In Search of an Energy Yardstick

New green-IT metrics help measure data center efficiency, but each has its strengths and weaknesses.

Earlier this fall, Google made an announcement that in many ways foretells the future of data center efficiency metrics. The search giant not only disclosed its total power consumption and carbon emissions (mostly attributable to its data centers), but also released estimates of its per-user and per-search energy consumption. With this information -- and given that a billion Google searches occur per day -- it was possible to calculate that searches account for 12.5 million watts of the company's 260 million watt total.

The idea of quantifying the kilowatts of energy required to perform a useful unit of work is now considered the holy grail of data center metrics. The most widely used metric today -- The Green Grid's Power Usage Effectiveness (PUE) measure -- compares the amount of data center energy consumed by IT equipment to the facility's total usage; but no metric yet reveals energy consumed per unit of work, mostly because it is difficult to define a standard "unit of work."

"For cars, it's miles per gallon, but for hardware, is it a MIP or bits transmitted per second? The industry hasn't zeroed in on what 'work' is," says Steve Press, executive director of data center facilities at Kaiser Permanente. "PUE doesn't tell you what work the data center is doing -- it could be blocking virus traffic or moving data for the National Weather [Service]. Once we settle on [that], it will have a big effect on overall competitiveness."

"If you're a bank or credit card company, your unit of useful IT work is a transaction, whereas if you're a Web company, it's Web pages served or ad click-throughs," says David Ohara, a green data center consultant who runs the Green Data Center Blog. "Ideally, there should be one fairly homogeneous definition."

The Green Grid and other industry groups are working on metrics that address the measurement of productive energy consumption, but issues remain. John Tuccillo, chairman of the board and president at The Green Grid, says, "Once that kilowatt gets to the rack, blade, processor or processor core, how can you then quantify how that unit of energy is being used for the intended purpose? That's the holy grail."

Meanwhile, companies are using combinations of available metrics, as well as coming up with their own metrics, to paint a picture of data center efficiency and productivity. "Overall, the industry struggles to create practical measures that provide the comprehensive insight that steers appropriate investment," says Simon Mingay, an analyst at Gartner.

With all this in mind, here is a compilation of metrics that data centers are using to calculate their energy efficiency.

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