A function that protects data stored on a disk-unit subsystem from being lost because of the failure of a single disc unit in the subsystem. When a disk-unit subsystem has device parity protection and one of the disc units in the subsystem fails, the subsystem continues to run. The disk-unit subsystem reconstructs the data after the disc unit is repaired or replaced. See also checksum protection, mirrored protection, Redundant Array of Independent Disks.