Method of reducing storage power consumption by use of prefetch and computer system using the same

Provided is a computer system including: a computer running as a DB server; a storage system including a plurality of disk drives for storing data; and a management module, in which: at least one of the plurality of disk drives stores data of a DB schema written by the computer; the management module specifies the DB schema to be accessed based on a received query, transmits, to the storage system, an instruction to copy at least a portion of the data of the specified DB schema from the disk drive to a memory, and transmits, to the storage system, an instruction to control an rpm of the disk drive that stores the data of the specified DB schema; and the storage system controls the rpm of the disk drive based on the instruction. Accordingly, power consumption of the storage system can be reduced even if installed disks increase in number.