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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Performance, Quality and Reliability | Redundant Arrays of Inexpensive Disks (RAID) | RAID Levels | Single RAID Levels ]

RAID Level 6

Common Name(s): RAID 6. Some companies use the term "RAID 6" to refer to proprietary extensions of RAID 5; these are not discussed here.

Technique(s) Used: Block-level striping with dual distributed parity.

Description: RAID 6 can be thought of as "RAID 5, but more". It stripes blocks of data and parity across an array of drives like RAID 5, except that it calculates two sets of parity information for each parcel of data. The goal of this duplication is solely to improve fault tolerance; RAID 6 can handle the failure of any two drives in the array while other single RAID levels can handle at most one fault. Performance-wise, RAID 6 is generally slightly worse than RAID 5 in terms of writes due to the added overhead of more parity calculations, but may be slightly faster in random reads due to spreading of data over one more disk. As with RAID levels 4 and 5, performance can be adjusted by experimenting with different stripe sizes.

This illustration shows how files of different sizes are distributed
between the drives on a four-disk RAID 6 array using a 16 kiB stripe
size. As with the RAID 0 illustration, the red file is 4 kiB in size; the blue
is 20 kiB; the green is 100 kiB; and the magenta is 500 kiB, with each
vertical pixel representing 1 kiB of space. This diagram is the same as the
RAID 5 one, except that you'll notice that there is now twice as much
gray parity information, and as a result, more space taken up on the
four drives to contain the same data than the other levels that use striping.

Controller Requirements: Requires a specialized (usually meaning expensive) hardware controller.

Hard Disk Requirements: Minimum of four hard disks; maximum set by controller. Should be of identical size and type.

Array Capacity: (Size of Smallest Drive) * (Number of Drives - 2).

Storage Efficiency: If all drives are the same size, ( (Number of Drives - 2) / Number of Drives).

Fault Tolerance: Very good to excellent. Can tolerate the simultaneous loss of any two drives in the array.

Availability: Excellent.

Degradation and Rebuilding: Due to the complexity of dual distributed parity, degradation can be substantial after a failure and during rebuilding. Dual redundancy may allow rebuilding to be delayed to avoid performance hit.

Random Read Performance: Very good to excellent; generally better for larger stripe sizes.

Random Write Performance: Poor, due to dual parity overhead and complexity.

Sequential Read Performance: Good to very good; generally better for smaller stripe sizes.

Sequential Write Performance: Fair.

Cost: High.

Special Considerations: Requires special implementation; not widely available.

Recommended Uses: In theory, RAID 6 is ideally suited to the same sorts of applications as RAID 5, but in situations where additional fault tolerance is required. In practice, RAID 6 has never really caught on because few companies are willing to pay for the extra cost to insure against a relatively rare event--it's unusual for two drives to fail simultaneously (unless something happens that takes out the entire array, in which case RAID 6 won't help anyway). On the lower end of the RAID 5 market, the rise of hot swapping and automatic rebuild features for RAID 5 have made RAID 6 even less desirable, since with these advanced features a RAID 5 array can recover from a single drive failure in a matter of hours (where without them, RAID 5 would require downtime for rebuilding, giving RAID 6 a substantial advantage.) On the higher end of the RAID 5 market, RAID 6 usually loses out to multiple RAID solutions such as RAID 10 that provide some degree of multiple-drive fault tolerance while offering improved performance as well.

Next: RAID Level 7

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