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:


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.



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

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