Patent ID: 7890716

Claim:
A method of managing differential snapshots in a storage system coupled to a host computer, the storage system having a disk drive for storing data, a memory configured to store a management information, and a disk controller for controlling data input and output to and from the disk drive, the differential snapshot management method comprising the steps of: providing a storage area of the disk drive as a plurality of logical volumes including an operational volume having a plurality of blocks for storing data requested by the host computer to be written, and a differential volume having a plurality of blocks for storing differential data between data in the operational volume and a differential snapshot of the operational volume, the differential snapshot being data of the operation volume at a point in time at which the differential snapshot is created, provided with data stored in the operation volume and data stored in the differential volume; managing the management information, in which, a time of update is stored for each of the plurality of blocks included in the operational volume when data is copied from the respective one of the plurality of blocks in the operational volume to one of the plurality of blocks in the differential volume, a creation time is stored for each differential snapshot when the differential snapshot is created, and an address information of one of the plurality of blocks in the operation volume or the differential volume corresponding to one of a plurality of blocks included in a differential snapshot is stored for each of the plurality of blocks in the differential snapshot; sending, in response to a request to read a block in a differential snapshot, data stored in the block specified by the address information corresponding to the requested block in the differential snapshot; judging, upon reception of a request to write a block included in the operational volume, using the creation time of each differential snapshot and the time of update of the block in which requested data is to be written in management information, whether there is a differential snapshot that was created after the time of update of the block in which requested data is to be written; copying, if there is a differential snapshot that was created after the time of update of the block in which requested data is to be written the according to the judgment, data from the block in which requested data is to be written to the differential volume and writing the requested data in the requested block; and writing, if there is not any differential snapshot that was created after the time of update of the block in which requested data is to be written according to the judgment, the requested data in the requested block without the copying.