues a physical block allocation request to the first block, and writes information on which indicates snapshot in the first block is dummy data in snapshot management information and writes the data in the first block without copying the data stored in the first block to the second block.