Verfahren zur Verwendung von nicht-aneinandergrenzenden reservierten Speicherplatz zur Datenmigration in einem hierarchischen redundanten Datenspeichersystem

A hierarchic disk array data storage system (10) has multiple storage disks (12) that define a physical storage space (34) and a RAID management system (16) that maps the physical storage space (34) into two virtual storage spaces. A RAID-level virtual storage space (40) presents the physical storage space as mirror and parity RAID areas where the mirror RAID areas store data according to RAID Level 1 and the parity RAID areas store data according to RAID Level 5. An application-level virtual storage space (50) presents the physical storage space as multiple virtual blocks. The RAID management system (16) migrates data between the mirror and parity RAID areas to optimize performance and reliability. To facilitate migration, the RAID management system (16) guarantees a minimum of unused storage space that can be used to empty a RAID area for the purpose of converting the RAID level or other storage attribute of the RAID area. The minimum unused storage space is preferably equivalent in size to at least one largest parity RAID area. However, the unused storage space is not in any one physical location, but can be allowed to distribute itself over various RAID areas in a non-contiguous manner across the storage disks.