Patent ID: 7143252

Claim:
A computer system comprising: a first storage system comprising a first disk controller for receiving data from a host computer and one or more first disks each of which is coupled to the first disk controller; a second storage system comprising a second disk controller and one or more second disks each of which is coupled to the second disk controller; and a network to which the first storage system and the second storage system are operatively coupled, wherein the first disk controller stores data received from the host computer to a first storage area of the first storage system and sends the data to the second storage system, wherein the second disk controller stores data received from the first disk controller to a third storage area of the second storage system, wherein, after the first storage system receives a first instruction from the host computer, the first disk controller: sends to the second disk controller, as received data, first data that is stored in the first storage area at a time when the first instruction was received; receives from the host computer update data corresponding to the first data; and manages the update data such that the update data can be distinguished from the first data, wherein the second disk controller stores the received data to the third storage area as second received data, wherein the first disk controller sends a second instruction to the second disk controller, and after the second instruction is sent, the first disk controller sends the update data to the second disk controller as received update data, wherein after the second disk controller receives the second instruction from the first disk controller, the second disk controller stores the second received data from the third storage area to a fourth storage area in the second storage system and manages the received update data such that the update data can be distinguished from the first data.