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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
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
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
40 l New-Tech Magazine Europe