Energy efficient IT and infrastructure for data centres and server rooms

Energy efficient IT and infrastructure for data centres and server rooms
Energy efficient
IT and infrastructure
for data centres and
server rooms
Responsibility: PrimeEnergyIT Project consortium, July 2011
Project coordination: Dr. Bernd Schäppi, Austrian Energy Agency, Vienna
Reprint allowed in parts and with detailed reference only. Printed on non-chlorine bleached paper.
The sole responsibility for the content of this publication lies with
the authors. It does not necessarily reflect the opinion of the European Union.
Neither the EACI nor the European Commission are responsible for any use that
may be made of the information contained therein.
Efficient technology for energy and cost savings in data centres
and server rooms
Energy consumption in data centres and server rooms has been increasing significantly during the last
decade. More powerful equipment and more complex IT services have been driving power demand. Since
infrastructure and energy costs in data centres have become a central factor in facility and IT management, a range of technologies has been developed to increase energy efficiency. New hardware and power
management options support energy saving strategies.
Overall, the energy saving potential in data centres and server rooms is high and may exceed 50% in many
cases, depending on the specific IT and infrastructure. In the past the focus of energy saving measures
has been on efficient solutions for power supply and cooling. More recently also measures addressing
IT hardware efficiency are considered. Current studies show that efficiency measures already lead to a
significant reduction of energy demand, compared to a business as usual scenario1. Nevertheless, the
remaining energy saving potential is still large and new technologies allow even more effective deployment of saving options.
This brochure provides a short overview of current technologies supporting energy efficiency both for IT
and infrastructure, with a focus on IT technology. It covers all essential IT technologies in the data centre,
including servers, data storage and network equipment. Efficiency approaches include effective system
design, power management from hardware to data centre level as well as consolidation and virtualisation
Recommendations for best practice highlight promising options to be considered in management and
procurement. A number of resources for further reading are indicated. The brochure provides a source of
basic information for IT and infrastructure managers to support energy- and cost-efficiency.
This brochure has been produced as
part of the international project PrimeEnergyIT
which is conducted within the framework of the
EU programme Intelligent Energy Europe.
1) Koomey, J. (2011): Growth in Data center electricity use 2005 to 2010, Jonathan Koomey, Analytics Press, Oakland,
CA, August 1, 2011
Monitoring of energy consumption in server rooms and data centres6
Monitoring concepts
Measurement devices
Server Equipment10
Energy efficiency and power management at the server and component level
CPU efficiency
Power supply efficiency
Power management at rack to data centre level
Capacity planning and energy management
Power capping
Specific power management options for blade servers
Blade chassis and blade components
Blade system - power and cooling issues
Server virtualization
Energy saving potential of virtualization
Requirements and tools for virtualization planning
Power management in virtualized environments – virtual server migration
Cooling and infrastructure for virtualized systems
Data Storage Equipment28
Storage devices
Tape based systems
Hard Disk Drives (HDDs)
Solid State Drives (SSDs)
Hybrid Hard Drives (HHDs)
Storage elements
Large capacity drives and small form factor
Massive Arrays of Idles Disks (MAIDs)
Efficient RAID levels
Horizontal storage tiering, storage virtualization and thin provisioning
Consolidation at the storage and fabric layers
Data De-Duplication
Network Equipment36
Technical and operational framework
Functional model
Network attributes
Balancing network performance and energy consumption
Improvement of energy efficiency
Merging traffic classes (I/O consolidation)
Network consolidation
Network virtualization
Components and equipment selection
Floor-level switching
Cooling and power supply in data centres and server rooms44
Cooling in server rooms
Split systems and portable systems
Measures to optimize energy efficiency
Cooling for medium to large data centres
General aspects
Temperature and humidity settings
Component efficiency - chillers, fans, air handling units
Free cooling
Rack based cooling / in row cooling
Power supply and UPS in data centres
Monitoring of energy consumption in
server rooms and data centres
Carlos Patrao, University of Coimbra
1.1 Monitoring concepts
The following typical approaches for monitoring may be applied:
Monitoring of energy consumption in server rooms
and data centres is essential to detect energy saving potentials and evaluate the effectiveness of
efficiency measures. Monitoring concepts should
be designed with care to ensure that right data is
collected, supporting effective measures.
