Patent ID: 7685387

Claim:
A first storage system adapted to be coupled to a second storage system and a third storage system, said second storage system including a plurality of second storage units and a second storage control unit which stores data received from a host computer into at least a portion of said second storage units related to a second volume, said third storage system adapted to be coupled to said second storage system and including a plurality of third storage units and a third storage control unit which receives data of said second volume from said second storage system and stores data received from said second storage system into at least a portion of said third storage units related to a third volume, said first storage system comprising: a plurality of first storage units; and a first storage control unit which receives data of said second volume from said second storage system and stores data received from said second storage system into at least a portion of said first storage units related to a first volume, wherein said first storage system is adapted to comprise a first bitmap and a second bitmap, said first bitmap storing information related to data of a plurality of write requests received from said second storage system during a first time period, said second bitmap storing information related to data of a plurality of write requests received from said second storage system during a second time period being different from said first time period.