PROBLEM TO BE SOLVED: To provide a hierarchical data storage system which optimizes performance and reliability by storing data by using different redundant technique. SOLUTION: A storage space reserve method where a data storage system forms a physical space 34, which is mapped to a RAID level virtual storage space 40 that has a mirror RAID area and a parity RAID area and furthermore, the latter is mapped to an application level virtual storage space 50 is provided with n pieces of storage disks in disk array and allocates virtual blocks in the space 50 to the mirror RAID area in order to store data according to mirror redundancy. Then, the data is stored in the virtual blocks and a maximum acceptable number in the mirror RAID area is calculated as a function of physical capacity, acceptable capacity and a storage disk number in the disk array.