How is parity 2 calculated?

[drumstyx]

Single parity seems to be a very easy concept, and I can run through theoretical failures in my head to understand how it works, but that second parity is making no sense to me.

 

I trust it works, and I have dual parity in my array, but I'm curious how the actual math works, especially how rebuilding occurs in a dual disk failure situation.

[Squid]

Not 100% if this is the exact method that unRaid uses, but it'll give a good basis on the math

 

http://igoro.com/archive/how-raid-6-dual-parity-calculation-works/