The following aspects are to be considered [1]:
Minimum Monitoring – Performing periodic
spot measurements with portable equipment is
mainly an approach for very small facilities. Some
data is acquired from manufacturer’s information
(power input, etc). The approach does not require
investment in permanently installed measurement
equipment and infrastructure.
State-of-the-Art Monitoring – Data is collected by the use of automated/permanent recording
systems in real time, with the support of online
software with extensive capability for analysis.
Modifications in the infrastructure are needed and
support from expert technical staff mostly will be
•Required accuracy and resolution of data
•Breakdown of data collection, ability to collect
Advanced Monitoring – Data is logged in real
time by using permanently installed equipment
not necessarily supported by online tools. Limited
modifications to the infrastructure should be expected.
The monitoring system must have the necessary
number of “info nodes” (or monitoring points) to
provide the required information for comprehensive energy consumption analysis. In larger facilities the “info nodes” selection should start with
the most representative subsystems (in terms of
power usage). Figure 1.1 shows the most important subsystems for which energy consumption
should be monitored. These subsystems can also
be considered as “info nodes” or “monitoring
data from all desired devices
•User friendliness and ease of integration of data
across devices and time scales
•Scalability for mass deployment and multi-site
•Adaptability to new measurement needs
•Data analysis options and integration with control systems
•Ability to detect problems and notify data centre operators
•Investment costs and pay-back
Building load
IT load
Total facility power
IT equipment power
Telco equipment
Free cooling
Fig. 1.1 Simple
schematic with the key data centre subsystems [Source: ASHRAE [2]].
Data collection, processing and evaluation is commonly supported by software tools. For example
the Save Energy Now Program (US Department
of Energy) has developed a tool suite called “DC
Pro”. The tool suite provides an assessment process, tools for benchmarking and performance
tracking as well as recommendations for measures. It is available for free.
•Power usage tool:
•PUE reporting tool
•PUE Scalability Metric and Statistics Spread-
Proper understanding of the overall goals for the energy monitoring is ­essential for designing
an effective monitoring concept.
Typical goals may be:
•Assessment of total IT and infrastructure energy consumption
•Analysis of energy consumption trends over time
•Understanding the instantaneous power demand of key equipment within the facility
•Calculating energy efficiency indexes and energy efficiency metrics
The software/hardware concept for energy monitoring shall provide the following
capabilities (Source ASHRAE):
•Reliable data collection and data storage at the required rates and accuracy
•Normalization of data from different devices, interfaces and p­ rotocols
•Data storage for long measurement periods
•Analysis and visualization of data in form of tables and graphs
•Scaling of the architecture with the data centre expansion
Key aspects to be taken into account when choosing the devices for the monitoring system
among others are instrument range, resolution and accuracy.
•PUE and DCiE Data Centre Efficiency Measurement
Tab. 1.1 Examples
for energy metering devices
Monitoring approaches
Portable power meters cover a range of products including handheld single phase multi-meters to sophisticated three phase power
analyzers with recording and triggering capabilities. Most of them
have a built-in display where the user can access the measured or
recorded data.
Minimum and
advanced monitoring.
Panel meters are usually permanently installed in the switchgear
measuring UPS systems, generators or other devices. These meters
have a display that shows the instantaneous measurements and
cumulative variables such as the total energy consumption. They
can be installed to measure overall and individual energy consumption of devices.
These meters can be
used for best practical and state-of-theart monitoring.
The revenue meters are mostly used by the electrical utilities, landlords and others who bill their customers. They are rarely used in
data centre monitoring systems, but they can provide data about the
overall energy consumption of the facility. In some cases utilities can
provide the access to the digital communication port, which gives
the ability to acquire and store it in a database for future analysis
(for instance every 15 minutes).
