AFCOM conference, a question of Power Usage Effectiveness
I recently spoke at AFCOM, presenting with Keith Gislason and Jeff Sturgeon from Emerson. We gave what AFCOM calls a “virtual tour” of the Emerson St. Louis Data Center. While at AFCOM, Keith, myself and others had a spirited discussion on the usefullness of PUE as a measure of energy efficiency. I’ll admit I was mostly listening.
First, some background. PUE is the ratio of total amount of power used by the data center divided by the power used by the servers. In a perfect world, all the power (in this case, electricity) used at the data center would be used to run servers. Unfortunately, when the servers use electricity, they also generate copious amounts of heat. This heat must be dissipated with equal amounts of cooling. There is also power lost to the UPS system – no UPS is 100% efficient. Finally, there’s power used for lighting, convenience outlets, building envelope heat gain etc. A green data center tries to minimize these loads, and works to get the most effective computing equipment possible, by doing things like virtualizing servers and using more efficient equipment.
The Emerson Data Center has been ranging around a PUE of 2. That’s not bad but certainly not as low as a lot of other energy efficient data centers. This number will drop as the center adds more servers – there’s still a lot of migration even a year after the building opened. However, as Keith likes to state, PUE has to be understood in the context of the Owner’s Project Requirements. Different criteria will greatly affect the PUE. There’s no standard for size of the facility, redundancy or work being completed.
Steve Hassell, President of Avocent and former Emerson CIO, has a great blog article talking about, among other things, PUE and the Emerson St.Louis Data Center. Redundancy (substitute “reliability, safety”) has a big effect on PUE. From his recent post on the “Efficient Data Center” blog:
Increasing the infrastructure safety margin is one way to address this problem, but it has the impact of decreasing utilization. Clearly, a more dynamic physical infrastructure is necessary to solve the problem of increasing agility while simultaneously increasing utilization/efficiency and preserving availability.
As the debate over PUE rages on, we’ll continue to focus on the big 4 in data center design – energy efficiency, reliability, maintainability and flexibility.