Patent ID: 8751767

Claim:
A computer system, comprising: a storage device including a plurality of storage areas; a virtual volume including a plurality of virtual logical areas; a plurality of pool volumes including the plurality of storage areas managed as a plurality of segments; and a controller configured to: manage the plurality of pool volumes as a group that relates to the virtual volume; allocate the plurality of storage areas of the plurality of pool volumes in the group to the virtual volume in segment units, when the virtual volume receives a write request to store data indicated by the write request, where an order of the allocation of the plurality of storage areas is determined based on a storage capacity ratio of the plurality of pool volumes in the croup, and store the data indicated by the write request; and when a new pool volume is added to the group, migrate data, in a part of the plurality of allocated storage areas, to a plurality of new storage areas in the new pool volume, and allocate the plurality of new storage areas to the virtual volume in segment units, based on a capacity of each of the plurality of pool volumes in the group and a number of segments in each of the plurality of pool volumes that are allocated to the virtual volume.