1. A computer system comprising: at least one storage subsystem including parity groups, and logically including a logical volume by arranging the parity groups; at least one application server; a management server storing mapping information indicating a mapping between the logical volume and the at least one application server, and performance information about the parity groups and logical volumes, wherein the management server is configured to: (A) acquire required I/O (Input/Output) performance information; and (B) determine a rearranging destination parity group from the parity groups, which is rearranging destination of the logical volume, based on the required I/O performance information, the mapping information, and the performance information, so as to reduce I/O performance of a predetermined high priority application server.