ZFS multi sized drives question

I know this has been asked repeatedly, but it’s hard to find an anwser for my specific setup.

Right now I have bare metal debian, with btrfs raid 5.(I know lol)

I’m moving from an external bay+usff to an upgraded closed system.

I did btrfs originally because I had no matching large drives.
I’ve now got these drives:

I know the pairs are fine, but can I combine them all with truenas scale+zfs raidz1(i think) to get a similar setup to my current? I don’t plan on adding any more drives for a while, and am trying to see my best options.

If so, can they be added after the initial setup? Say I add a 20TB+5TB drive(5tb usable space?), then add the rest after I transfer some stuff?


There is no raidz expansion in TrueNAS yet. It’s predicted to appear in the next major release, Electric Eel, later this year.

RaidZ expansion means you could have three drives in a raidz1, and then add a fourth. But you wouldn’t be able to go from RaidZ1 to RaidZ2

Meanwhile, if you setup a raidz1 with 2 5TB drives and a 18TB, then you’d have 10TB of space (as the 18TB would be treated as a 5TB drive)

If you then replaced the two 5s with two 20s, you’d have the equivalent of a 3way raidz1 with 18TB drives (smallest drive is 18TB) for 36TB of space.

And then if you bought another 5TB drive you could creat a 3 way raidz1 to add another 10TB using your 5TB drives.

OR you could use mirrors.

My personal gut reaction given this mix of drives:

A single pool consisting of:

  • Mirror-0 - 2x 5TB
  • Mirror-1 - 1x 18TB + 1x 20TB
  • Spare - 1x 20TB.

This gives you a total of 23TB of useable space and a hot spare.

There are probably some horrible hybrids with multiple RAID Z1 vDevs but I am not sure that they would actually give you any/much more useable space.

What about a general answer? When using drives of different sizes in the same vdev, the smallest size is applied to all drives. Mixing vdevs of diffrent capacities is no issue.

A single vdev with all your drives would be 5*5 TB. Not good.
2*20 TB + 2*5 TB (mirrors) is possible (25 TB)… but the 20 TB part is somewhat risky due to the large size of the drives.
You could do a 3-way mirror with the larger drives (18 TB capacity, good resiliency), and possibly stripe it with a 2-way mirror of the 5 TB drives (ideally adding a third drive here as well for consistency). 23 TB capacity.

Best advice: Get at least one more 18-20 TB drive, and preferably 3, so you can do a raidz2.

If you really want to use your weird mix of drives, forget ZFS and look into Unraid.


Thank you, that was the most direct response I’ve gotten. Coming from just plain debian+docker for years, all the other options can be a little daunting while researching.

I was wanting to go with truenas, but had a feeling with my current setup it wouldnt be ideal.

I don’t think I’ll be getting any more drives for a while, but I’ll think it over. Thanks again everyone :heart: