Neil Barton

Tuesday, March 16, 2010

Measuring Data Centre Efficiency

I found a rich seam of practical and clear white papers on measuring Data Centre efficiency. They come from Neil Rasmussen at APC, a supplier of Data Centre cooling and power equipment.

I had noticed the PUE ("Power Utilisation Effectiveness") metric being mentioned more often recently, but Neil describes in detail what is being measured, and how. Essentially it measures how much electrical power is consumed by the power and cooling equipment in a Data Centre, and is therefore not used to power the computing equipment. He argues for an alternative metric, DCiE (Data Centre Infrastructure Efficiency), but this is measuring the same thing in a different way. A follow-on paper by Victor Avelar has some practical insights on how to measure this.

It shows how important the whole issue of Data Centre costs has become. A few years ago, the TCO of a server could be roughly split 30% on hardware, 30% on software, and 30% on labour, with data centre hosting costs lost in the remaining 10% along with other overheads. Three things have changed:
  • As servers get more powerful, they consume more power (and therefore need more cooling, which consumes more power ...)
  • Power itself is getting a lot more expensive
  • Other elements of server TCO are falling (hardware is getting cheaper, server management is being off-shored), making data centre hosting a much larger percentage of server TCO.
More white papers from APC here and here, the latter including a discussion of Data Centre TCO. However, those I have read so far have left me hungry. They describe PUE and DCiE as measuring "Data Centre output" or "useful computing work". However what they actually measure is output of power to the servers, not output of computing from the servers to users. Granted, this is a notoriously difficult thing to measure: after 20 years of debate about MIPS, SPECints, TPC, and SAPS, there is still no generally accepted industry measure of server computing power. Indeed, server power has been so hard to count that some have semi-seriously suggested it would be easier to measure Data Centres by their storage capacity in Terabytes, rather than their computing capacity. Perhaps, even more flippantly, we can expect in the future to measure Data Centre costs in terms of kW consumed per user per day?

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