Can be used in all the
Intelligent or metered rack power distribution units (PDUs) provide
active metering to enable energy optimization and circuit protection. Metered rack PDUs provide power utilization data to allow data
centre managers to make informed decisions on load balancing and
right sizing IT environments to lower total cost of ownership.
PDUs may be equipped with real-time remote unit-level and individual outlet-level power monitoring of current, voltage, power, power
factor and energy consumption (kWh) with ISO/IEC +/– 1% billinggrade accuracy. Users can access and configure metered rack PDUs
through secure Web, SNMP-, or Telnet interfaces.
Can be used in all the
metering feature
Server-embedded power metering feature
Minimum and
advanced monitoring.
Power transducer
The power transducer is usually referred as an equipment with no
display that is permanently connected in a switchgear like the panel
meters. Such devices are often used by monitoring systems to acquire power measurements from various points of a data centre.
Can be used in all the
Portable meter
Source: Chauvin Arnoux
Panel meter
Source: Chauvin Arnoux
Revenue meter
Source: Itron
Source: Raritan
Source: Chauvin Arnoux
1 Monitoring of energy consumption in server rooms and data centres
1.2 Measurement devices
Further Reading
A large number of types of measurement devices
is available for measuring key variables such as
energy consumption, temperature, flow rate and
Some examples for energy measuring devices are
presented in table 1.1 (left, on page 8). For further
reading see the sources indicated in the following section or access the “Technology Assessment
Report” available at the PrimeEnergyIT website.
ASHRAE (2010): Real-Time Energy Consumption Measurements in Data Centres, ASHRAE –
American Society of Heating, Refrigerating and
Air-­Conditioning Engineers, 2010.
ISBN: 978-1-933742-73-1
Stanley, J. and Koomey, J. (2009): The Science
of Measurement: Improving Data Centre Performance with Continuous Monitoring and Measurement of Site ­Infrastructure, Stanley John and
Koomey Jonathan, October 2009
Rasmussen N. (2010): Avoiding Costs From
Oversizing Data Centre and Network Room Infrastructure, Neil Rasmussen, APC by Schneider Electric, 2010. White paper #37 – Revision 6
Ton, M. et al (2008): DC Power for Improved
Data Centre Efficiency,Ton, My, Fortenbery, Brian
and Tschudi, William, Ecos Consulting, EPRI, Lawrence Berkeley National Laboratory, March 2008
Webinar: „The Data Centre in Real Time: Monitoring Tools Overview & Demon“
Schneider Electric (2011): E-learning website
(Energy University) that provides the latest information and training on Energy Efficiency ­concepts
and best practice
The Green Grid (2008): Green Grid Data Centre
Power Efficiency Metrics. White Paper 6, The Green
Grid, White Paper 6. December 30, 2008
Rasmussen N. (2009): Determining Total Cost
of Ownership for Data Centre and Network Room
Infrastructure, Neil Rasmussen, APC by Schneider
Electric, White paper #6 – Revision 4
[1] Stanley, J. and Koomey, J. (2009): The
­Science of Measurement: Improving Data Centre
Performance with Continuous Monitoring and
Measurement of Site Infrastructure. October 2009.
(2010): Real-Time Energy
­Con­­sump­tion Measurements in Data Centres:
ASHRAE- American Society of Heating, Refrigerating and Air-Conditioning Engineers, 2010.
ISBN: 978-1-933742-73-1.
2 Server Equipment
Bernd Schäppi, Thomas Bogner, Hellmut Teschner, Austrian Energy Agency
Server equipment consumes about 30–40% of the total energy used in
data centres and server rooms. Therefore, it is one of the primary areas
to implement effective energy saving measures. Typical server equipment
in common server rooms and data centres includes standard rack servers,
blade servers, as well as pedestal servers and multi-node servers.
