Prep work for Network aware scheduling

Change how we talk to neutron to allow us in the future to implement network aware scheduling.

Problem description

Some IP subnets can be restricted to a subset of hosts, due to an operators network configuration. In this environment, it means you could build somewhere that has no public IPs available. This is being added into Neutron by the Routed Networks feature.

To make this possible, we need to know the details of all the user’s requested ports, and what resources are required, before asking the scheduler for a host. In addition, after picking a location, we should check that there is an IP available before continuing with the rest of the build process.

As an aside, the allocate_for_instance call currently contains both parts of that operation and has proved very difficult to maintain and evolve. Splitting that code into a separate get/create ports and bind ports phase should help make that code easier to navigate.

Use Cases

This is largely a code refactor.

Proposed change

The first step is to split the nova-compute manager’s allocate_for_instance into two distinct phases: get/create ports and bind ports. Currently we create ports or update ports, depending on if they were passed in. This changes the logic to optionally create a port, then later always update the port with the binding details.

Second, we want to move the get/create ports before the scheduler is called. In terms of upgrades, we need to ensure old compute nodes don’t re-create ports that the scheduler has already created. Similarly, when deleting an instance, the old node should correctly know which ports were created by Nova and can be deleted. For nova-network users, the get/create ports can be a noop.

The third step is to move the port update into the conductor, right after the scheduler has picked an appropriate host. We will not be able to run this code until all compute nodes have been upgraded to the newest version. Until all nodes have been upgraded, the new nodes will still have to run this code on the Compute node. While annoying, this move is only to help with faster retries, and as such, should not block any progress. Note this port update step includes setting the host on the port, and in the future will be the point an IP is assigned, if the port does not yet have an IP.

There are still some open questions around MAC address updates that occur currently during the port creation. This process is only required for ironic based VMs. The current state of this code is breaking our ability to boot ironic instances with ports created outside of Nova. Resolving this issue may delay this effort. DNS updates are not expected to change during this transition, but as this work progresses I expect other issues similar to the MAC address issue to come to light.

For nova-network, we can run the existing allocate-for-instance logic in the conductor, after the scheduler is called. For cells v1 users, this should correctly be in the child cell conductor. For cells v2 users, it doesn’t matter that this is in the api conductor.


We could attempt to add more complexity into the existing allocate_for_instance code. But history has shown that is likely to create many regressions.

Data model impact


REST API impact


Security impact

Eventually it could mean we don’t need any neutron related credentials on any of the compute nodes. This work will not achieve that goal, but it is a step in the right direction.

Notifications impact

Notifications may now have a different host and service, but they should be otherwise identical.

Other end user impact


Performance Impact

Currently the neutron port binding is done in parallel with other long running tasks that the compute node performs during the boot process. This moves the port creation and binding into the critical path of the boot process.

When Nova is creating ports for users, instead of just calling port create with all the parameters, will now first create the port and later update the port. This will slightly increase the load on the Neutron API during the boot process. However this should be minimal, as we are not duplicating any of the expensive parts of the process, such as port binding and IP allocation.

This also generally moves more load into the nova-conductor nodes, but on the upside this reduces the load on the nova-compute nodes.

Other deployer impact

We will need the neutron credentials on nova-conductor, which may not currently have been happening.

Developer impact

Improved ability to understand allocate_for_instance, and its replacements.



Primary assignee:

John Garbutt (IRC: johnthetubaguy)

Work Items

  • Resolve how MAC address allocation will work in this new system

  • Split allocate_for_instance into two functions

  • Move create/get port call into the conductor, before calling the scheduler, such that allocate_for_instance no longer creates ports, no op for nova-net. This is likely to be achieved by adding a new method into the network API for both neutron and nova-net.

  • Move the remainder of allocate_for_instance call into conductor, for both nova-net and neutron




Grenade + neutron should cover this well.

Documentation Impact






Release Name