MTBF: RAID-Z3 vs 2x RAID-Z2

[netchris]

I am about to embark on my first FreeNAS project and would like some input from the forum. My primary goal is high reliability and I'm looking at the following options:

• 11 drives in a single RAID-Z3 vdev (8 data 3 parity)
• 12 drives in two RAID-Z2 vdevs (8 data 4 parity)

Which do you guys think is the better option for reliability? for performance? which would you go with?

 

[BigDave]

• 12 drives in two RAID-Z2 vdevs (8 data 4 parity)

A pool with 2 vdevs (RAIDZ2) will still fail with the loss of three drives within the same vdev,
the failure of one vdev will mean the loss of the entire pool. Not 4 parity!

 

[netchris]

Agreed--not 4 parity drives for a single array, just 4 parity drives in total.

I guess my question, phrased another way, is something like "what's more reliable--striping across two 6 disk RAID-Z2s or a single 11 disk RAID-Z3." It's not clear cut to a noob such as myself, so I'm reaching out for any input/thoughts/advice/thoughtless ramblings (both welcome and encouraged)

 

[Ericloewe]

It's hard to provide a straightforward answer.

Performance will clearly favor more vdevs.

Reliability will probably favor one big RAIDZ3 vdev, but I haven't run the numbers. I wouldn't worry with either alternative, in a regular Home/SMB setting.

 

[cyberjock]

Well, @DrKK had a link a few months back for ZFS MTBF ratings. I can't remember if it supported multiple vdevs though. But, all things being equal I'd exepect that the two-vdev would provide higher reliability because you are breaking the pool into two smaller vdevs with RAIDZ2.

 

[enemy85]

Consider that a raidz3 would require a longer resilvering process compared to a raidz2 with 6disks. I'll go with the 2 raidz2 vdevs