RAID storage system and method for the parallel execution of a rebuild process

A rebuild control unit splits a rebuild process into partial processes and instructs an access processing unit to execute the partial processes. In the rebuild process, data identical to data recorded in a first storage device that constitutes RLU #1 is generated based on data read from, among storage devices constituting RLU #1, storage devices other than the first storage device and written to a spare storage device included in a storage system. Each partial process includes a combination of an operation of reading data from a split range created by splitting a data readout target range into fixed-size intervals and an operation of writing data in another storage device based on the data read from the split range. In response to the instruction from the rebuild control unit to execute the multiple partial processes, the access processing unit executes the instructed partial processes in parallel.