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 .