Which RAID level is best: the dual parity of RAID 6 or the mirrored data of RAID 10? |
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By Antony Adshead, UK bureau chief, storage
01 Jun 2009 | SearchStorage.co.UK |
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Many storage managers have a hard time deciding which redundant array of independent disks (RAID) level to use for the best availability and reliability. Should they use the dual parity of RAID 6 or the mirrored data of RAID 10?
RAID 6 stripes data across disks and calculates dual distributed parity. Distributed parity provides fault tolerance against two drive failures. Dual parity means that while a failed disk is being rebuilt the array is still protected by the remaining parity data.
RAID 1+0 (RAID 10) is mirrored sets in a striped set. RAID 1+0 creates a striped set from subsets of mirrored drives. If disks fail, RAID 1+0 allows all of the remaining disks to continue in use. The array can suffer multiple drive failures as long as no mirror set loses all of its drives.
To help you choose the RAID level that best meets the needs of your organisation, let's take a look at some of the advantages of RAID 6 and RAID 10.
RAID 6 gives more usable capacity the more disks you add. Because RAID 10 mirrors everything, an array requires double the disk capacity of the data to be stored. The remainder of the capacity constitutes the mirror. If a RAID 6 array comprises four disks, only 50% of that space is available as usable capacity, but the proportion of usable space increases as you add more drives.
That means half the total capacity of a RAID 10 array will always be dedicated to protection, but with a RAID 6 array the usable capacity grows as the number of drives increases. For example, if you increased the number of disks in a RAID 6 array from four to eight, the space consumed by parity data would decrease from 50% to 25%.
RAID 6 requires more processing power. RAID 6 makes two parity calculations for each write operation, so it's slower to write than most other RAID levels.
RAID 6 can always protect against two simultaneous disk failures. Because RAID 6 doubles up its parity data, it can withstand two disks failing at the same time. Whether RAID 10 can handle two disk failures simultaneously depends on where they occur. If both the disks that fail are located in the same mirror, the other set can step in. You will lose all data if the same disks in both mirrors fail within the rebuild window (which should be relatively short, however).
RAID 10 rebuild times are faster. RAID 10 has among the fastest rebuild times possible because it only has to copy from the surviving mirror to rebuild a drive, which can take as little as 30 minutes for drives of approximately 1 TB. The key drawback of RAID 6 is that the time it takes to rebuild the array after a disk failure is lengthy because of the parity calculations required, often up to 24 hours with even a medium-sized array.
RAID 10 doesn't need special hardware. Most controller hardware will support RAID 10 with good performance. Because RAID 6 doubles the parity calculations for every write, it requires specially designed controller hardware.
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