Method for adding storage disks to a hierarchic disk array while maintaining data availability

A method is described for enlarging the storage capacity of a hierarchical disk array data storage system which stores data according to different levels of redundancy, or RAID levels. The method includes first mapping the physical storage space (34) into a first virtual storage space (40) having a plurality of RAID areas that extend across multiple storage disks (0-3) in the disk array (11) and comprise multiple stripes of segments from each storage disk. The first virtual storage space (40) is mapped into a second virtual storage space (50) having multiple virtual blocks. To enlarge storage capacity, one or more storage disks (4) are added to the hierarchic disk array. To assimilate the new storage capacity, data in a selected existing RAID area is moved to another area on the disk array of equal or greater storage capacity. The mapping of the second virtual storage space to the first is updated to reflect the data movement. The physical storage space corresponding to the selected RAID area is then remapped into an expanded RAID area which spans across all storage disks, including the new disks. The process is repeated until all RAID areas in the hierarchic disk array have been expanded to include regions on the new storage disks. The capacity available to the second virtual storage space (50) is modified to reflect the enlarged storage capacity. According to this method, data remains available to the user at all times during the enlargement process.