Patent ID: 8504765

Claim:
A computer system comprising: a first storage system coupled to a host computer; and a second storage system coupled to the first storage system, wherein the first storage system comprises at least one first storage device including a plurality of first storage areas, and a first controller configured to provide a first virtual volume including a plurality of first virtual storage areas to the host computer with the plurality of first storage areas, wherein the second storage system comprises at least one second storage device including a plurality of second storage areas, and a second controller configured to provide a second virtual volume including a plurality of second virtual storage areas with the plurality of second storage areas, wherein the first controller is configured to: allocate one of the first storage areas to a first virtual storage area which receives data from the host computer, in response to receiving the data, manage allocation information indicating whether the first storage area is allocated to the plurality of first virtual storage areas, transfer data stored in the first storage area, which is allocated to the first virtual storage area, to the second storage system to make the second virtual volume as a copy pair of the first virtual volume based on the allocation information, wherein the second controller is configured to: manage differential information indicating a second virtual storage area to which a data is written when the copy pair is suspended, wherein the first controller is configured to: receive the differential information to start a copy process between the copy pair, specify the first virtual storage area to which the first storage area is not allocated and which corresponds to a second virtual area which has data written when the copy pair is suspended based on the allocation information and the differential information, and transfer an instruction to release an allocation of the second virtual storage area which corresponds to the specified first virtual storage area.