Michael has been posting very informative blogs on the summary of the mid-cycle 
meetups for Nova. The one on the Nova Network to Neutron migration was of 
particular interest to me as it raises a number of potential impacts for the 
CERN production cloud. The blog itself is at 
http://www.stillhq.com/openstack/juno/000014.html

I would welcome suggestions from the community on the approach to take and 
areas that the nova/neutron team could review to limit the impact on the cloud 
users.

For some background, CERN has been running nova-network in flat DHCP mode since 
our first Diablo deployment. We moved to production for our users in July last 
year and are currently supporting around 70,000 cores, 6 cells, 100s of 
projects and thousands of VMs. Upgrades generally involve disabling the API 
layer while allowing running VMs to carry on without disruption. Within the 
time scale of the migration to Neutron (M release at the latest), these numbers 
are expected to double.

For us, the concerns we have with the 'cold' approach would be on the user 
impact and operational risk of such a change. Specifically,


1.      A big bang approach of shutting down the cloud, upgrade and the 
resuming the cloud would cause significant user disruption

2.      The risks involved with a cloud of this size and the open source 
network drivers would be difficult to mitigate through testing and could lead 
to site wide downtime

3.      Rebooting VMs may be possible to schedule in batches but would need to 
be staggered to keep availability levels

Note, we are not looking to use Neutron features initially, just to find a 
functional equivalent of the flat DHCP network.

We would appreciate suggestions on how we could achieve a smooth migration for 
the simple flat DHCP models.

Tim

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