Hierarchical RAID system including multiple RAIDs and method for controlling RAID system

A record medium capable of being read through a computer so as to use at least one RAID having a plurality of disks as a virtual disk, in a hierarchical RAID system having a microprocessor, the record medium characterized in that the computer has a writing of a program to realize:

• a first function of giving a read-command for existing data and parity from an upper level RAID controlling unit to a lower level RAID controlling unit when a write-command for new data is given from a host computer;
• a second function of performing an exclusive OR of the new data, existing data and the parity, and executing a write-transmission in the lower level PAID controlling unit so as to perform a renewal of the parity and the data according to the execution result of the exclusive OR operation;
• a third function of generating a command for reading the parity and data of a hard disk in the lower level RAID controlling unit according to the read-command of the first function, and transmitting the command to the upper level RAID controlling unit;
• and
• a fourth function of evoking a reading for the data and a parity value and performing the exclusive OR so that the write-command given from the upper level RAID controlling unit becomes the write-transmission to each corresponding lower upper level RAID controlling unit, and executing the write-transmission for the renewal of parity and data.