Time to Make Peace with PUE?

Arthur Cole

It looks as if the Power Usage Effectiveness (PUE) benchmark has become the de facto standard for gauging data center efficiency, which is surprising, considering so many experts say it provides an incomplete picture of energy use.

It is well-known that by simply establishing a ratio of overall data center power consumption to that which is going directly to data services, the PUE is open to easy manipulation. In general, the standard tends to give more weight to lowering overall consumption, primarily by reducing the cooling load, than to making more efficient use of available resources through virtualization and consolidation.

According to Intel's Winston Saunders, this is primarily due to PUE being a "system-level indicator" that is only truly effective under optimal conditions. But in many cases, increasing system optimization will also increase your PUE rating, making it seem as if IT resources are less efficient than they actually are. A more accurate measure would be one that accounts for data loads and other indicators of actual productivity, giving IT managers a better handle on bottom-line energy costs.

The problem with that is there is no effective way to measure "productivity," according to eWEEK's Andew Donoghue. Such a system would require so many metrics and so many variables that any results would be open to a wide range of interpretations. In the end, we'd be no closer to a comparable measurement framework because organizations would be able to put just about any spin they chose on the test results. Still, work continues to find an equitable solution, with groups ranging from the British Computer Society to The Green Grid itself looking to expand on PUE's limited range.

Let's hope they come up with something soon because now that the EPA has decided to base its data center Energy Star ratings on PUE, it has enormous power to direct the investment patterns of the IT industry. And as a new set of calculations on Vertatique show, PUE can, in fact, rise even though energy consumption goes down. It all depends on how you implement the changes. It would be a shame if some truly revolutionary technologies were to fall by the wayside simply because the government is picking winners and losers based on a flawed methodology.

But perhaps PUE's only flaw is that it is being implemented incorrectly. A company called Trendpoint Systems is hoping to drill the measurement down to individual cooling units. This kind of "Micro PUE," the company argues, would concentrate the metric onto what it is best at: measuring the efficiency of cooling equipment. Note, however, that Trendpoint specializes in energy-management technology and would benefit handsomely under a standardized measurement regime.

However PUE evolves over the next few years, one thing is clear: Energy efficiency is a lot more complicated than it seems. But the fact that there are no easy answers does not mean green IT is not worth the effort. And if all else fails, you can always gauge progress through your monthly energy bill.

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Sep 9, 2010 10:31 AM Gary Anderson Gary Anderson  says:

While PUE may not be ideal, it's what is commonly available and easily implementable right now.  There are two main issues that cause the heartache in the industry.  First, PUE is an inaccurate way to compare data centers.  A data center in Phoenix is not going to have the same PUE as an identical data center in Buffalo, let alone a data center with different cooling technologies, power distribution schemes or usage loads.  The second issue is how data centers use their IT equipment.  If the servers aren't doing productive work, they are just wasting energy.  Emerson Network Power has proposed a measure called CUPS (http://bit.ly/a2M58W) which addresses this and provides a much better measure of energy use and productive work.

In the meantime, companies seeking to reduce energy usage should focus on the meter at the entrance to the building and drive down what they can through the Energy Logic approach (http://bit.ly/mJwFX).  Implementation of data center best practices will really deliver the savings if they are implemented properly.

Sep 13, 2010 11:03 AM Vertatique Vertatique  says:

Thanks a good survey of the issues surrounding PUE and for noting our recent PUE calculations.  Since the article's formatting offsets the link to our work from the text about it, here it is directly: http://vertatique.com/gaming-pue-cooling-green-data-center-investment

Your readers may also be interested in our simulations of ENERGY STAR fro Data Centers using the EPA's Portfolio Manager:


Dec 8, 2010 7:52 AM Winston Saunders Winston Saunders  says:

It's not that PUE is bad. It's a great indicator. NO single indicator can create the whole picture. Just as you can't judge a company just by profit margin alone (you also need to look at revenue and Cost of Sales, for instance), you need to look at several metrics to get the whole picture for a data center.


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