I assume Thornwood will reply and confirm this, but as I read his post, he used 2 4TB drives, but configured two arrays on those drives -- a 4TB RAID-0 (using 2TB from each of the drives) for parity, and a 1TB RAID-1 (using 500GB from each drive) for cache.    This would leave 1.5TB unused on each drive -- but would also enhance the performance, since the drives were "short-stroked" in this configuration and never accessed the slower inner cylinders.

 

This is not how RAID 1 works. RAID 1 requires 100% extra capacity for replication. Thus a 1TB cache LUN would require 2TB from the disk pool.

Yes, I'm well aware of how RAID works -- clearly I was asleep when I wrote that 

... or perhaps Thornwood had an erroneous post on that and has since deleted it ??    I don't see ANY mention of his RAID setup in this thread now ... but clearly I was referring to something he had said !!