Open Stack

Hello stackers, TC, Neutron contributors,

At the Nova mid-cycle meetup last week in Oregon, during the discussion 
about the future of nova-network, the topic of nova-network -> Neutron 
migration came up.

For some reason, I had been clueless about the details of one of the 
items in the gap analysis the TC had requested [1]. Namely, the 5th 
item, about nova-network -> Neutron migration, which is detailed in the 
following specification:

https://review.openstack.org/#/c/101921/12/specs/juno/neutron-migration.rst

The above specification outlines a plan to allow migration of *running* 
instances from an OpenStack deployment using nova-network (both with and 
without multi-host mode) to an OpenStack deployment using Neutron, with 
little to no downtime using live migration techniques and an array of 
post-vm-migrate strategies to wire up the new VIFs to the Neutron ports.

I personally believe that this requirement to support a live migration 
with no downtime of running instances between a nova-network and a 
Neutron deployment *is neither realistic, nor worth the extensive time 
and technical debt needed to make this happen*.

I suggest that it would be better to instead provide good instructions 
for doing cold migration (snapshot VMs in old nova-network deployment, 
store in Swift or something, then launch VM from a snapshot in new 
Neutron deployment) -- which should cover the majority of deployments -- 
and then write some instructions for what to look out for when doing a 
custom migration for environments that simply cannot afford any downtime 
and *really* want to migrate to Neutron. For these deployments, it's 
almost guaranteed that they will need to mangle their existing databases 
and do manual data migration anyway -- like RAX did when moving from 
nova-network to Neutron. The variables are too many to list here, and 
the number of deployments actually *needing* this work seems to me to be 
very limited. Someone suggested Metacloud *might* be the only deployment 
that might meet the needs for a live nova-network -> Neutron migration. 
Metacloud folks, please do respond here!

In short, I don't think the live migration requirement for nova-network 
to Neutron is either realistic or valuable, and suggest relaxing it to 
be good instructions for cold migration of instances from an older 
deployment to a newer deployment. There are other more valuable things 
that Neutron contributors could focus on, IMO -- such as the DVR 
functionality that brings parity to Neutron with nova-network's 
multi-host mode.

Thoughts?

-jay

[1] 
https://wiki.openstack.org/wiki/Governance/TechnicalCommittee/Neutron_Gap_Coverage