> From: Michael Still [mailto:mi...@stillhq.com] 
> Sent: 25 August 2014 23:38
> To: OpenStack Development Mailing List (not for usage questions)
> Subject: Re: [openstack-dev] [nova][neutron] Migration from nova-network to 
> Neutron for large production clouds


> Mark McClain and I discussed a possible plan for nova-network to neutron 
> upgrades at the Ops Meetup today, and it seemed generally acceptable. It 
> defines a "cold migration" as
> freezing the ability to create or destroy instances during the upgrade, and 
> then requiring a short network outage for each instance in the cell.
> This is why I'm trying to understand the "no downtime" use case better. Is it 
> literally no downtime, ever? Or is it a more simple "no simultaneous downtime 
> for instances"?
> Michael

The simultaneous downtime across the cloud is the one we really need to avoid. 
Short network outages (depending on how you define short) can be handled along 
with blocking API operations for short periods.

The other item was how to stage the upgrade.. with a cloud of a significant 
size and some concerns about scalability, we would like to be able to do the 
migration as a set of steps rather than a big bang. During the gap between the 
steps, we'd like to open the APIs for usage, such as new VMs get created on 
Neutron hypervisors. Would that be a possibility ?

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