Ceilometer is the telemetry component in OpenStack. While all the other components in OpenStack are busy managing virtual resources, Ceilometer keeps a watchful eye over them and measures the usage of resources, what resources are being used, and how they are being used within the cluster. In this chapter, we are going take a look at what is being measured and how to query the telemetry data. Then, we will use gnuplot to plot some of the data on a graph.