The Benefits of Double Parity or RAID 6

RAID 6 otherwise known as ‘double parity’ uses two stripes on each hard disk drive. The advantages are that the system can afford a double drive failure and still recover. Why is this important? Many users choose RAID 5 to increase their data storage performance but it really isn’t very reliable and here is why…

If two hard disk drives fail in a RAID 5 array then data loss will prevail. The array can recover after one drive failure due to the inherent redundancy, but if two drives fail the result is terminal. The only way to get your data back is to engage data recovery specialists. Some users may set up a ‘mirrored’ system known as RAID 1. The downside of this is that it effectively halves the amount of available storage. Performance is also effected as the drive has to write data twice! However it doesn’t need to be this way. For the price of another hard drive, the user can configure RAID 6 to give them the performance of RAID 5 and the stability of RAID 1.

You may well ask, “what is the likelihood of two drives failing simultaneously anyway and why should I be concerned?” Quite frankly the risks are very real and occur frequently. When configuring hard drives in a RAID array, it is of paramount importance that the hard drive architecture is the same between all models in the array. This means that all the hard drives must not only have the same model number, but the same firmware, capacity and circuitry otherwise the array will encounter problems. One of the most common causes of mechanical hard drive failure is through manufacturing defects. Hence if one of your hard drives in the array has a defect, all the others are likely to have inherited the same defect. Furthermore, when one drive fails it puts a strain on the others as they try to recover. This strain is often the catalyst for a second failure. As the array tries to recover but is unable, logical damage can exasperate the mechanical failure.

RAID 6 is identical to RAID 5 other than the addition of a second parity block, distributed across all disks. There is no performance loss for read operations. However write operations are slowed due to the increased parity calculations and the requirement for more processing power. Performance does vary greatly depending on the manufacturer’s storage architecture, but generally it is slower to write than any other RAID level. Due to the ‘double parity’ the user must remember that with only four disk arrays, only half of the total disk capacity is available as usable data storage. As you add more drives though, the user will benefit from the full capacity of those hard disk drives.

Aran Pitter of Data Recovery Specialists says “whenever we have completed a RAID 5 data recovery, we always recommend RAID 6 as an option to the client. Hard drives are so cheap in comparison to a user’s data, that it is a no brainer! If you are configuring a RAID 5 array, give some thought to RAID 6. The investment may well save you in the long term!”

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