The energy efficiency potential is high and depending on the type of IT
2.1 Energy efficiency and
­power management at the
server and component level
system and the measures applied, energy savings of 20–60% or even
beyond can be achieved. The primary approaches for improving energy
efficiency involve energy efficient hardware selection and system design,
power management at all levels from the hardware component to the total
system and last but not least hardware consolidation and virtualization.
The following chapter provides information on power saving technologies and options
from the component to the system level. Energy efficiency issues and possible measures
for improvement are provided from the server to the rack and data centre level. Two
specific sections address blade server technology and server virtualization as potential
efficiency strategies. Specific recommendations for best practice options are highlighted
in boxes.
Tab. 2.1 Energy
Star Idle Power Criteria
Number of
Managed server
Base Idle
State Power
Allowence (W)
Tab. 2.2 Concept
of SERT assessment tool
Energy efficiency of servers has been strongly improved in the last years, mainly due to develop­
ment of effective power management for hardware components. To date, server energy efficiency
is assessed and declared based on Energy Star requirements and the SPECpower benchmark (SPEC:
Standard performance evaluation corporation).
The current ENERGY STAR requirements for
enter­prise servers [1] stipulate energy efficiency
criteria for rack and pedestal servers containing up to 4 processor sockets. The requirements
­define maximum levels for power consumption
in On Idle Mode for 1- and 2-CPU socket servers as well as criteria for power supply efficiency
and power management features (see Table 2.1
and Table 2.4). The idle mode criteria are primarily
useful as an efficiency indicator for low average
load conditions close to idle operation. Such low
loads on servers (e.g. < 15%) are still quite common, ­although hardware consolidation to achieve
higher load levels should be a general goal.
Server energy efficiency at higher workloads and
for consolidated systems is addressed with the
SPECpower–benchmark, which however is focused more on CPU-related efficiency and CPU
intense workloads (see information below). A
comprehensive Server Efficiency Rating Tool (SERT)
addressing all major server hardware components
at different load levels is currently in development by SPEC [2] and will be available in winter
2011/2012. The SERT tool will assess server efficiency based on partial benchmarks for CPU,
memory, storage and system (Table 2.2). The tool
will support IT managers in selecting energy efficient hardware for specific applications.
Benchmark result system
Benchmark result
Benchmark result
Benchmark result
Performance scale
Performance to Power Ratio
3,197 overall ssj_ops/watt
Target Load
SPECpower_ssj2008 [2] was the first standard
fig 2.1
benchmark supporting energy efficiency assessment of volume class servers. It addresses mainly
CPU related efficiency and thus provides a good
assessment regarding CPU intensive workloads.
The benchmark is published by manufacturers
only for selected hardware though. Fig. 2.1 shows
one example of SPECpower results for a volume
server. The typical SPEC graph provides information on the average performance per watt across
the range of loads as well as values for ten different load levels. Thus servers can be compared at
different load levels from idle to 100%. For procurement purposes the complete SPECpower information (also containing detailed configuration
information) should be requested from suppliers.
Furthermore it should be considered that products
are often tested in low configurations.
Overall benchmark score
(SPECpower_ssj2008 metric
is calculated as the sum of all
ssj_ops scores for all target
loads, divided by the sum of
all power consumption averages (in watts) for all target
loads, including the active idle
measurement interval)
Load scale
(from idle to
100% utilization
in 10% steps)
Performance bar graph
Power consumption line
50 75 100 125 150 175 200 225
Average Active Power (W)
Power consumption scale
Fig. 2.1 SPECpower
diagram and key information
Recommendations for best practice
Energy efficiency criteria and benchmarks for hardware ­selection
•Use the efficiency criteria from Energy Star for procure-
ment if applicable. For servers operated at low loads,
­Energy Star Vers.1 requirements for idle mode may serve
as reasonable efficiency indicators. Requirements for
­power supplies can be used for any type of equipment.
