Using metering for compute resources

You can collect usage data of compute resources using Gnocchi. This time series database processes and stores measurement data of compute resources and provides access to it via REST API or the command-line tool.

Measurements can be sampled from such compute resources as virtual machines, VM disks and interfaces, compute networks, volumes, etc. All resources are being revised: if any attribute of a resource changes, this change is recorded in the history of the resource. For a VM, for example, you can measure the amount of allocated memory and virtual CPUs, as well as the memory and CPU usage.

An entity storing aggregates composed of a timestamp and value is called a metric. A metric is attached to a specific resource and associated to an archive policy. A policy defines how long aggregates are kept in a metric and how they are computed (minimum, maximum, average, etc.).

The following metrics are available for aggregation:

Compute resource metrics

Metric	Type	Resource type	Description
memory	gauge	instance	Amount of RAM allocated to the VM, in megabytes
memory.usage	gauge	instance	Percentage of RAM used by the VM
vcpus	gauge	instance	Number of virtual CPUs allocated to the VM
cpu	cumulative	instance	Amount of CPU time used by the VM, in nanoseconds
disk.device.read.requests	cumulative	instance_disk	Number of read requests
disk.device.write.requests	cumulative	instance_disk	Number of write requests
disk.device.read.bytes	cumulative	instance_disk	Amount of data read, in bytes
disk.device.write.bytes	cumulative	instance_disk	Amount of data written, in bytes
network.incoming.bytes	cumulative	instance_network_interface	Incoming network traffic, in bytes
network.outgoing.bytes	cumulative	instance_network_interface	Outgoing network traffic, in bytes
network.incoming.packets	cumulative	instance_network_interface	Incoming network traffic, in packets
network.outgoing.packets	cumulative	instance_network_interface	Outgoing network traffic, in packets
image.size	gauge	image	Size of the uploaded image, in bytes
volume.size	gauge	volume	Size of the volume, in gigabytes
snapshot.size	gauge	volume	Size of the volume snapshot, in gigabytes
magnum.cluster	gauge	coe_cluster	Number of Container Orchestration Engine (COE), that is, Kubernetes clusters

Cumulative metrics are polled every 5 minutes and increase over time, while gauge metrics are updated on events and show fluctuating values.

For gauge metrics, the default archive policy is ceilometer-low, and for cumulative metrics, it is ceilometer-low-rate. These policies imply that all computed aggregates are kept for one day with 5-minute granularity and for one month with 1-hour granularity. The difference between them is in aggregation methods they use: the ceilometer-low policy stores only mean values, while ceilometer-low-rate stores mean values, as well as the average of delta values per interval.