Adding additional disks to existing RAIDZ2 VDEV to increase storage space

Darren David

Feb 27, 2014
I've got an 8x10TB RAIDZ2 VDEV, and I just attached 2 new 10TB disks to the system. Both disks show up, and I have the option to add them to the existing pool. This prompts me to create a new VDEV, but I'm told I need at least 4 disks to proceed.

Now that I'm on SCALE instead of CORE (which I have been running for the past several years), I'm curious if I now have the long-hoped-for power to expand the size of the pool and have it resilver across all disks. However, the online documentation assumes I know more about VDEVs than I actually do, so I'd like to better understand what's actually possible here in SCALE before I go out and by two more disks. What I'm looking for is to increase the size of the original pool.


Dec 30, 2020
Short answer: NO.
Vdev expansion is still not available, and ZFS does not have more abilities in SCALE than in CORE anyway.

You indeed need at least four drives, and much preferably eight, to add a further raidz2 vdev to your pool. Or eight much larger drives (say, 18-20 TB) to replace all drives in the current vdev. Other options, including the two drives as a mirror vdev, would leave you with a lopsided pool, decreased reliability and no way out, except backing up and destroying the pool.


Jul 5, 2022
Yeah, it's still the case that expansion should be considered when initially setting up Truenas (really ZFS). An 8 wide Raidz2 is not ideal for this. Unfortunately, as noted, your options are fairly limited. As Etorix mentioned, you'd ideally you'd add 8 more drives.

This is why a mirror setup is often a good choice if you are going to have a high likelihood of expansion. But in larger environments, Raidz2/z3 are fine as more budget!
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Mar 5, 2013
You seem to have a misconception that SCALE is some superior (upgrade) version of TrueNAS. It is not, they just serve different needs and each has its own pros/cons. Regardless, ZFS works (mostly) the same on both variants. Well, ZFS performs better on CORE due to ARC being able to scale much much more aggressively than on SCALE (yeah I know, ironic, isn't it?), but most other things are more or less equal.

I run mirrors really for this reason. Expansion and resilver times. It's much much easier to upgrade/expand a mirror array (2 drives at a time) and the resilver times are way orders of MAGNITUDES shorter, so you see that expanded pool size way quicker also.The performance of the pool while resilvering also won't go to sh*t like it tends to do in RAIDZ.