39 / 84
Figure 1. Data center monthly-amortized costs (source: James Hamilton’s blog)
industry faces pressure to scale
data centers, one of the most
constrained resources is power. It
is often the case that the power
capacity of existing data centers is
exhausted well before they run out
of storage or processing capacity.
The two main factors of this power
capacity limitation have been the
need to provide supply redundancy
and the way power is partitioned
within data centers, both of which
take up significant space but more
importantly leaving untapped power
sources idle. And this is despite the
fact that current server designs
are far more power efficient than
previous generations and have
significantly lower idle power
consumption.
Providing additional power capacity
within a data center is also time
consuming and expensive even
assuming that the local utility can
supply the additional load, which
IDC forecasts could double from
48GW in 2015 to 96GW by 2021 for
a typical data center. From a capital
expenditure standpoint, as shown
in figure 1, the power and cooling
infrastructure cost of a data center
is second only to the cost of its
servers.
The nature of Cloud services also
means that demand can fluctuate
dramatically with a significant
difference between the peak and
average power consumed by a
server rack. Consequently, providing
enough power to meet peak-load
requirements will clearly result in
underutilization of the installed
power capacity at other times. Also,
lightly loaded power supplies will
always be less efficient than those
operating under full-load conditions.
Clearly any measure that can even
out power loading and free up
surplus supply capacity has to be
welcome in enabling data center
operators to service additional
customer demand without having to
install extra power capacity.
With
regard
to
efficiency
considerations,
servers
and
server racks use distributed
power architectures where the
conversion of power from ac to dc
is undertaken at various levels. For
example, a rack may be powered
by a front-end ac-dc supply that
provides an initial 48 Vdc power
rail. Then, at the individual server
or board level, an intermediate
bus converter (IBC) would typically
drop this down to 12 Vdc leaving
the final conversion, to the lower
voltages required by CPUs and other
devices, to the actual point-of-load
(POL). This distribution of power at
higher voltages helps efficiency by
minimizing down-conversion losses
and also avoiding the resistive
power losses in cables and circuit
board traces, which are proportional
New-Tech Magazine Europe l 39