Patent ID: 8769235

Claim:
A computer system, comprising: a storage device including a plurality of storage areas; a virtual volume to which access from a host system is permitted; a plurality of logical volumes each including a plurality of storage area units and constituting a plurality of pool groups; and a controller for controlling allocation of the storage area units from the plurality of logical volumes in the pool to the virtual volume, wherein the controller is configured to: manage, for each of the plurality of logical volumes in the pool, information regarding a capacity of each of the plurality of logical volumes and a number of storage area units allocated to the virtual volume; allocate at least one storage area unit, which has not been allocated from the pool to the virtual volume, to the virtual volume when the host system accesses the virtual volume; detect a state where an imbalance of a usage rate of the logical volumes, which is determined based on the capacity of each of the plurality of logical volumes and the number of storage area units allocated to the virtual volume, occurs among the plurality of pool groups; and change the allocation of at least one storage area unit of the plurality of logical volumes allocated to the virtual volume to a storage area unit of another logical volume in another pool group to balance the usage rate among the plurality of pool groups in the pool.