I have a 4TB drive that is nearing max capacity and would like to move the data to a RAID or ZFS (the host OS is ProxMox 7.1).

I have 3 other identical (WD Blue) 4TB drives (I might be able to get 1 more drive soon, but Amazon has a cap on how many you can purchase right now).

Ideally the ZFS or RAID configuration would afford some level of whole drive redundancy/fault tolerance for convenience.

But here's the (maybe?) tricky part: I'd like to utilize all 4 drives and in the end have ~8TB or more available for storage.

I don't yet know enough about RAID and ZFS to understand the complexities of this, and have read online that RAID 5 is bad, and maybe so is RAID 6, and that ZFS just can't do what I'm asking as the pool will become unbalanced. So I'm wondering if this data dance is even possible without a 5th drive to temporarily store the data. This seems to me like it should be possible (maybe with some partition dancing), but my own searches just left me with more questions and doubts that it can work.

As a bonus, and perhaps a bit of a stretch, I'd love to be able to (easily/simply) further expand the capacity of the array with additional 4TB drives in the future (maybe once every few years).

Is it possible to do this or do I absolutely need to offload all the data to a separate location first?

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    "RAID 5 is bad": That's a generalization that does not hold. "So is RAID 6": let's discuss this when you have a use case for that kind of RAID. Not everything fits your use case, but it doesn't make it "bad". "ZFS just can't do what I'm asking": um, far as I can tell, it does? And, 4×4 TB = 16 TB, not 8 TB, so I think you might be making assumptions on your wanted storage features that you just forget to tell us! Mar 7, 2022 at 20:05
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    @Marcus at larger (multiples of TB), RAID 5 becomes a liability. Even RAID 6 starts to become questionable with 10 or 12 TB disks Mar 7, 2022 at 20:38
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    Possible duplicate - Migrate Single Disk to RAID Mar 7, 2022 at 22:13
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    Finally, btrfs offers similar features to zfs but makes it easier to add and remove drives (of same or even different sizes) and has a great btrfs balance feature to rebalance the data across all the drives. Unfortunately, that's the one thing that btrfs has that zfs doesn't....in pretty much all other ways, zfs is superior. Still, that rebalance feature is very useful in a home/low-budget setting so btrfs is worth considering.
    – cas
    Mar 8, 2022 at 0:24

1 Answer 1


You can do this with ZFS, you can choose from usage of FreeNAS or Solaris and it support your configuration. Proxmox also support ZFS.

If you choose to use RAID-Z (one parity disk) the minimum number of disks is 3 and you can add later more disks. Such array can survive fail of one disk. Similar is RAID5 arrays which you can do using md tool in Linux.

If you need more reliable array you can use RAID-Z2 which ask for minimum of 4 disks and have two parities so it can survive fail of 2 disks. Similar is RAID6.

In ZFS you can grow the array by adding new disk(s). Also you can speedup the array by adding separate caching disks and separate log devices (for example SSD)

  • I can't remember where, but I thought I read that when you add a new drive to RAID-Z it doesn't have a way to rebuild its parity/redundancy (or whatever it may be calls in ZFS). Do you know anything about that? Mar 7, 2022 at 22:56
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    @DavidMurdoch you are correct. resizing in ZFS is a little complicated. A pool is made up of one-or-more vdevs, which each consist of one-or-more drives. The vdevs can be single-drives (no redundancy!), mirrors (two or more drives, similar to RAID-1), or RAID-Z (minimum of 3 vdevs). You can add a vdev to a pool, but you can not remove one. You can always attach an extra drive of same size (or larger) to a single-drive or mirror vdev to increase reduncancy. To increase the size of a vdev (mirror or raidz) you have to replace all of the drives in that vdev with larger drives.
    – cas
    Mar 8, 2022 at 0:01
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    oops. s/minimum of 3 **vdevs**/minimum of 3 **drives**/
    – cas
    Mar 8, 2022 at 0:33
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    I ended up going with a Pool consisting of 2 devs of 2x4TB mirrors. I'll add another 2x4TB mirror in the near future once I get an additional drive. The balance will be off, but it looks I can sort of "rebalance" things by rewriting all data a few times, which should improve read performance on par with how it would have been had I just started with all 3 vdevs in the first place. Mar 11, 2022 at 19:17

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