•Request SPECpower_ssj2008 (and SPEC-SERT as soon as
available) benchmarking results from manufacturers. For
SPECpower, consider the following issues:
■ It is a CPU centric benchmark, thus most representative
for CPU intense workloads.
■ Servers may have been tested at rather low configuration (thus check configuration).
■ To arrive at robust interpretation, consider not only the
overall score (overall operations per watt) but also the
detailed benchmarking data.
2.1.1 CPU efficiency
CPUs are the most energy consuming components
in servers, thus energy efficient CPU models with
effective power management can strongly support
CPU energy consumption depends on the specific
voltage and the clock frequency. Power management at CPU or core level therefore is based on
Dynamic Voltage and Frequency Scaling (DVFS) or
switch-off of cores. Energy consumption of CPUs is
often compared on the basis of the thermal design
power (TDP) that indicates the maximum power
the cooling system in a server is required to dissipate. However, TDP provides only limited information, as overall efficiency also strongly depends
on power management. Manufacturers offer specific low power CPU versions that allow significant
­energy savings in practice, if the specific performance requirements can be met.
Energy efficiency of CPUs strongly depends on
the effective implementation of power management. Common operating systems support power
management based on the Advanced Configura-
tion and Power Interface (ACPI) specifications for
processor performance states and power consumption (P-States) and thermal management
states (C-States). The new system and component
controls enabled by ACPI Vs 3 provide higherlevel power management engines allowing a finer
grained power and performance adjusting based
on demand. In many recent server models, predefined power profiles can be applied, e.g.:
■ “High
performance” (appropriate for servers
that run at very high utilisation and need to
provide maximum performance, regardless of
power costs)
■ “Power saver mode” / “Minimum power usage” (applied to servers that are run at low
utilization levels and have more performance
capability than really needed, using this mode
may provide incremental power savings)
■ “Balanced power and performance”
G7 (3.07 GHz, Intel Xeon X5675)
Target Load
Performance Performance
to Power Ratio
to Power Ratio
250 0
500 250 750 500 1,000750 1,250
734 overall ssj_ops/watt
734 overall ssj_ops/watt
50 0 75 25100 50125 75150100
225175 200 225
Average Active
(W) Power (W)
Fig. 2.2 Example
Target Load
Target Load
3,197 overall ssj_ops/watt
3,197 overall ssj_ops/watt
Target Load
For hardware configuration in procurement, it is
generally essential to check for concrete performance requirements to be met by the hardware
G5 (2.66GHz, Intel Xeon L5430)
Performance Performance
to Power Ratio
to Power Ratio
Figure 2.2 shows the positive effects of modern
CPU power management in benchmark results
(SPECpower) for the server product family HP ProLiant DL 380: the ratio of idle power to full load
power has been strongly reduced from genera­
tion G5 to G7 of the specific server model. For
the DL 380 G5 server, idle power (no load) was
33% (170 watts) lower than full load power (253
watts). For G7, it is about 75% lower compared
to maximum power. This shows that new server
technology is much more energy efficient at low
load or idle operation, due to intelligent power
management at CPU level. The computing performance for the specific server model on the other
hand has been increased by more than a factor
of three.
50 0 100 50 150 100 200 150 250 200
Average Active
(W) Power (W)
of SPECpower-benchmark for different server generations (G5, G7 Server from HP) [SPEC (2010,]
2 Server Equipment
components. Different types of server workloads
set different requirements regarding hardware
performance that should be considered for efficient hardware configuration. A rough indication
of hardware performance requirements for different workloads is given in Table 2.3.
2.1.2 Power supply efficiency
The Energy Star program for servers [1] has set
requirements for power supply efficiency defining
levels for 10%, 20%, 50% and 100% load. The 80
PLUS Certification Scheme [3] also provides ­energy
efficiency requirements for server power supplies
but excludes the 10%-load level. For practical purposes and procurement, it is r­e­commended to order power supplies that meet at least the 80 PLUS
Gold level, which corresponds to 88% efficiency at
20% load and 92% efficiency at 50% load.
