New-Tech Europe Magazine | Feb 2017

Figure 2. A 2N 100% redundancy power architecture for a tier 3/4 data center

to current and distance. The more recent migration to digitally controllable power supplies has allowed the introduction of Software Defined Power® techniques that can monitor and control the loading of all the power supplies. This allows intermediate and final load voltages to be varied so that the various supply stages can always operate as efficiently as possible. Nevertheless, further improvements in hardware performance are reaching their limits and other solutions are needed. The problem with existing data center power provisioning Traditional data center power supply architectures are designed to provide high availability using supply redundancy to cope with mission critical processing workloads. This is illustrated by figure 2, which shows a 2N configuration that provides the 100% redundancy requirements expected of a tier 3 or tier 4 data center. As can be seen, for a

linear equation: P_server=P_idle+u(P_full- P_idle) where Pidle is the server power consumed when idle and u is the CPU utilization. With new technology delivering lower idle consumption the difference between idle and full power becomes ever more significant. This spread becomes larger still at the rack level, making power capacity planning based on an assumed CPU utilization figure very challenging. Furthermore, the type of workload exacerbates the variability in power consumption. For example, Google found that the ratio between average power and observed peak power for servers handling web mail was 89.9% while web search activity resulted in a much lower ratio of 72.7%. So provisioning data center power capacity based on the web search ratio could result in underutilization by up to 17%. Unfortunately, it does not end there. The fear is that actual peaks might exceed those that have been modeled, potentially overloading

dual-corded server this provides independent power routing from separate utility supplies or backup generators with the additional protectionof intermediateredundant uninterruptable power supplies. Even single-corded servers have the security of a backup generator and uninterruptible power supplies (UPS). However, implicit in this approach is the usually false assumption that all the servers are handling mission critical tasks and that the loading on each (and hence the power demand) is equal. In reality up to 30% of the servers could be handling development or test workloads meaning that half the power provisioned for them is not really required i.e. 15% of the total data center power capacity is blocked from use elsewhere. The other issue is that, conventionally, supply capacity is designed to provide sufficient power for peak CPU utilization. The variability in server power consumption that this results in can be simply modeled by the following

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