Patent ID: 7266654

Claim:
A storage system comprising a server apparatus including at least one storage interface, and, at least one disk apparatus connected to the storage interface including an operational volume which performs ordinary read/write and a differential volume which stores differential data of snapshot of the operational volume, wherein: the server apparatus includes: a file server part which accesses the disk apparatus in response to a file access request sent to the server apparatus; a data protection part which controls taking of snapshot; a file system processing part which issues an access processing to a snapshot management part in response to a request from the file server part and the data protection part; and a snapshot management part which manages the differential volume and maintains taken snapshot; and the snapshot management part: provides a preceding copy area to store data, when the data is written and stored in the operational volume in the differential volume and records location information of an area in the operational volume in which said data was stored; and discards the stored location information and uses the preceding copy area as an free area if free area in the differential volume becomes insufficient, wherein said snapshot management part includes a differential block management table comprising a plurality of columns each having fields for differential volume block address, possession bit map, and preceding copy area address for recording said location information; and wherein said differential volume address represents a differential volume block address in said differential volume, said possession bit map represents whether the block in said differential volume indicated by said differential volume block address is used in respective snapshots, and said preceding copy area address indicates whether a block in said differential volume indicated by said differential volume block address is being used as a preceding copy area or not, and if being used as a preceding copy area, a block address of the copied block in said preceding copy area is indicated in the field of said preceding copy area address.