The Character of PUE

Technological Maturity

The consistent change and advancement in the technologies that a data centre depends upon also has a bearing on PUE.

As discussed in ‘Occupancy’, a data centre’s design can significantly affect its ability to benefit from newer, more efficient technologies. Therefore the maturity of a data centre's installed technology will have a significant impact on PUE and how much it can be enhanced.

In a data centre of traditional design it is both difficult and costly to implement new infrastructure technologies, although the adoption of some technologies and practices, such as hot/cold aisle containment and blanking plates, can reap quick rewards. However, there is a limit to the level of investment that should be considered before it makes more sense to refit, or rebuild, the data centre.

In such a case, the potential of lowering the PUE may be extremely limited and occasionally the upgrades may even cause the PUE to rise. Investment in data centre infrastructure has traditionally been addressed long-term, and it is not unusual to amortise infrastructure costs across decades. Consequently, as the data centre must take its fiscal status into account as it evolves, assessment is becoming more important in order to understand how to maximise the facility’s energy efficiency.

Conversely the change cycle associated with IT Equipment is rarely more than five years. This reflects the pace of change for server, storage and networking technologies, their ease of deployment and the fiscal and business benefits they accrue. Businesses are therefore able to, and invariably do, take advantage of these technology advances at a much faster rate than they can enhance their data centre’s capability to host the equipment and their newer energy and cooling challenges.

For these reasons it is important to understand how any technological enhancement may affect PUE. Furthermore, as it is becoming a more important and visible management metric it is necessary to ensure that the effect of any change is monitored and expectations are correctly set.

To demonstrate, let's consider two data centres of comparable occupancy and workload. We could conclude that their PUE would be similar, but without taking into account the maturity of both the infrastructure and IT equipment technology deployed, we cannot be certain.

Both data centres have a traditional data centre design, but one has implemented virtualisation and de-duplication technologies, while the other one still operates old legacy systems. In this situation it is quite feasible for the data centre with the modern, and green, IT equipment technologies to have a higher PUE than the data centre running on older, outdated equipment.

This is because the relationship between ITEP and TFP has changed. In our modern data centre the newer equipment has a reduced ITEP value. However the traditional design of the data centre limits the potential for drastically reducing the TFP. Therefore as we divide the static TFP value by a lower ITEP value, the PUE will increase.

The converse is also true. If HVAC and power delivery infrastructure (M&E) components were upgraded, then the PUE may substantially reduce as the TFP is likely be a lower number. However, through increases in workload, the data centre could still consume the same amount of energy.

Other aspects of technological maturity, such as how often the TFP and ITEP metrics are taken and, operational procedures and processes can also affect PUE in similar ways.

Variations of this nature could prove extremely embarrassing if the business case for IT investment was based on reducing the PUE. It is also paradoxical when considering the adoption of the metric by government agencies world-wide as a measure of energy efficiency.

As a result, businesses are confronted with the question: ‘What upgrade/s will give me the best ROI?’. The answer to this question is not always obvious and a considered programme of change based on regular assessment of your energy efficiency status is necessary to avoid poor investment returns.