35
Overall
Physical infrastructure
PUE gets worse with
efficiency
Worse (higher PUE)
virtualization alone
Large unclaimed
entitlement here
gets somewhat better with
virtualization
But will get much better
if physical infrastructure
(PUE) is optimized too
IT
efficiency
MUCH better
IT efficiency
(The subject of this paper)
hin
r
r
Overall efficiency
efficiency
Somewhat better
Watts
IN
Physical
Infrastructure
infrastructure
rtualnter
indicator for how “green” a particular data center is. The purpose is to show how efficient the
power and cooling systems are for a given IT load. Figure 2 illustrates the effect of virtualENERGY MANAGEMENT
ization on data center efficiency.
gets much better with
virtualization
IT
watts
IT
IT
Always a large efficiency
gain here
Useful
computing
Typical effect of virtualization on data center efficiency showing context of PUE within overall data center efficiency
Virtualization’s track on the infrastructure efficiency curve
Power Usage Effectiveness (PUE) remains a simple and
popular metric for the electrical efficiency of a data centre.
If the power and cooling infrastructure is left alone as it was before virtualization was
As a simple ratio of the total electrical power consumed by a
implemented,
PUE will
data centre to the
power expended onthen
IT equipment
alone,worsen after the physical consolidation of servers and storage
it provides a convenient
standard
against which
data centre
has taken
place.
Inherent
in unused power and cooling capacity is what is known as “fixed
management can measure how effectively they’re using
losses”. This is basically power that is consumed by the power and cooling systems regardpower in respect of their facility’s main purpose.
less
ofsufficient
what metric
the IT
The more power and cooling capacity that exists, the more fixed
PUE is not, in itself, a
for load
overall is.
power
management; after
all, it doesn’t
concernAs
itselfthe
withIT load shrinks (e.g., from consolidation) these fixed losses become
losses
will exist.
the total poweraconsumption
of a facility, only
how
higher proportion
ofwith
the
total data center energy use. This means PUE will worsen. This
efficiently that power is directed towards the IT equipment
means
that focus
PUEonis
always better at higher IT loads and worse at lower loads. Figure 3
at the heart of also
operations.
An exclusive
reducing
PUE, therefore, shows
is insufficient
for maintaining
a typical
PUEcost-effective
curve illustrating the relationship between efficiency and the IT load.
power management and reducing the environmental impact
of data centres.
It is, perhaps, in part for that reason that the newly
published Energy Standard for data centres (number 90.4)
thought to be synonymous, or at least in harmony, with
from the American Society of Heating, Refrigerating and
efficient power consumption. But, in fact, if you downsize
Air-Conditioning Engineers (ASHRAE) does not mandate a
your IT load – through virtualisation or outsourcing – your
set PUE figure to ensure compliance.
PUE will get worse assuming you leave your physical
Nevertheless, PUE remains an important metric for those
infrastructure as it was.
Worse
concerned with energy efficiency
of the power and cooling
Efficiency degrades dramatically at low loads
systems. This point is reinforced by the US Government’s
Abstracting the physical
due
“fixed losses”
inherent
unused
Data Center Consolidation Initiative (DCOI) which does
Virtualisation,
theto
abstraction
of physical
server,in
storage
power/cooling
capacity
require US Federal agencies and departments to achieve
and networking resources, greatly increases the ability of
and maintain a PUE rating of 1.5 or less for existing tiered
data centres to scale up services to customers and achieve
data centres, and requires newly built data centres to adhere more complete utilisation of the physical equipment at their
to a rating of less than 1.4.
disposal. The benefits are obvious but some of the drawbacks
Although the DCOI is not binding on international
are less apparent, especially from the power management
organisations, or indeed on non-governmental organisations
point of view.
in the US, it is commonplace historically for initiatives such
Consequences include greater server density as racks
as this to spill over into best practice
recommendations
likely
become
more heavily populated leading to a greater need
Better
to be followed by leading companies and organisations.
for cooling, and greater variability in IT loads as demands on
Consequently, achieving and maintaining a low PUE will
service fluctuate over time, leading in turn to variations in
remain the focus of those intent on managing data centres
PUE, particularly as loads are reduced.
to the highest standards.
Despite the fact that initial physical server consolidation
0%
20%
40%
60%
80%
100%
With that in mind it’s worth paying attention to the fact
results in lower overall energy use, a subsequent reduction
No load
Full load
current strategies that are essential to the effective provision in IT load will diminish the lower portion of the PUE ratio
IT Load
of colocation and cloud services, in particular virtualisation,
leading to a subsequent rise in PUE if the upper portion
%
of
data
center’s
power
capacitymust
can have a negative effect on PUE ratings. This may come
is not reduced accordingly. Therefore,
management
as a surprise to many, given that a low PUE rating is often
be alert to the need to ‘right-size’ power and cooling
"Virt ualisation, the abstraction of physical server,
storage and networking resources, greatly
increases the ability of data centres to scale up
services to customers and achieve more
complete utilisation of the physical equipment"
5
4
PUE
3
2
1
www.networkseuropemagazine.com
Schneider Electric – Data Center Science Center White Paper 118 Rev 5