@vikingred If you are interested in some good reading on how FreeNAS (ZFS Pools) work and the understanding behind vdevs and such - check out this fourm post from Tom, as well as the docs he links to. I read this stuff through when I began with FreeNAS and it helped clear up a lot of confusion.
@Greg_E
While I agree that RaidZ (or RaidZ2) would provide more space than a mirrored pairs configuration - my desire to avoid it is that it makes expanding the pool far more difficult. In a business environment (or maybe someone else’s home environment!) purchasing 8x 8TB drives at once is not a problem. I prefer not to spend that kind of money on storage all at once if I do not need it all right now. If I were in a different financial situation where 8x 8TB drives was no problem, then I would be inclined to agree- RaidZ2 would be my preference as it results in a higher percentage of usable space.