Power Usage Effectiveness (PUE) is the current metric for measuring data-center efficiency. Some debate its usefulness, others swear by it. Until something better comes along that all parties agree to, data-center managers will continue to base their operational efficiency on PUE.
It's interesting when operators live stream their data center's PUE, because it can be a double-edged sword. If the displayed metric is not considered acceptable, the entire world knows that something or someone messed up. The flip side is a data-center operations team can feel good about their facility if the PUE is close to the Holy Grail number of 1.0.
Several major data-center operations track and publish their PUE online, including Facebook's Prineville data center shown above. Averaging a PUE of less than 1.1 is a remarkable feat considering the Prineville data center encompasses more than 300,000 sq ft.
Something else remarkable
It is one thing for major operations like Facebook, Apple, or Google to maintain a low PUE — they get to tweak their custom-built computing equipment, squeezing out every drop of efficiency they can. Not so for smaller operations — in particular, colocation data centers (colo) where operators rent space to customers who bring their own commodity-brand servers and ancillary equipment. With all the device variety, is it even possible for colos to achieve a respectable PUE?
It just so happens that running a low PUE is possible. In fact, there's a data center in Kent, U.K. that live streams the colo's PUE to prove it.
Custodian Data Centre
The company I'm referring to is Custodian Data Centre. Its Kent facility maintains a PUE of 1.2, a number I did not think was possible. To find out how the company manages it, I contacted Robert Williams, technical director for the Kent data center. I started out by asking why the company decided to live stream the data center's PUE.
Williams said it just made sense from a technical and commercial standpoint. "The live PUE feed is viewed by our technical team as a working tool, and they didn't want to compromise its integrity," added Williams. "The live feed enables us to demonstrate how good our PUE is, without hiding behind annualized or theoretical figures."
I then asked Williams how Custodian technicians use the real-time PUE as a tool. "With the PUE being calculated every minute, we can track everything in real time," explained Williams. "And the technicians can determine if their efforts improved the situation or made it worse."
I was curious as to the extremes recorded by the PUE system at Custodian's Kent data center. Williams mentioned the data center's running low of 1.14 occurs during the winter, and the high reading of 1.7 occurs when the techs force all systems to pull their full design power during backup generator load testing.
Williams (picture at left) talked about the system accuracy. "The system is extremely accurate," said Williams. "We know that because when testing the data center's UPS [uninterruptable power supply], the PUE is less than 1.0."
At first, I did not see the significance, but Williams explained, "Having the UPS online, if only for a few minutes, reduces the grid load and the PUE reacts to the change, demonstrating the system is metering correctly and accurately."
Method of recording PUE
I asked Williams how Custodian determined PUE. He answered, "To obtain the 'IT-only load,' we put in-house metering devices on every distribution supply point and outlet within the data center, including all rack feeds. We measure the incoming 3-phase supplies from the various 11kV transformers and subtract the IT-only load from that value. This provides the raw PUE on the company website.
"In addition, we monitor other systems such as generator heaters, lighting, fan motors, and security systems. That allows the analysis of losses across the distribution system (total incoming supply minus total metered load), providing information about the power usage of every system on our electrical grid."
Williams offered this example, "The system enabled technicians to 'tune' the UPS room temperature to where the air-conditioning system provides the required cooling and no more, which saves energy."
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