Patent ID: 7613896

Claim:
A storage system coupled to a plurality of computers, the storage system comprising: a controller managing a plurality of virtual volumes, a volume pool used for the plurality of virtual volumes and a plurality of pooled volumes included in the volume pool, and the controller managing, for each of the plurality of virtual volumes, a virtual capacity for providing to at least one of the plurality of computers and an assigned capacity, which indicates a total capacity of two or more logical storage areas assigned to one of the plurality of virtual volumes and is different from the virtual capacity; and a plurality of disk drive providing a plurality of physical storage areas to the controller, so that the controller uses the plurality of physical storage areas for a plurality of logical storage area of the plurality of pooled volumes, wherein the controller, in response to receive an access request sent from a first computer of the plurality of computers to a first virtual volume of the plurality of virtual volumes, allocates a first logical storage area of the plurality of logical storage areas from the volume pool to the first virtual volume for writing data of the access request to the first logical storage area, if necessary, wherein the controller, if the first logical storage area is allocated to the first virtual volume, increases the assigned capacity, of the first virtual volume, of a capacity of the allocated first logical storage area, wherein the controller, if the assigned capacity of the first virtual volume exceeds a threshold being lower than the virtual capacity of the first virtual volume, migrates data from the first virtual volume to another volume so that the controller can continue to cause the first computer to access the migrated data, wherein the controller uses a volume, selected among a first type of volume, a second type of volume and a third type of volume, as another volume, wherein the first type volume is a real volume, which is not provided to the plurality of computers and is already assigned to other logical storage areas using the other physical storage areas provided by the plurality of disk drives before using as a volume for writing data, and a formatted capacity of the real volume is same as a capacity, allocated to the other logical storage areas, and of equal to or higher than the virtual capacity of the first virtual volume, wherein the second type of volume is first pooled volume of the plurality of pooled volume, a formatted capacity of the first pooled volume being same as a capacity, already allocated to two or more of the plurality of logical storage areas using two or more of the plurality of physical storage areas, and of equal to or higher than the virtual capacity of the first virtual volume, and wherein third type of volume is a second pooled volume of the plurality of pooled volumes, a formatted capacity of the second pooled volume being indifferent about the virtual capacity of the first virtual volume.