The problem with RAIDZ or why you probably won't get the storage efficiency you think you will get

nice writeup, but I humbly suggest that this isn't a representation of a "problem" as the subject suggests. What you've done is quantify how zfs utilizes vdevs to provide usable storage- no more, no less, user expectation notwithstanding.
  • Like
Reactions: IsThisThingOn
Thank you guys for your feedback! Glad that you seem to like it.

I personally think that most mirror proponents focus to much on performance. That leads beginners to believe, that because they don’t need the performance, they will get away with the increased efficiency. They would say stuff like „I only run 5 VMs“ or „only Plex files“ but don’t understand how big of a role geometry plays or what advantages a dataset could offer to them.

So my main point is, that you choose RAIDZ with all its disadvantages over mirror for storage efficiency but unless you really know what you are doing and have a suiting workload, you won’t even get that!
That to me is the „problem“ with RAIDZ.
That to me is the „problem“ with RAIDZ.
the problem with RAIDz is that it has the bare minimum of fault tolerance, and is one drive fault away from operating without a safety net. Any "advantages" it has are effectively overshadowed.

Parity raid is not good for virtualization payloads for many reasons; raidz2/z3 have use cases for bulk storage.
  • Like
Reactions: Kingneutron
Well, the same could be said for mirror. There is 3 way mirror and RAIDZ2-3 if you want to survive multiple drive failures.

But I agree on that RAIDZ is not good for virtualisation for many reasons and the only advantage of RAIDZ (Storage efficiency) is sometimes even not true.
  • Like
Reactions: leesteken
Well, the same could be said for mirror.
yes, but not to the same extent. a missing member on a raidz vdev in endangering the whole vdev. ANY other failure will take the whole volume down. A missing drive on a striped mirror will endanger only the one vdev in the group, which means ONLY the failure of the surviving partner will take the array down and not any other drive in the array.

the purpose of disk aggregation is not to eliminate risk- this is not possible, but to minimize and allow for mitigation. A striped mirror, for example, will resilver a lot faster than a raidz volume. In a large enough array, you can stripe across multiple parity raid vdevs and get the best of all worlds, but that would require a lot of disks to be effective- and at that point you may be looking to move your failure domain up to the host level and move to ceph anyway.

-edit- I noticed you said it could be said for a single vdev mirror; while true, you dont have any options with only two drives.
Last edited:


The Proxmox community has been around for many years and offers help and support for Proxmox VE, Proxmox Backup Server, and Proxmox Mail Gateway.
We think our community is one of the best thanks to people like you!

Get your subscription!

The Proxmox team works very hard to make sure you are running the best software and getting stable updates and security enhancements, as well as quick enterprise support. Tens of thousands of happy customers have a Proxmox subscription. Get yours easily in our online shop.

Buy now!