Patent ID: 8516215

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