After upgrading LXD from 2.0.x to 3.2, with everything at first looking hunky dory, I discovered after a few days that I couldn't create new containers any more!

$ lxc launch ubuntu:bionic testcontainer
Error: Failed container creation: No root device could be found

WTF!

On another host which received the same upgrade, however, everything still worked fine. After checking if there were any differences in the configuration, I found something missing in the default profile, in the devices section.

On the host where everything worked:

devices:
  eth0:
    name: eth0
    nictype: bridged
    parent: br0
    type: nic
  root:
    path: /
    pool: default
    type: disk

On the one which DIDN'T work:

devices:
  eth0:
    name: eth0
    nictype: bridged
    parent: br0
    type: nic

AHA! The entire 'root' section is gone! Why didn't it get added in the first place? I don't know.

Adding it was easy though, just did "lxc profile edit default" and pasted it in, and everything works fine.

Weird stuff.