18Software Installation and User’s GuideIntroductionFigure 1-2. RAID 1 - Disk MirroringFigure 1-3. RAID 1 - Disk Duplexing1.3.3 RAID 4 - Data Striping With a Dedicated Parity DriveRAID 4 works in the same way as RAID 0. The data is striped across the hard disks andthe controller calculates redundancy data (parity information) that is stored on a separatehard disk (P1, P2, ...). See Figure 1-4. Should one hard disk fail, all data remains fullyavailable. Missing data is recalculated from existing data and parity information.Unlike in RAID 1 only the capacity of one hard disk is needed for redundancy. Forexample, in a RAID 4 disk array with 5 hard disks, 80% of the installed hard disk capacityis available as user capacity, only 20% is used for redundancy. In systems with manysmall data blocks, the parity hard disk becomes a throughput bottleneck. With large datablocks, RAID 4 shows significantly improved performance.
