Agreed, mirror vdevs are the way to go. Here's a post that explains the tradeoffs between RAIDZ and mirror vdevs in great detail: http://jrs-s.net/2015/02/06/zfs-you-should-use-mirror-vdevs-...
$ zpool status
pool: BoxODisks
state: ONLINE
scan: scrub repaired 0 in 9h20m with 0 errors on Sun Jul 31 01:31:31 2016
config:
NAME STATE READ WRITE CKSUM
BoxODisks ONLINE 0 0 0
mirror-0 ONLINE 0 0 0
gptid/18059e22-b6a4-11e5-9cca-0cc47a6bbf34 ONLINE 0 0 0
gptid/194649b1-b6a4-11e5-9cca-0cc47a6bbf34 ONLINE 0 0 0
mirror-1 ONLINE 0 0 0
gptid/1a86b3cc-b6a4-11e5-9cca-0cc47a6bbf34 ONLINE 0 0 0
gptid/1bcd3ad6-b6a4-11e5-9cca-0cc47a6bbf34 ONLINE 0 0 0
cache
diskid/DISK-S24ZNWAG903847Lp1 ONLINE 0 0 0
I think I have mirrored vdevs.What does that mean for replacing failed disk/s? What does that mean when I want to upgrade to 8TB drives? The biggest "mistake" I made was buying a very expensive 4 bay enclosure - 4 drives is not enough.