I've recently been looking at zfs a bit for my lxd containers and I'm thinking the COW feature would save me a ton of space on my ssd. Currently running 10 or 11 containers in dir mode. My server has a 120gb ssd with all but 2gb for the root partition. 2gb swap I believe.

I'd like to reduce my root partition down to 20gb or so and make a new partition which will ultimately be zfs to hold my containers. is this feasible or even easy to do via live boot / chroot. Or would it be easier to just wipe it out and reinstall properly? I have another machine that can hold my containers for the time being (I love lxd for it's network ability).

You would need to use a live cd as you can't resize a mounted partition. Should be fairly easy with GParted if you have that on a live cd. I don't use containers so I'm not sure what effect that would have.

Yes.
You don't state what filesystem you currently have on your root partition. xfs cannot be resized; ext* can be using a livecd as noted above. However, resizing may change your root partition uuid which would then potentially require changing fstab and grub to accommodate the new uuid - see where I'm going with this. It's certainly doable if you are careful but I think you are better off just doing a reinstall.

This is wrong in both cases. Shrinking an ext4 filesystem won't result in changing its UUID.

For an xfs filesystem you will have to format a new xfs filesystem as xfs cannot be shrunk. However you can easily set the UUID of the new filesystem to the old UUID with

You can easily get the old UUID string from fstab or grub.cfg so you don't have to change those files. It is a 1-2 minute fix to set the UUID to the old one already in fstab and grub.cfg.

Actually I don't. Since your 'problem' doesn't really exist there is no need to idiotically do a unnecessary re-install.

And it should be noted that you can also resize (even shrink) partitions of an active system - including the root.
It's not something I encourage, and if you have to ask how, don't even contemplate it. LiveCD are a godsend for things like this.

Re-install is for when you properly screw things up - which is always a possibility when playing with partitions.

Forgot to mention ext4. I'll keep in mind. Probably do a reinstall. As if I screw it up ill end up doing that anyway.

Better to make the attempt - that way you'll learn something. If you just re-install, next time you'll be asking again.

Shrink?? Really?? Is that something new?

gparted won't let you do it - think non-GUI.

Good point. I'll get into it sunday probably if i do it..