Remove a port to the external bridge with openvswitch

Question

I was setting up a openstack configuration using this document: http://docs.openstack.org/juno/install-guide/install/apt/openstack-install-guide-apt-juno.pdf and got to page 72 where it says to do this:

# ovs-vsctl add-port br-ex INTERFACE_NAME

but i put the wrong INTERFACE_NAME, and now i have to correct that mistake (it is a real interface, but the wrong one, i was supposed to put another one in there).

But i have trouble undoing that.

I tried:

# ovs-vsctl del-port br-ex INTERFACE_NAME

but it tells me that /etc/openvswitch/conf.db (or something like that) is read-only

i then tried

# ovs-vsctl del-br br-ex

Bu then it says that you cant just delete a port, you need to delete the entire bridge (or something like that). Which is wierd to me, i thought that that command would delete the bridge...

So does anyone know the correct way to delete the port i mistakenly made?

EDIT: And i tried all that as root.

EDIT2: I just tried on a practice machine do the same thing, make the same error and then fix it with:

# ovs-vsctl del-port br-ex INTERFACE_NAME

and it worked, no read-only nonesense, so i really dont get it. Any suggestions?