Whether you're building a public cloud or a private cloud, one of the most critical capabilities of the system is tracking the usage of the virtual objects that are provisioned by the users or tenants. Usage in OpenStack is tracked largely in the same way it is in Amazon Web Services. For compute resources, the system tracks when a particular instance was started and when it was terminated. The cost of a compute resource is associated with the number of CPU cores or the amount of memory in the flavor associated with the instance. Some organizations may also associate a cost with the image as well.
For example, an instantiation of a Red Hat Enterprise Linux image may cost a certain number of cents per hour, whereas an instantiation of a Microsoft Windows image may cost something different. Other resources may be billed based on when they're provisioned...
