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
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