RAID 6 works in the same way as RAID 5, except that RAID 6 includes a second parity element to enable survival if two disk failures occur in a RAID set. Therefore, a RAID 6 implementation requires at least four disks. RAID 6 distributes the parity across all the disks. The write penalty in RAID 6 is more than that in RAID 5; therefore, RAID 5 writes perform better than RAID 6. The rebuild operation in RAID 6 may take longer than that in RAID 5 due to the presence of two parity sets. RAID 6 is based on polynomial erasure code technique which are generally used in future RAID techniques, data protection and in cloud based architectures.
The parity in RAID 6 is distributed across all members of the RAID set. RAID 5 reserves the equivalent of one drives worth of blocks for parity whereas RAID 6 reserves two drives worth of block for two discrete sets of parity. This enables RAID 6 to suffer two failed drives without losing data. This makes RAID 6 an extremely safe RAID level.
RAID 6 is generally considered extremely safe and RAID 6 performs well in high-read situations. It can suffer two drive failures in the set without losing data. This makes it an ideal choice for use with large drives or for use in pools with large numbers of drives. Large drives tend to be slow drives in terms of revolutions per minute (RPM) and therefore take a lot longer to rebuild than smaller, faster drives. For this reason, the window of exposure to a second drive failure while performing a rebuild is greatly increased. RAID 6 gives an extra safety by allowing you to suffer a second drive failure without losing data.
The major downside of RAID 6 is performance, especially performance of small-block writes in an environment experiencing a high number of writes. Because RAID 6 doubles the amount of parity per RAID stripe, RAID 6 suffers more severely from the write penalty than RAID 5. The four I/Os per small-block write for a RAID 5 set becomes six I/Os per write for a RAID 6 set. Therefore, RAID 6 is not recommended for small-block-intensive I/O requirements, and in the vast majority of cases, it is not used for any heavy write-based workloads.
RAID 6 Use Cases
RAID 6 is a good all-round system that combines efficient storage with excellent security and decent performance. It is preferable over RAID 5 in file and application servers that use many large drives for data storage.
Advantages
- Like with RAID 5, read data transactions are very fast.
- If two drives fail, you still have access to all data, even while the failed drives are being replaced. So RAID 6 is more secure than RAID 5.
Disadvantages
- Write data transactions are slowed down due to the parity that has to be calculated.
- Drive failures have an effect on throughput, although this is still acceptable.
- This is complex technology. Rebuilding an array in which one drive failed can take a long time.
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