RAID level 6 has been one of the orphan levels of RAID. Like RAID 2 -- and to a lesser extent RAID 4 -- storage administrators, generally, don't use it. The reason for this is that RAID 6 adds an extra disk and a good deal of overhead to write operations in return for the ability to survive the failure of two drives in the array. So far, most storage admins haven't viewed the tradeoff as worthwhile.
RAID 6 is basically RAID 5 with extra parity. Like RAID 5, it stripes both data and parity blocks across the disks in the array. However, unlike RAID 5, it calculates two sets of parity data, organized so a single drive failure cannot eliminate them both.
These extra parity operations are the source of the strengths of RAID 6 as well as its weaknesses. Because it has dual parity, RAID 6 can survive the failure of two disks, whereas RAID 5 can only survive the failure of one. However, calculating the second parity adds a significant amount of computational overhead to the write operations, and this reduces write performance considerably.
Two factors have changed the equation for RAID 6: Application Specific Integrated Circuits (ASICs) and the emergence of SATA arrays. New RAID 6 controllers from companies such as Nexsan Technologies Ltd. and Areca Technology Corp. use ASICs to handle the parity calculations. The performance of these companies' RAID 6 controllers is roughly on par with RAID 5.
Not that ASICs are new. They have been available for a long time and ASIC parity engines could have been added to RAID controllers years ago. However, one reason that companies have started incorporating ASICs on RAID controllers is the growth of Serial ATA (SATA) RAID.
SATA drives offer SCSI-like performance at a price only slightly more than conventional ATA drives. SATA drives are cheap because they are usually built to ATA standards rather than the more demanding SCSI specifications. This means there is a significantly increased chance of drive failure with SATA drives. Given the built-in redundancy of
RAID, this is not much of a problem, but storage administrators who truly did not want to run the risk of data loss have shied away from SATA RAID because of its reliability issues.
Putting RAID 6 on a SATA array provides a means of having SATA and more reliability. At the cost of a somewhat more expensive controller, a RAID 6 array can offer added protection against data loss while still providing the benefits of SATA. A Nexsan official said that the price difference between the company's RAID 5 and RAID 6 controllers is minimal. It is worth noting, however, that RAID 6 does require an additional hard drive in the array.
Rick Cook has been writing about mass storage since the days when the term meant an 80 K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last 20 years he has been a freelance writer specializing in issues related to storage and storage management.
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