Complicated things, datacentres, with lots and lots of hardware boxes and software applications and network links and power supply units and battery backups, etc. Yet at the end of the day you want it to do work and there is one input power supply. So measure the efficiency with which the latter is turned into the former.

That is the principle behind the The Green Grid's Power Usage Effectiveness (PUE) metric.

It is defined as a fraction; total datacentre power divided by IT equipment power. The total datacentre power is the electrical power dedicated only to the datacentre, which includes everything that supports the IT equipment, as well as the IT equipment itself.

IT equipment includes:-

- server computers,
- storage equipment such as disk drive arrays and tape libraries,
- Networking routers and swiches,
- keyboard and mice (KVM) and screen and notebook computer used for monitoring and managing the IT equipment.

The remaining powered kit in the datacentre is everything else; items used to enable the IT equipment to be operated:-

- power supply components such as cables and individual power supply units (PSU), generators, switching boxes, batteries, uninterruptible power supplies (UPS),
- Cooling devices such as Computer room Air Conditioning (CRAC), chillers, pumps, cooling towers and direct expansion air handler (DX) boxes and any water cooling equipment,
- Datacentre lighting.

The Green Grid has also defined a DCE, a DataCentre Efficiency metric. This is the reciprocal of the PUE and is the total IT equipment power divided by the total datacentre power, or one divided by the PUE value.

A longer term aim is to produce a DCPE metric, a Datacentre Performance Efficiency index, one calculated by dividing total data centre power into the number representing the useful work done by the datacentre. Well, there is no definition of 'useful work' done by a datacentre and no obvious way of measuring such a construct.

Another possible measure is to split the PUE into a cooling load factor (CLF) and a power load factor (PLF).

Two windows

The PUE and DCE are different windows through which to look at datacentre power usage.

Let's assume a PUE of 3.0 and realise that it indicates the datacentre needs three times more power to operate than the IT equipment inside it needs.
We can look at what that means at a device level. A storage array taking in 500 watts means this datacentre needs to have 1500 watts delivered so that it can get 500 watts to the storage array.

Clearly the lower the PUE value the more efficient the datacentre infrastructure is at taking electrical power into the datacentre and delivering it to IT equipment.

The closer a PUE value is to one (the theoretical ideal) the greater proportion of power entering the datacentre gets to the IT boxes. It is hoped that a PUE value of 2.0 might be generally achievable with better values possible in some cases. Measurements by Lawrence Berkeley National Laboratories of 22 datacentres revealed a spread of PUE values from 1.3 to 3.0.

The DCE number can be used to view the situation from another angle. A DCE value of 0.5 means that the IT equipment is using 50 percent of the power coming into the datacentre. The larger this number the less power is consumed by support (non-IT) equipment.

To calculate PUE and DCE values means knowing how much power enters the datacentre, a meter at the door so to speak, and how much is used by each device in it. To monitor usage patterns you also need meters, instrumentation, on kit showing power usage. A single power supply unit for all IT equipment power needs would be a good measurement point.

An advantage of the PUE and DCE numbers is that they enable different datacentres to be compared along the same spectrum with green goodness at one end and sinning at the other; sinning being the wastage of power. There is no baseline set of PUE and DCE values available yet to indicate today's datacentre efficiencies, and that allow us to say what is definitely a poor datacentre and what value indicates a good one.

If the Green Grid has its way then they will come. The harder value, the DPCE, will take a lot longer to appear than the PUE and DCE metrics. Those should provide a useful insight into power usage and a practical way to measuure the effectiveness of changes.