Using ports with resource request

Starting from microversion 2.72 nova supports creating servers with neutron ports having resource request visible as a admin-only port attribute resource_request. For example a neutron port has resource request if it has a QoS minimum bandwidth rule attached.

The Quality of Service (QoS): Guaranteed Bandwidth document describes how to configure neutron to use this feature.

Resource allocation

Nova collects and combines the resource request from each port in a boot request and sends one allocation candidate request to placement during scheduling so placement will make sure that the resource request of the ports are fulfilled. At the end of the scheduling nova allocates one candidate in placement. Therefore the requested resources for each port from a single boot request will be allocated under the server’s allocation in placement.

Resource Group policy

Nova represents the resource request of each neutron port as a separate Granular Resource Request group when querying placement for allocation candidates. When a server create request includes more than one port with resource requests then more than one group will be used in the allocation candidate query. In this case placement requires to define the group_policy. Today it is only possible via the group_policy key of the flavor extra_spec. The possible values are isolate and none.

When the policy is set to isolate then each request group and therefore the resource request of each neutron port will be fulfilled from separate resource providers. In case of neutron ports with vnic_type=direct or vnic_type=macvtap this means that each port will use a virtual function from different physical functions.

When the policy is set to none then the resource request of the neutron ports can be fulfilled from overlapping resource providers. In case of neutron ports with vnic_type=direct or vnic_type=macvtap this means the ports may use virtual functions from the same physical function.

For neutron ports with vnic_type=normal the group policy defines the collocation policy on OVS bridge level so group_policy=none is a reasonable default value in this case.

If the group_policy is missing from the flavor then the server create request will fail with ‘No valid host was found’ and a warning describing the missing policy will be logged.

Virt driver support

Supporting neutron ports with vnic_type=direct or vnic_type=macvtap depends on the capability of the virt driver. For the supported virt drivers see the Support matrix

If the virt driver on the compute host does not support the needed capability then the PCI claim will fail on the host and re-schedule will be triggered. It is suggested not to configure bandwidth inventory in the neutron agents on these compute hosts to avoid unnecessary reschedule.

Extended resource request

It is expected that neutron 20.0.0 (Yoga) will implement an extended resource request format via the the port-resource-request-groups neutron API extension. As of nova 24.0.0 (Xena), nova already supports this extension if every nova-compute service is upgraded to Xena version and the upgrade_levels.compute configuration does not prevent the computes from using the latest RPC version.

The extended resource request allows a single Neutron port to request resources in more than one request groups. This also means that using just one port in a server create request would require a group policy to be provided in the flavor. Today the only case when a single port generates more than one request groups is when that port has QoS policy with both minimum bandwidth and minimum packet rate rules. Due to the placement resource model of these features in this case the two request groups will always be fulfilled from separate resource providers and therefore neither the group_policy=none nor the group_policy=isolate flavor extra specs will result in any additional restriction on the placement of the resources. In the multi port case the Resource Group policy section above still applies.