US Patent 7181477 Storage system and storage control method

A primary volume group 30 includes a primary volume 31 and a differential volume 32. A secondary volume group 40 includes a primary volume 41 and a differential volume 42. Multiple generations of differential snapshots D1 to D3 are accumulated in the differential volume 32. When a new snapshot obtaining request is issued while a usage rate has reached a given value, the volume groups 30 and 40 are synchronized with the volume snapshot (a), and then split (b). After being split, the differential volume 32 is initialized, and the secondary volume group 40 is backed up (c). Accordingly, a new differential snapshot can be created and held in the differential volume 32.