To optimize performance and power consumption of a storage system having many disk drives, the storage system contains a plurality of volumes. A first number of the volumes belong to a first volume set. The first number of the remaining volumes belong to a second volume set. The volumes that belong to the first volume set are allocated dispersedly to a second number of disk drives. The volumes that belong to the second volume set are allocated dispersedly to a third number of disk drives, the third number being larger than the second number. A computer selects one of the first volume set and the second volume set based on a predetermined condition to store data dispersedly in the volumes belonging to the selected volume set. The computer stops spinning of disks in the disk drives to which none of the volumes belonging to the selected volume set are allocated.