Computer system and its control method

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.