Computer system and method for reducing power consumption of storage system

A disk rotation control part for controlling the rotations of a plurality of disk devices is provided. The configuration is such that a disk of a disk device for which an access will not be generated is rotated at a second rate of speed, which is faster than 0, and slower than a first rate of speed, which is the rotation speed at access time. DB management information comprises information denoting which DB schema inside a DB is stored in which location. The disk rotation control part, based on a query plan of a query received by the DBMS, and DB management information, specifies the storage location of a DB schema to be accessed when this query is processed, and rotates a disk of the disk device comprising the specified storage location at a first rate of speed.