RAID 5 or RAID 6: Which should you select?


Takeaway: Many RAID controllers now support both RAID 5 and RAID 6. IT pro Rick Vanover explains what RAID 6 is and when to select it over RAID 5.

When it comes to architecting a storage solution, planning is key. In addition to selecting the storage protocol (iSCSI, fibre channel, NAS, etc.) and disk type to use (SAS, SATA, SSD, etc.), you should also give a lot of thought about what RAID algorithm to use.

I used to try to keep it simple and stick to either RAID 1 or RAID 5 when I wasn’t working with larger amounts of storage. However, when it comes to provisioning larger storage in any SAN or NAS environment, it may be worth selecting RAID 6 over RAID 5 for larger arrays. To help you understand how RAID 5 and RAID 6 differ, we’ll explain each RAID type.

RAID 5 is an array that has a distributed parity bit across the array. Figure A is a representation of RAID 5.

Figure A


The grey blocks are the parity bit through the array. RAID 5 takes a minimum of three drives to implement; this example uses four drives to give an easier visual comparison to RAID 6.

RAID 6 uses two independent parity schemes that maintain array integrity. Figure B represents RAID 6.

Figure B


The grey blocks are the parity bits, but the two parity algorithms exist separately on their blocks. RAID 6 has more overhead in terms of usable storage compared to the raw amount, as well as a more complex RAID controller algothrithm. According to the AC&NC RAID information sheet, RAID 5 has better write performance.

I prefer building for RAID 6 in spite of the RAID 5 write performance advantage. This is primarily due to the rebuild times being less impactful on disk with RAID 6 even though they take longer, which is an important factor considering we now have 2 TB or larger disks in use in many NAS and SAN storage systems. RAID 6 has additional protection against block failures and controller errors by the extra parity. If zero loss of data is a priority, this is something to consider. I’ve had controllers have logic errors and data loss even when the actual disks are healthy.




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