Last week I posted the following discussion question in our Computer Room Design networking group at LinkedIn.com. I’m really impressed with the response from group members, so I’d like to share their thoughts with you here:
How can the industry address problems with the reporting of Power Usage Effectiveness (PUE) without undermining the usefulness of the metric?
In a recent post in Data Center Knowledge, Rich Miller points out that the value of Power Usage Effectiveness (PUE) as the leading ‘green data center’ metric “has become fuzzy due to a disconnect between companies’ desire to market their energy efficiency and the industry’s historic caution about disclosure.” [Source: http://www.datacenterknowledge.com/archives/2009/07/13/pue-and-marketing-mischief/]
What are your thoughts on redefining PUE? Are additional refinements the answer? Or does increasing the complexity of PUE undermine the usefulness of the metric?
• Gordon Lane, Facilities Coordinator at Petro Canada, explained:
I don’t see a real value in PUE.
If you leave unused servers powered on you can keep your PUE low.
Assume you have a PUE of 2
2MW total power consumption gives you 1 MW for servers.
If you can reduce your server consumption to 0.75MW by turning off comatose servers total consumption reduces to 1.75MW and gives you a PUE of 2.33
I know there would be some reduction in a/c power usage due to less heat output from the turned off servers but if you are using legacy a/c units with no VFD style control then you will not get a corresponding electrical consumption reduction.
• Scot Heath, Data Center Specialist, weighed in with:
PUE is difficult to measure in mixed facilities, is muddied by configurations such as the Google every-server-has-a-battery and varies widely with Tier level. A universal measurement that combines both IT capability (total Specmarks for example) and availability with respect to energy consumption would be most useful. PUE does have the advantage of being quite easily understood and for controlled comparisons (like tier level, etc.) is very useful.
• Dave Cole, Manager of Data Center Maintenance Management and Education Services at PTS, responded:
Gordon and Scot bring up very good points. I have mixed feelings about PUE. The concept is easily understood – we want to maximize the power that is actually used for IT work. The interpretation of the value is easy to understand – lower is better (or higher is better in the case of DCiE). The problem I see is that it’s almost been made too simplistic. You still have to know your data center and the impact of the decisions you make in regards to design and operation. You can actually raise your PUE by virtualizing or by turning off ghost servers as Gordon pointed out. What needs to be understood is that when you lower the demand side, you should also be making corresponding changes to the supply side. At the end of the day, PUE can be valuable as long as you are also looking at what impacts the value. You need to be able to answer the question of WHY your PUE is changing.
What are your thoughts on the value of Power Usage Effectiveness (PUE) as a metric? Please share your experience by posting a comment here, or by continuing the discussion in the Computer Room Design Group on LinkedIn.