Patent ID: 8370590

Claim:
A storage system providing a first volume for storing data transmitted from a host system and a second volume for storing data which is pre-updated data stored in said first volume prior to update executed after a prescribed timing, comprising: a management unit for managing said data written into a first block area configured in said first volume and said second volume, or into a second block area configured in said first block area of said second volume which is smaller than said first block area; a snapshot acquisition unit for acquiring a snapshot of said first volume at said prescribed timing; and a transfer unit for transferring said written data of said snapshot acquired by said snapshot acquisition unit stored in said first volume and said second volume to an external device including a volume also configured with said first block area and said second block area to backup said written data and said snapshot therein, wherein data in said first volume to be backed-up to the external device is transferred in a data size of said first block area, data in said second volume to be backed-up to the external device is transferred in a data size of the first or second block area depending upon a size of said data, said transfer unit transfers said data in said second volume with said first block area when said data is not being managed with said management unit in said second block area, and when a total value of the areas of said second block area is greater than a prescribed threshold value, said management unit deletes all areas of said second block area that it is managing.