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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.

  • Remember to buy the disks each from a different supplier (or to wait one-two weeks between each purchase) so that you don't get disks from the same batch. Or they may fail all within days from each other. – FarO Feb 9 at 8:23
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Both RAIDZ3 and 4-way mirrors can lose up to 3 drives without data loss. However, you must consider that RAIDZ3 will have much lower read IOPS and higher resilver time after a disk replacement.

Anyway, I really think 4-way mirrors is way too much (other things will broke before four drives die). If IOPS are not a concern, I would use a RAIDZ2 array.

That said, if you are sure that no additional space will be required, for a 4-bay system capable to sustain 3 disk failures, I vote for 4-way mirror rather than RAIDZ3.

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    Aldo worth pointing out its a non-optimal solution to get 4 drives of the same model as it increases the risk of failure ofultiple disks at about the same time. – davidgo Feb 5 at 11:17
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With 4-way mirror you will get 1 x speed of write but you will also get 4 x speed for reading (because ZFS will treat them for reading as RAID0).

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