Open Stack

Hi,

Dnia poniedziałek, 22 sierpnia 2022 22:32:35 CEST Christian Rohmann pisze:
> Hello openstack-discuss and neutron experts,
> 
> I'd like to ask for your input and discussion on the idea of changing 
> the ML2 driver for an existing cloud, read: changing a tire while still 
> riding the bike.
> 
> I actually like to find out if it's feasible to switch from the trusted 
> Linuxbridge driver to the more modern SDN stack of OVN - in place.
> With all existing user networks, subnets, security groups and (running) 
> instances in the database already.  And while I know there a push of 
> migrating
> from OVS to OVN and a clear migration path documented 
> (https://docs.openstack.org/neutron/latest/ovn/migration.html), but they 
> are much more similar in their data plane.
> 
> And to get this out of the way: I am not asking to do be able to do this 
> without downtime, interruptions, migrations or any active orchestration 
> of the process.
> I just want to know all the possible options apart from setting up a new 
> cloud and asking folks to migrate all of their things over...
> 
> 1) Are the data models of the user managed resources abstract (enough) 
> from the ML2 used?
> So would the composition of a router, a network, some subnets, a few 
> security group and a few instances in a project just result in a 
> different instantiation of packet handling components,
> but be otherwise transparent to the user?

Yes, data models are the same so all networks, routers, subnets will be the same but implemented differently by different backend.
The only significant difference may be network types as OVN works mostly with Geneve tunnel networks and with LB backend You are using VXLAN IIUC your email.

> 
> 2) What could be possible migration strategies?
> 
> While it might be a little early to think about the actual migration 
> steps. But just to consider more than a full cloud shutdown following a 
> cold start with modified neutron config then using the OVN ML2.
> I know there is more than just the network and getting a virtual layer 2 
> network. There are DHCP, DNS and the metadata service and last but not 
> least the gateway / router and the security groups.
> But if using VXLAN for OVN (as Linuxbridge uses currently) could the 
> shift from one implementation to the other potentially be done node by 
> node? Or project by project by changing the network agents over
> to nodes already running OVN?

Even if You will keep vxlan networks with OVN backend (support is kind of limited really) You will not be able to have tunnels established between nodes with different backends so there will be no connectivity between VMs on hosts with different backends.

> 
> If not, is there any other hot cut-over approach that would avoid having 
> to shutdown all the instances, but only cause them some network downtime 
> (until the next DHCP renew or similar?)

TBH I don't know about any way to do that. We never tried and tested migration from LB to OVN backend. The only currently supported migration is from ML2/OVS to ML2/OVN backend and it depends on the Tripleo framework.

> 
> 
> Has anybody ever done something similar or heard about this being done 
> anywhere?

I don't know about anyone who did that but if there is someone, I would be happy to hear about how it was done and how it went :)

> 
> 
> 
> Thanks for your time and input,
> 
> 
> Christian
> 
> 
> 
> 


-- 
Slawek Kaplonski
Principal Software Engineer
Red Hat
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