Patent ID: 7114044

Claim:
A method of controlling a storage system comprising a first storage device having a first storage volume provided at a first site, a second storage device having a second storage volume provided at a second site, and a third storage device having a third storage volume provided at a third site, wherein the method includes the steps of: storing a copy of data stored in the first storage volume in the second storage volume with an asynchronous remote copy procedure; writing the copy of data written in the first storage volume into the third storage volume with a synchronous copy procedure; relating data written in the first storage volume after a first time to a storage area of the first storage device; controlling by the third storage device to store a first differential management table on which a write history of the data written in the first storage volume is recorded after the first time and a second differential management table on which a write history of the data written in the first storage volume is recorded after a second time subsequent to the first time; transferring data related to the storage area of the first storage device from the first storage device to the second storage device with the asynchronous remote copy procedure after the second time; and controlling by the third storage device to make contents of the data stored in the second storage volume consistent with contents of the data stored in the third storage volume by using the first differential management table and the third storage volume of the third storage device, if the first site has a disaster.