1. A storage system coupled to a host computer, comprising: a non-volatile medium that stores data; a disk cache that temporarily stores data stored in the non-volatile medium, where the disk cache is divided into a plurality of independent disk cache partitions; a control unit that controls an input and an output of data to and from the non-volatile medium; and a memory unit that stores information used by the control unit, including consistency control information setting respective commands permitted for each of the disk cache partitions, to guarantee consistency of the data; wherein the control unit is configured to determine whether or not to execute a requested command for a given disk cache partition, by referring to the consistency control information setting respective commands permitted for each of the disk cache partitions.