Standard rack servers commonly operated at low
loads are often equipped with over-provisioned
­redundant power supplies. This results in signifiTab. 2.4 Efficiency
requirements of different server applications [5]
File/print server
Mail server
Virtualization server
Web server
Database server
Application server
Terminal server
cant energy losses due to a very low operating
point of the equipment. Thus right sizing of power
supplies is essential. It is supported for example
by online power configuration tools offered by
manufacturers and by tools for power capping
Some manufacturers (e.g. HP ProLiant G6 and G7
server series) provide specific hardware features to
overcome unnecessary losses for redundant p­ ower
Hard disks IO
supplies. Such hardware offers an operation mode
that allows use of only one power supply until load exceeds a certain threshold. The second
power supply stays in standby maintaining redundancy. This mode provides full power redundancy
in case of a power supply or circuit failure.
requirements for power supplies in the Energy Star programme and the 80 PLUS initiative [1, 3]
Energy Star Vs1
Energy Star Vs2 Draft
Tab. 2.3 Performance
Power Supply Type
Rated Output
10% Load 20% Load 50% Load 100% Load
Multi-output (AC-DC & DC-DC)
All Output Levels
Single-output (AC-DC & DC-DC)
≤ 500 W
>500–1,000 W
> 1,000 W
Multi-output (AC-DC & DC-DC)
All Output Levels
Single-output (AV-DC & DC-DC)
All Output Levels
All Output Levels
All Output Levels
All Output Levels
All Output Levels
2.2 Power management at rack
to data centre level
2.2.1 Capacity planning and energy
Going beyond hardware components and single
server units, power management at the system
level is also important to optimize overall energy
As indicated above, the majority of servers are
still utilised at modest workloads, thus there is
large potential for energy savings to be achieved
by hardware consolidation (see next chapter) or
by power management at system level. As for the
component level, power management at higher
levels adjusts performance and power draw to
the actual demand and powers off or throttles resources if not needed. Table 2.5 shows the various
approaches of power management at different
levels [7]. Some of the options are addressed in
the following sections and in later chapters.
Server management software provides essential
tools for secure server operation but also for holistic power management. Server management tools
can effectively help to reduce energy consumption
as they facilitate the implementation of energy
policies throughout the server system and provide
features like provisioning, monitoring and configuration management that can strongly support
system efficiency. Major features commonly are:
Tab. 2.5. Power
•configuration management
•update control
•power management
•workload management
All larger hardware suppliers offer powerful server
management tools. IBM (Systems Director) and
HP (Systems Insight Manager including Insight
Dynamics) offer very comprehensive management solutions capable of integrating third party
systems. Fujitsu (Server View Site) offers products
with basic functionalities that can be integrated
in established management consoles from other
suppliers. DELL is using the Altiris Total Management Suite. Sun and Acer provide consoles for
their own environments.
Energy Management Suites (e.g. IBM Energy
Among many other features, this type of tool
supports monitoring and collecting power consumption data, managing power including setting
power savings options and power caps as well as
automating power-related tasks. The latter include
configuration of metering devices such as PDUs
and sensors, setting thresholds, creating and setting power policies, calculating energy costs. For
further information on energy management suites,
see below.
management options from component to data centre level [7]
Component Level
System Level
Rack Level
• CPU (Package/core C-states,
• S-states
• System or node management
P-states, T-states, Thermal throttle) • Platform-based power management • Application/load balancing
• Other components (D-states, L-states) • Workload schedulers
• Chassis management
• Fan speed control
Data centre Level
• Application/load balancing
• Facilities and equipment monitors
• Data de-duplication, etc.
