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Hello Tripleo team,

I have few questios regarding migration from neutron ML2OVS to OVN. Below
are some of the requirements

 - We want to migrate an existing depoyment from Neutroon default ML2OVS to
OVN
 - We are targetting this for tripleo Queen's release.
 - The plan is to first upgrade the tripleo deployment from Pike to Queens
with no changes to neutron. i.e with neutron ML2OVS. Once the upgrade is
done, we want to migrate to OVN.
 - The migration process will stop all the neutron agents, configure
neutron server to load OVN mechanism driver and start OVN services (with no
or very limited datapath downtime).
 - The migration would be handled by an ansible script. We have a PoC
ansible script which can be found here [1]

And the questions are
-  (A broad question) - What is the right way to migrate and switch the
neutron plugin ? Can the stack upgrade handle the migration as well ?
- The migration procedure should be part of tripleo ? or can it be a
standalone ansible script ? (I presume it should be former).
- If it should be part of the tripleo then what would be the command to do
it ? A update stack command with appropriate environment files for OVN ?
- In case the migration can be done  as a standalone script, how to handle
later updates/upgrades since tripleo wouldn't be aware of the migration ?


Request to provide your comments so that we can move in the right direction.

[1] - https://github.com/openstack/networking-ovn/tree/master/migration

Thanks
Numan
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