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Power usage in the data centre

Power usage in the data centre

Jun 1, 2013

Corporate data centres are the focus of considerable investment as their uptime and efficiency are keys to business continuity and profitability, writes Jack Ward, MD of Powermode.

Today’s challenge is to maintain uptime in the face of increasingly uncertain utility power supply reliability characterised by “brown-outs”, rolling black-outs and other forms of crippling power supply interruptions.

Another challenge is to conserve energy, as data centres and the servers that power them are recognised for their high power consumption.

In this light, uninterruptible power supply (UPS) systems and generators have become essential elements of the data centre infrastructure, not only to maintain continuity of operation in times of power outages, but also to help conserve electricity and reduce costly overheads.

In order to conserve power, it first has to be accurately measured and the data centre’s efficiency levels determined. Since 2007, the accepted standard for power usage effectiveness (PuE) is determined by dividing the amount of power entering a data centre by the power used to run the IT infrastructure within it.

What the PuE tells in simple terms is how much extra energy is needed for the IT equipment, in terms of KWh, with due consideration for the power going into cooling, power distribution and other losses.

The PuE metric was developed by a consortium called The Green Grid. PuE is the inverse of data centre infrastructure efficiency (DCiE). The formula is: PuE equals total facility power over IT equipment power.

PuE is expressed as a ratio, with overall efficiency improving as the quotient decreases towards 1. Having a PuE ratio measurement of 1 is the ultimate goal; however, in reality it is only possible to get down to 1,2 or possibly (in rare cases) 1,1.

A PuE of 1 would mean that there was only an IT load and no other power being used within the facility and that there were no system losses. This is not a practical approach for data centres that (alongside promoting energy efficiency) also have to ensure uptime and availability for their customers.

Therefore PuE has to be considered as part of the overall data centre strategy. The primary task of the data centre must be enabling their clients’ businesses.

Data centre PuE can be improved by stripping out levels of resilience. N+1 is more efficient than N+N, for example, but N+1 is not reliable enough for data centres higher up the tier scale.

But how can users make any real improvements if they don’t have an accurate log of true electricity consumption? To make significant improvements in efficiency, power consumption must be tracked and measured in realtime.

Measuring the IT load has not always been straight forward and often data centres have had to invest in expensive DCIM (data centre infrastructure management) software, intelligent power distribution units (PDUs) with simple network management protocol (SNMP) capabilities.

A select number of new-generation UPS systems now incorporate these capabilities and are “PuE-ready”. This means they provides a kWh measurement of their output as standard, allowing data centre managers to see the total energy kWh drawn by their IT load easily and in realtime.

This data can also be logged to enable load profiling and identify utilisation and energy usage over a set period.

Another metric for measuring data centre efficiency is “GreenPuE” or GPuEx – claimed to be an “update” of the PuE definition. In essence, it looks at the power source, giving the “x” a value of between zero and three, with “zero” being linked to 100% (green) renewable power.

For example, the weight for un-scrubbed coal is 1,050 (kg of CO2/KWh) while hydroelectric river generation has a weight of 0,013.

The GPuEx metric is, in essence, a way to “weight” the PuE calculation to identify data centres that are “green” in the sense that they indirectly cause the least amount of CO2 to be emitted by their use of dirty or clean energy.

Truly green data centres will have a GPuEx close to their PuE values, and dirty energy data centres won’t.


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