A while back I ran into a customer who was using RAID 6.  Say what?!?  They hadn’t yet encountered poor IO performance.  But they’d only just RTM’ed their application and would soon encounter performance bottlenecks once they tried to scale.

The typical recommendation is to stick with RAID 5 when money is tight or to use RAID 1 and/or RAID 10 if you have more money for the IO subsystem.  But why not RAID 6?

I like how Steve Kass, a SQL Server MVP I hold in very high regard puts it:

“RAID 6 setup with N drives dedicates N-2 of the drives to data and 2 drives to parity. The minimum number of drives for a RAID 6 array is 4, and with 4 drives, 50% of the space is available for data. With more drives, a higher percentage of space is available for data (with 8 drives, 75% is data and 25% is parity).”

Another source sheds some more light on RAID 6:

"RAID-6. Is actually Dual Parity and takes the parity region from RAID-5 and duplicates it so each disk has two parity regions that are calculated separately. As a consequence a RAID-6 array can recover from the loss of two drives, but performance is impacted and an extra drive is required which increases the cost of implementation."

Wow – that’s a lot of unnecessarily consumed space without much, if any, improvement in IO speed. 

Read more details at http://www.3ware.com/intranet/pdf/RAID_6_TechBrief.pdf and http://www.acnc.com/04_01_06.html.