• Multi-rack management, dynamic
2 Server Equipment
Capacity planning tools
(e.g. HP Capacity Planner)
Capacity planners, among other features, support
IT managers with increasing server utilisation,
reducing energy consumption and enhancing application performance. They allow collection of utilisation data for CPU cores, memory, network, disk
I/O and power. Furthermore, they support workload planning or system changes and assessment
of impact on resource utilisation. They also evaluate trends to forecast resource needs. For further
information on capacity planning tools, see below.
Based on utilisation logs, the HP tool for example provides a good decision basis for consolidation measures by assessing the resource demand
for merged applications. Figure 2.3 shows the
example of a comparison of the utilisation of
two systems indicating that peak performance is
­occurring at different times and the average load
would increase only modestly in case of hardware
Number of Cores
Peak utilisation of system 2
Peak utilisation of system 1
21 Feb
28 Feb
System 1
System 2
Fig. 2.3 Comparison
6 March
13 March
of CPU utilisation for “system 1” and “system 2” (see HP Capacity Planner)
2.3 Specific power management options for blade servers
2.2.2 Power capping
Active allocation of power budgets to servers is
also known as power capping. IT managers can
specify power caps for servers according to real
power requirements. Dynamic power capping reduces the maximum power demand of the system
and thus optimises power provisioning beyond the
level typically supported by power configurators
offered by manufacturers.
The concrete savings achieved in practice depend
on the level of the cap. The caps should be set in
a way that power peaks are capped but computing performance is not visibly affected. Optimised
capping requires an assessment of the workload
and power consumption pattern. For relatively
uniform workloads, caps can be set at average
server load without significantly affecting performance. As a rule of thumb, caps should not be
set lower than about midway between minimum
and maximum power consumption of the servers.
Some ­management tools also provide the option
of time dependent specification of capping that
defines different caps for different periods in the
day ­depending on load pattern, power costs etc.
Blade server technology is deployed both in data
centres and server rooms. The blade server market
has been the fastest growing market segment in
the last few years and it is therefore important that
the technology is as energy efficient as possible.
Blade chassis (see Figure 2.4.) typically include
7, 14 or more blade server modules, one or more
management modules as well as KVM interfaces.
Chassis support server-, storage- and network
modules and may be optimised for specific applications and user types. Compared to standard
rack servers, blade technology allows a reduction
of some hardware components like power supplies, network I/O and wiring which are shared by
several servers in the common enclosure.
Energy efficient DC planning and management
•Use server management tools for capacity planning, workload and power monitoring and specific
power management. Detailed descriptions and recommendations on use of power management
features are supplied with the technical documentation of the server management suites.
•Use application and load balancing to optimise use of hardware resources.
•Use power capping to keep power demand at desired levels for the whole system.
• Benefit from optimised IT hardware resilience levels. Evaluate the level of hardware resilience actually justified in view of expected business impact of service incidents for each deployed service.
•Decommission unused services and completely remove the hardware. Assess the options for decommissioning low business value services by identification of those services which do not justify
the financial and environmental cost.
Fig. 2.4 Blade
Major benefits of blade systems are:
•High computing density and low space demand
•Reduced time for maintenance and upgrade
of the system due to hot-plug replacement of
modules and integrated management features
•Slightly higher energy efficiency as compared to
rack servers if power management and cooling
is optimised
Fig. 2.5 Dual-node
blade server
2 Server Equipment
Dual-node and multi-node concepts are partly
based on a similar philosophy as blade servers. In
the multi-node concept, a fixed number of server
units (commonly 2 or 4) is combined in one rackmounted chassis. Similar to blades, the servers
share powersupplies and fans, however there are
few expansion options. Thus multi-node technology is an approach to implement higher computfig 3.7
ing density at comparably low cost, often designed
for purposes of small and medium enterprises.
