A method completes background processing within a shortest possible time without degrading an efficiency of main processing executed in a system. The system includes a control apparatus including a plurality of hard disks and a memory controller configured to control an access to the hard disk. The memory controller changes a unit of the background processing, which is changed according to a state of a plurality of storage devices. In changing the unit of processing, the memory controller monitors the presence or absence of an access for the main processing coming from a main controller, and changes the unit of the background processing to a first unit of processing or to a second unit of processing, which is smaller than the first unit of processing according to a result of the monitoring.