Double-parity RAID

Double-parity RAID (redundant array of independent disks), also called diagonal-parity RAID, Advanced Data Guarding (RAID_ADG), or RAID-6, is a method of protecting against multiple storage drive failures by creating two sets of parity data on an array of hard disks.

In a conventional RAID, data is stored in different places on multiple hard disks, thereby increasing the aggregate mean time between failures (MTBF) and improving the fault-tolerance. In the approach known as RAID-4, the number of bits in data blocks on multiple disks is added up, and the total is kept on a disk called the parity disk. If a drive fails, data recovery is facilitated by using the bits stored on the parity disk and bits remaining on the surviving drives. In RAID-5, the parity information is stored diagonally across all the disks in the RAID set. If a single drive fails, the original data is calculated from the parity information remaining on the surviving disks in the set.

Because hard disk storage capacity continues to grow, and because RAID systems are using larger numbers of disks, the risk of a double-disk hit (failure of two disks at the same time) is increasing. In a conventional RAID, such an event can result in extended downtime. In a double-parity RAID, this risk is minimized by the use of two parity disks. The number of bits is added up in a redundant fashion, and two sets of parity information is stored diagonally across rows of data blocks. As a result, data can be recovered easily, even if two hard disks fail simultaneously.

RAID-6 (double-parity RAID) systems are offered by several vendors. The main advantage is enhanced data protection as compared with single-parity RAID systems. Some engineers estimate that the MTBF in a double-parity RAID is over 100 times the MTBF in a single-parity RAID. Conversion from a conventional RAID to a double-parity RAID is said to be straightforward and not overly expensive. Though double parity RAID exacts more of a performance penalty than RAID-5, it is said to offer twice the protection. Double-parity RAID also allows up to 56 physical drives (a minimum of four drives are required), offering an advantage in logical unit (LUN) sizes from a single RAID-set over RAID-5 implementations, which are typically 16 drives or less.