Before we start exploring Ceilometer, it is important to know that, by default, Ceilometer uses MongoDB to store all of its telemetry data. This data store can grow very rapidly and can use excess space. It is in your interest to keep Mongo's data store separate from the root partition of your control node so that the telemetry data does not fill up your control node's root disk. OpenStack has a horrible time functioning without a disk to write to. Mongo's data store is /var/lib/mongodb/ by default. A simple way to be sure that the node's root disk doesn't fill up would be to mount another partition, logical volume, or some other external storage to /var/lib/mongodb/. If there isn't any important data in Ceilometer, you can even stop the MongoDB service, delete the contents of the data store directory, mount the new storage, ensure the ownership is correct, and restart the central and collector services of both the MongoDB and Ceilometer APIs. The files that...