By “existing” I meant that the root device exists in the instance’s config (rather than the profile config applied to this instance).
You can see an instance’s config (without applying the profile config) by running:
lxc config show <instance>
And with profile config applied:
lxc config show <instance> --expanded
If the root device wasn’t existing in the non-expanded instance config then you would not be able to modify it using
set, as it doesn’t exist yet.
lxc config device add is fine to use, the
override option is just a shorthand that will effectively do an
add using the existing options from the profile and then overriding the specific key(s) you specify.
The end result is that the instance has a root device in its own config.
I’m not sure what you mean by quota in the context of VMs. They use fixed size volumes so the VMs cannot grow beyond that size (effectively they all have a fixed quota of their VM root size).