[openstack-dev] [nova][neutron] Migration from nova-network to Neutron for large production clouds

Tim Bell Tim.Bell at cern.ch
Wed Aug 20 15:17:40 UTC 2014

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.


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