It's not just the size of your PUE, it's what you do with it

Whether opting for colocation or an in-house data centre, enterprises are faced with a dizzying array of statistics and measurements designed to sell them on the efficiency of a particular choice. Power Usage Effectiveness (PUE) is a term often...

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Whether opting for colocation or an in-house data centre, enterprises are faced with a dizzying array of statistics and measurements designed to sell them on the efficiency of a particular choice. Power Usage Effectiveness (PUE) is a term often cited in the industry as the de facto indicator of data centre performance, but the definition has been abused to the point where it is almost unusable.

At its simplest, PUE is the ratio of the energy taken in by a data centre to that actually used by IT - with 1.0 being the impossible ideal. While this can give an initial indication of efficiency, it does not provide the full picture. Instead of focusing on abstract measurements like PUE, organisations should concentrate instead on delivering the best cost for the business.

PUE Pitfalls
A ‘design’ PUE is also not very useful, as rather than giving a realistic view of data centre performance it will generally reflect how a data centre is run in its optimum state. Unsurprisingly, this can be a state which a data centre never achieves in its lifetime, let alone immediately after it opens for business.

The problem is that PUE assumes all IT load is good, and that all IT equipment is of equal efficiency and value. If I have two data centres, one with a good PUE but with IT equipment with no power management and hosting non-critical development platforms and another with a worse PUE but correct power management and host only essential business IT servers, I need to know more to work out which is the most efficient overall.

Similarly, a half full (or half empty) data centre will have a very different PUE to one running at full capacity since efficiency improves with load, yet it won’t state which of those is actually costing the enterprise more. Another counter-intuitive example is if equipment is turned off overnight to reduce costs and emissions - this will deliver a worse PUE since IT load has dropped while the power consumption of the data centre remains constant, meaning managers may receive perverse reports that power saving measures are actually reducing efficiency.

Make metrics work for you
This isn’t to say that PUE is completely worthless - used correctly it can give an at-a-glance impression of data centre efficiency. However, enterprises need to be sure that they are using metrics that will show the true value of their data centres. The first step is establishing the Total Cost of Ownership (TCO) of the data centre. While it is all very well aiming for better efficiency, if this doesn’t improve the profitability of services or reduce overall costs then the whole pursuit is rather pointless.

Secondly, enterprises need to consider what value their data centre brings to the organisation. The way to do this is to use metrics that work in the context of what the organisation is trying to achieve. For example, a company like eBay will be concerned about the cost per transaction made, while other enterprises would like to know the precise cost of each email inbox or other crucial business service. Once enterprises know these costs, they can then aim to optimise them; from which improving efficiency and reducing carbon emissions naturally follow.

Behind any IT investment the ultimate decision lies with the CFO, whose only concern is delivering IT at the lowest cost to their business and avoiding unnecessary investment. Being able to provide the precise, real-word costs for individual IT services, instead of more abstract figures such as PUE, will provide a huge benefit to IT departments pushing for investment (and fending off the challenge of the cloud with its clearly delineated costs).

A measured response
Unless PUE is used correctly and in the context of the cost to the business, it will be a meaningless marketing number. By aiming at delivering the best TCO, efficiencies in power consumption and management will inevitably follow. By understanding the TCO of the entire data centre estate, deciding whether this is in line with the expectations of the business, and knowing what investment (if any) is required, organisations can make the best use of their resources well into the foreseeable future.

Posted by Zahl Limbuwala, CEO, Romonet

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