Need Help Understanding ZFS Expansion

[Jath]

Hello,

In a few months, I am looking at making a DIY NAS. I don’t plan on buying all the HDDs up front (considering the prices lately…), and was planning on going with UNRAID. I have since learned that TrueNAS SCALE either has or is planning to release the ability to expand RAIDZ storage pools.

I am very interesting in utilizing TrueNAS, but want to make sure this is something I can do.

I did a quick test with VMware Player, and I saw that under Storage in 25.04, there’s an “expand” option, but I am unsure if it does exactly what I’m looking for - adding a drive to expand capacity as necessary. However, the “expand” options I’m seeing talk about “rewriting” the data to get the full capacity.

Is the option to expand a ZFS pool now a thing where I can add drives as needed? I was planning on buying four upfront, and then adding two more as time went on, that way I could set it up with RAIDZ2 up-front, and take time to get money for the other two drives.

If anything, I may just wait an extra month or so and just buy all six drives at once.

Thank you.

[LarsR]

@Jath

Is the option to expand a ZFS pool now a thing where I can add drives as needed?

Yes, it’s been released since 24.10.
The only storage layouts that can be expanded are raidzx (x= 1,2 or 3) but not mirrors
But there is still a drawback to raidz expansion which is discussed at length here

24.10 RC2 Raidz expansion caused miscalculated available storage

I have a server with 6x18tb (16.37TiB) drives in RAIDz2, and due to hardware limitations, needed to start with 4 drives and expand later. Using 24.10 RC2, I extended my vdev twice, and during the second vdev expansion, I rebooted the system (as zfs documentation said it was safe to reboot during expansion, and I had a need to reboot). After all expansion was completed, it seems like the usable capacity value wasn’t updated to properly reflect the new capacity. [image] I did try the zfs rebal…

TLDR (as i understood it) the data written after expansion gets written with a different parity ratio then the old data was written with, which leads to a falsly displayed available space.