I was discussing the topic of RAID reliability and I found out I don't know how to calculate the URE probability for systems tolerant to multiple drive failures (RAID6/Z2, RAIDZ3, RAID1 with 3 or more drives per vdev).
For normal systems this is easy: error_prob = 1-(1-p)^bit but what about multiple drives?
Is the second one read again from the beginning or, in ZFS, are only the failed reads taken from the second drive? in this case the probability of success would increase a lot!
I asked the same question on superuser, I repeat it here because the question is very ZFS specific and I think there is a lot of ZFS knowledge in this forum.
I'm interested in the answer both for pure curiosity, but also because I bought a small server (HP ML110 G7) and I have to decide whether to go for a 2+2 RAID1 (6 TB are enough for me) or to jump immediately to 4+2 RAIDZ2 (it means another 300 Euro and a lot of useless storage). In case I go for 2+2 I will have in each vdev a WD Green and a WD Red (mixing them lowers the probability of hardware failure at about the same time) and about 21% probability to fail a rebuild.