The Power Usage Effectiveness (PUE) metric has become the de facto standard for measuring data center energy efficiency. PUE compares the total power going into a data center with the amount of power used to power IT equipment (servers, storage, and network). There is increasing pressure being exerted on data center managers to take measures to reduce the PUE. Unfortunately, the proper usage of PUE is often misunderstood and, by focusing solely on this single metric, it may mean data center managers are missing out on other opportunities to affect sustained reductions in energy use.
The Power Usage Effectiveness (PUE) metric was introduced by the Green Grid, an association of IT professionals focused on increasing the energy efficiency of data centers. In the white paper Green Grid Data Center Power Efficiency Metrics: PUE and DCiE (Belady, Rawson, Pfleuger, & Cader, 2007), the authors lay out the case for the introduction of metrics to measure energy efficiency in the data center.
The Green Grid believes that several metrics can help IT organizations better understand and improve the energy efficiency of their existing datacenters, as well as help them make smarter decisions on new datacenter deployments. In addition, these metrics provide a dependable way to measure their results against comparable IT organizations.
There is a great deal of truth in the adage "You can't manage what you can't measure". In order to manage energy efficiency in the data center, it is imperative to have metrics in place to measure the impact of changes. There were two primary metrics introduced, PUE and DCE (Data Center Efficiency). The latter was later changed to DCiE (Data Center Infrastructure Efficiency). Both metrics measure the same two parameters, the total power into the data center and the IT equipment power.
While both metrics had their supporters, PUE became the standard metric.
A PUE value of 1 would represent the optimal data center efficiency. In practical terms, a PUE value of 1 means that all power going into the data center is being used to power IT equipment. Anything above a value of 1 means there is data center overhead required to support the IT load.
What components make up this overhead? Let's look at where the power going into a data center is consumed.
Ideally, we would like all power entering the data center to be used to power the IT load (servers, storage and network). This would result in a PUE value of 1. Realistically, however, some of this power must be diverted to support cooling, lighting and other support infrastructure. Some of the remaining power is consumed due to losses in the power system. The remaining power then goes to service the IT load.
Let's look at an example to see how PUE is calculated. If the power entering the data center (measured at the utility meter) is 100 kW and the power consumed by the IT load (measured at the output of the UPS) is 50 kW, we would calculate PUE as follows:
A PUE value of 2.0 is fairly typical for a data center. This means that for every watt required to power a server, we actually consume 2 watts of power. It is important to remember that we are paying for the power entering the data center, so every watt of overhead represents an additional cost. Reducing this overhead will reduce our overall operating costs for the data center.
If we want to improve data center energy efficiency, there are two areas in which we can affect change. If we can reduce the power going to the support infrastructure or reduce losses in the power system, more of the power entering the data center will make it to the IT load. This will improve our energy efficiency and reduce our PUE.
PUE is a great tool for the facilities side of the data center. It allows facility engineers to measure the impact of changes they make to the infrastructure, things like raising the data center temperature, upgrading to a higher efficiency UPS, increasing voltage to the rack and so on. PUE must be used with care, however. It must be understood that IT changes can have a dramatic impact on PUE.
Under pressure to reduce costs, and in some cases to try to match the reported PUE from other companies, data center managers are being pushed to significantly reduce their PUE value. Unfortunately, this is not always the right approach. The drive to reduce PUE at all costs can actually have a negative impact. If data center managers focus only on reducing PUE, they may inadvertently use more energy and increase data center costs.
Let's run through an example on how this can happen. Suppose we have a data center which has input power of 100 kW, 50kW of which is being used to power IT equipment. As previously illustrated, this would give us an initial PUE value of 2.0.
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