Nova introduced the placement API service in the 14.0.0 Newton release. This is a separate REST API stack and data model used to track resource provider inventories and usages, along with different classes of resources. For example, a resource provider can be a compute node, a shared storage pool, or an IP allocation pool. The placement service tracks the inventory and usage of each provider. For example, an instance created on a compute node may be a consumer of resources such as RAM and CPU from a compute node resource provider, DISK from an external shared storage pool resource provider and IP addresses from an external IP pool resource provider.
The types of resources consumed are tracked as classes. The service provides a set of standard resource classes but provides the ability to define custom resource classes as needed.
The placement-api service must be deployed at some point after you have upgraded to the 14.0.0 Newton release but before you can upgrade to the 15.0.0 Ocata release. This is so that the resource tracker in the nova-compute service can populate resource provider (compute node) inventory and allocation information which will be used by the nova-scheduler service in Ocata.
http://docs.openstack.org/developer/nova/placement.html
It's not entirely clear yet whether this will be a "hard" switch in Ocata, but at the very least it seems it would be prudent to deploy and run the placement stack on Ocata and possibly even stable/newton...
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