RAID 6 Replacing RAID 5 in Enterprise Environments
Modern large drive capacities and the large RAID arrays used in modern servers create two problems (discussed below in Problems with RAID). First, in almost all arrays the drives are fitted at the time of manufacture and will therefore wear at similar rates and times. Therefore, the times of failure for individual drives correlate more closely than they should for a truly random event. Second, it takes time to replace the faulty drive and to rebuild the array.
Rebuilding a RAID 5 array after a failure will add additional stress to all of the working drives because every area on every disc marked as being "in use" must be read to rebuild the redundancy that has been lost. If drives are close to failure, the stress of rebuilding the array can be enough to cause another drive to fail before the rebuild has been finished, and even more so if the server is still accessing the drives to provide data to clients, users, applications, etc. It is during this rebuild of the "missing" drive that the entire raid array is at risk of a catastrophic failure. The rebuild of an array on a busy and large system can take hours and sometimes days and therefore it is not surprising that when systems need to be highly available and highly reliable or fault tolerant RAID 6 is chosen.
With a RAID 6 array using drives from multiple sources and manufacturers it is possible to mitigate most of the problems associated with RAID 5. The larger the drive capacities and the larger the array size, the more important it becomes to choose RAID 6 instead of RAID 5.
A disadvantage of RAID 6 is extra cost because two redundant drives are required. In small arrays, this can add significantly to the production cost and also to the ongoing cost because of the additional power consumption and additional physical space required. RAID 6 is a relatively new technology compared to RAID 5, and therefore the hardware is more expensive to purchase and drivers will be limited to a smaller range of operating systems. In software implementations of RAID 6, the algorithms require more CPU time when compared to RAID 5, because the algorithms are more complex and there is more data to be processed. Therefore, RAID 6 in software implementations may require more powerful CPUs than RAID 5.
RAID 6 also suffers a greater write performance penalty than RAID 5. For small (non-full stripe in size) write operations, which are the dominant size in transaction processing systems, the spindle operation overhead is 50% greater and latency will be slightly higher than with RAID 5. Providing the same write performance as a RAID 5 array requires that a RAID 6 array be built of approximately 50% more spindles and this impacts the cost of performance.
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