Thursday, 13 January 2011

Types of RAID6

1. P+Q RAID6

Each data stripe of a P+Q double parity RAID6 has a standard RAID5 parity and a ‘Q’ parity which is generated by Reed-Solomon coding algorithm. It allows up to two drives failure. The minimum number of disks is 4.

The HP ADG RAID6 is a special type of P+Q RAID6. Within each standard data stripe, there are an integer number of smaller data stripes where the P and Q parities remain on the same disk. This is also called delay parity. Generally speaking, this type of RAID6 has a delay parity value of 16.

2. NetApp Double XOR RAID 6

It is also called RAID-DP and has a similar data structure of RAID4. P and Q parities are generated and stored on the same drives. It allows up to two drives failure. The minimum number of disks is 4.

3. X-Code RAID6

It is also called vertical XOR RAID6. The minimum number of disks is 4. It allows up to two drives failure. Each data stripe group has N (disk number) of data stripes. For example, if it has 5 disks, each data stripe group has 5 rows of data stripes as shown in the figure below.

4. ZZS Code RAID6
The minimum physical number of disks of ZZS code RAID6 is 5 and it has to be a prime number (5, 7, 11, 13, 19...). But the ZZS coding algorithm also allows one of the drives contains zeros. In another word, the minimum logical number of disk is 4. In ZZS coding algorithm, it also defines the calculation unit (one cycle) is equal to (n-1)/2 if there is n physical disks in the RAID6. For example showing below, it has 7 disks, thus, each cycle has three data stripes.

5. Park Code RAID6
Park is an engineer of IBM. He designed a new coding algorithm to implement a type of RAID6, supports disk number of 3 to 38. Park code RAID6 is always using three data stripes as a calculation unit.

Currently, I don't find any software application support the ZZS and Park RAID6. But, with all the disks intact, the data are still retrievable by customised RAID of R-studio. Leave your comments if you know any other types of RAID6 or solutions.

Written by: Zijian Xie (R&D Manager, MSc,BEng)


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