Open Stack

Hi,

The Neutron team wants to propose a switch of the default Neutron backend in
Devstack from OVS (neutron-ovs-agent, neutron-dhcp-agent, neutron-l3-agent) to
OVN with its own ovn-metadata-agent and ovn-controller.
We discussed that change during the virtual PTG - see [1].
In this document we want to explain reasons why we want to do that change.


OVN in 75 Words
---------------

Open Virtual Network is managed under the OVS project, and was created by the
original authors of OVS. It is an attempt to re-do the ML2/OVS control plane,
using lessons learned throughout the years. It is intended to be used in
projects such as OpenStack and Kubernetes. OVN has a different architecture,
moving us away from Python agents communicating with the Neutron API service
via RabbitMQ to C daemons communicating via OpenFlow and OVSDB.

Here’s a heap of information about OpenStack’s integration of OVN:
* OpenStack Boston Summit talk on OVN [2]
* Upstream OpenStack networking-ovn documentation [3] and [4]
* OSP 13 OVN documentation, including how to install it using Director [5]

Neutron OVN driver was developed as a Neutron stadium project,
"networking-ovn". In the Ussuri cycle, networking-ovn was merged into the main
Neutron repository.


Why?
----

In the Neutron team we believe that OVN and the Neutron OVN driver are built
with a modern architecture that offers better foundations for a simpler and
more performant solution. We see increased participation in kubernetes-ovn,
resulting in a larger core OVN community, and we would like OpenStack to
benefit from this Kubernetes driven OVN investment.
Neutron OVN driver currently has got some feature parity gaps comparing to
ML2/OVS (see [6] for details) but our team is working hard to close those gaps
and we believe that this driver is the future for Neutron and that’s why we
want to make it the default Neutron ML2 backend in the Devstack configuration.


What Does it Mean?
------------------

Since most Openstack projects use Neutron in their CI and gate jobs, this
change has the potential for a large impact.
But this backend is already tested with various jobs in the Neutron CI and it
works fine. Recently (See [7]) we also proposed to add an OVN based job to the
Devstack’s check queue.
Similarly the default Neutron backend in TripleO was changed in the Stein cycle
and there were no any significant issues related strictly to this change. It
worked well for other projects.
Of course in the Neutron project we will be still gating other drivers, like
ML2/Linuxbridge and ML2/OVS - nothing will change here, except for the names of
some of the jobs.
The Neutron team is *NOT* going to deprecate any of the other existing ML2
drivers. We will be still maintaining Linuxbridge, OVS and other in-tree
drivers in the same way as it is now.


Action Plan
-----------

We want to make this change before the Victoria-2 milestone to not make such
changes too late in the release cycle. Our action plan is as below:

1. Share the plan and get feedback from the upstream community (this thread)
2. Move OVN related Devstack code from a plugin defined in the Neutron repo to
   Devstack repo - we don’t want to force everyone else to add “enable_plugin
   neutron” in their local.conf file to use default Neutron backend,
3. Switch default Neutron backend in Devstack to be OVN,
   a. Switch definition of base devstack CI jobs that it will run Neutron with
      OVN backend,
4. Propose DNM patches depend on patch from point 3 and 3a to main OpenStack
   projects to check if it will not break anything in the gate of those projects.
5. If all will be running fine, merge patches proposed in points 3 and 3a.

[1] https://etherpad.opendev.org/p/neutron-victoria-ptg - Lines 185 - 193
[2] https://www.youtube.com/watch?v=sgc7myiX6ts
[3] https://docs.openstack.org/neutron/latest/admin/ovn/index.html
[4] https://docs.openstack.org/neutron/latest/ovn/index.html
[5] https://access.redhat.com/documentation/en-us/red_hat_openstack_platform/13/html/networking_with_open_virtual_network/
[6] https://docs.openstack.org/neutron/latest/ovn/gaps.html
[7] https://review.opendev.org/#/c/736021/

-- 
Slawek Kaplonski
Senior software engineer
Red Hat