RAIDZ2 vs mirrored vdevs? Recommendation please.

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Oct 27, 2015
I about to build a 4 drive RAIDZ2 pool. Now it seems a pool of 2 mirrored vdevs may be an option as well. I got a recommendation (sorry, I lost the source), that the replacement of a failed drive would be much much quicker than in a RAIDZ2. The long duration of a drive replacement in a RAIDZ2 would decrease the overall reliability of the RAIDZ2.

Is that true?

The RAIDZ2 option should provide higher reliability: It will survive the loss of up to any two drives while a pool with 2 mirrored vdevs will die on failure of 2 drives depending on which drives fail.

Other aspects to consider:
  • Capacity: No difference in this case (4 drives of equal capacity).
  • Performance: The Guide claims improved performance of mirrored vdevs in specific scenarios (not the most important aspect for me though).
  • The mirrored vdevs have another advantage: The migration is much easier because I can create a pool of one mirrored vdev, migrate data to this, wipe the existing drive, create another mirrored vdev of the existing drive and another new drive and expand the pool with this. (I'm limited to 4 drive bays ...)
  • Also mirrored vdevs should make it easier to expand capacity in the future: I can replace 2 drives of one vdev with larger drives and thus increase the pool size. With a RAIDZ2 this would require the replacement of all drives.
Are mirrored vdevs a commonly used setup or should I stay clear? Did I miss any important aspects?


Server Electronics Sorcerer
Aug 5, 2013
In this case it's false.

Both solutions are commonly used; the mirrors are preferred where you need a lot of IO/s (VMs, ...) or you want to easily expand your pool later.
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Aug 29, 2015
Mirrored vdevs are frequently used, especially for IOPS-heavy workloads (like VM datastores).

As far as your assertions...
Capacity - yes
Performance - yes
Multiple vdevs - yes
Expanding in the future - yes, you will have to replace all of the drives in the vdev. With 2 way mirrors, this is 2 drives. With RAIDZ2, this is however many drives you have in the vdev.
Shorter resilver time - yes, mirrors are much faster to resilver

Now, the big downside... RAIDZ2 will survive the loss of *any* 2 drives. Striped mirrors will survive the loss of 1 drive per vdev... so you can lose 2 drives, but ONLY if one drive is in each vdev. Lose two drives in the same vdev and you're toast.

The most common RAIDZ2 pool seen around here uses 6-drive-wide vdevs... in that case, you get substantially more capacity going RAIDZ2 versus striped mirrors.
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