I know RAIDZ1 is ZFS variant of RAID5 and RAIDZ2 is ZFS variant of RAID6; I will use all these terms interchangeably because I'm not too interested in the ZFS special sauce here.
In the early 2000s, a lot of people were pushing RAID5. Having worked in a hosting / colocation data centre for many years I had witnessed many RAID5 failures. What would happen is an array would degrade, and more often than not a secondary drive will fail due to undue load on the array as part of the degraded status. Also a lot of times failure would happen on the rebuild process because a lot of the HW implementations were flakey -- but again also the undue stress on all drives as you rebuild. This is why I would suggest a RAID10 setup at the time because of the double lucky failure, and more importantly because you can trivially use a software implementation which is much more safe. Also a lot of the motherboards at the time were offering RAID but this was really just a binary blob in the kernel doing software RAID with a facade of making it appear like hardware which fooled a lot of people.
Well we've finally done away with hardware / proprietary RAID and we have ZFS, mdadm, etc. I've normally dismissed RAID6/RAIDZ2 because of the parity/rebuild process and concerns of putting undue stress on the drive. But I think maybe this is premature and that I didn't really understand the consequences of a single drive failure versus a double drive failure. So this is kind of what I want to know:
1. When a single drive fails, is there any undue stress on the array, or because the array can pretty much operate unaffected, there's actually no performance degradation until you rebuild the missing drive, and in the case of software it's really just a negligible hit on the CPU if it has to do hashing/erasure coding/etc. I guess the rebuild process is really just the cost of a zfs scrub at this point but at least it is on a healthy array.
2. The good news of RAID6 over RAID10 is you can always survive two drive failure; but I think this is where things are concerning because the rebuild across two drives places a lot of undue stress on the remaining disks, and if any of those disks die then you're shit outta luck. This scenario is much more similar to a single drive failure in a RAID5 failure. But again, I think the rebuild cost is that of a zfs scrub but with the minimal set of disks. So RAIDZ2 would be a much more solid choice over RAID10 right; at least you will always know you can survive a two drive failure?