Is there a difference in durability between a RAID-Z3 and a 4-way mirror? AFAIU, both can lose three drives without losing data. Is there anything special with the parity that would make a RAID-Z3 safer? (or maybe the speed of resilvering would make a 4-way mirror safer?)
Background: I'm helping a non-profit that may not be able to quickly respond to drive issues. They almost lost a RAID-Z1 after a drive started failing and taking no action for multiple months.
They have a four bay FreeNAS Mini and only need to store ~2.5TB, growing ~200GB/yr. I think a 4-way mirror might be an awesome option in this case (we're eyeing 4x HGST HE10 10TB drives)? I could also build them something with more bays and do a RAID-Z3 + hot spare, but they seem to find hardware like that more difficult (intimidating?) to maintain (I already have eight 4TB HGST drives that are great for ZFS and a Supermicro X10SDV-7TP4F laying around; only thing we'd need is a quiet-ish rackmount chassis).
Our goal is to make the pool itself more redundant, and then to replicate to a second box, then to sync offsite to Amazon S3. We're using FreeNAS 11.3.