However, there are also special high performance
dual-node servers available for example for blade
systems which combine two server nodes in one
blade. The main benefits of standard dual and
multi-node systems are:
•Lower cost and space demand as compared to
standard rack servers
•Slightly lower energy consumption due to
shared power supplies and fans
2.3.1 Blade chassis and blade components
Larger power supplies are often more efficient,
thus a lower number of larger power supplies
in blade systems can increase energy efficiency
compared to rack servers. However efficiency in
practice also depends on the power demand in
relation to the power supply capacity. Figure 2.6
shows the efficiency curve of a platinum labelled
power ­supply [3] of 2,990 W rated power for a
blade chassis indicating efficiencies between 92%
and 95% across the load range. Efficient power
supplies for blades should reach energy efficiency
levels above 90% between 20% and 100% load.
For new product generations of blade and multinode servers, some manufacturers provide several
power supply models with different rated power
that allow right-sizing according to the power demand. Power supply selection is supported by online power configurators offered by manufacturers.
Fewer and more efficient power supplies, more
efficient fans and extended energy management
options in the blade chassis offer higher energy
efficiency as compared to standard rack servers in
principal. However, efficiency in practice strongly
Efficiency of the Power Supply
Efficiency (%)
However, if high blade densities are implemented,
this results in high demand for infrastructure and
cooling. High computing density increases power
densities to 10–25 kW/rack. Consequently, standard cooling in data centres and server rooms is
often not sufficient and specific cooling concepts
are required. Thus energy efficiency of a blade concept also strongly depends on the overall system
Selection of blade technology based on clear decision criteria
Loading (% of Rated Output Power)
•Define and assess the main reasons for implementing blade technology in
Fig. 2.6
Blade power supply efficiency [3]
the data centre, e.g. space restrictions.
•Assess the benefits that are expected in comparison to rack technology
and check if expectations are realistic.
•Check if virtualization may be an alternative solution considering the
defined objectives.
• Evaluate the expected Total Cost of Ownership (TCO) and energy efficiency
compared to other options (based on information provided by suppliers).
Dell M610 Blade server
R610 1U rack server
Performance to Power Ratio
Performance to Power Ratio
3,093 overall ssj_ops/watt
Target Load
Target Load
2,938 overall ssj_ops/watt
Average Active Power (W)
25 50 75 100 125 150 175 200 225 250
Average Active Power (W)
Fig. 2.7 SPECpower_ssj2008 for a Dell M610 Blade server and R610 1U rack server. The blade system includes 16
blades with identical processor configuration as the rack server (2 x Intel Xeon 5670, 2.93GHz). SPEC (2010,
depends on the configuration of the chassis as
well as on the use of the power management
­options. Chassis configured with only a few blades
will clearly be less efficient, due to over-provisioning of cooling, power and network capacity.
indicating that the performance per watt or the
energy efficiency at maximum load is 4% better in
the blade system than in the rack solution. The difference increases to about 8% for low loads (10%
load) and to 11% for idle operation.
For approximate comparison of the energy
­efficiency of blade servers versus standard rack
servers, a fully configured blade system may be
considered. Such a rough comparison based on
energy efficiency data published by Dell is shown
in Figure 2.7. Dell has published SPECpower-data
(SPECpower_ssj2008) for blade systems and comparable rack servers in 2010 (
Although this simple comparison must not be over
interpreted (as SPECpower does only assess part
of the server efficiency), it suggests that blade
systems, even if fully configured and optimised for
testing, show only slightly better energy ­efficiency
than standard rack servers, especially at high
loads. The difference is more significant at low
load levels indicating a better overall power management in the blade system at low load.
The SPEC results show a maximum performance of
3,885 ops/watt at 100% load for the blade system
and 3,739 ops/watt for the rack server system,
Thus, blade solutions seem to offer only limited
potential for increasing energy efficiency for ex-
ample compared to virtualization. Similar to rack
servers, there is also the option to combine blade
hardware with virtualization, which allows for
strong improvement of energy efficiency.
Challenges related to high heat densities at rack
and row level are addressed in section 2.3.2
Modern blade chassis contain management hardware and software that in combination with remote access controllers in the server blades allow
a power inventory and power management of the
individual blades. Specific management cards support a hardware and power demand inventory of
the different blades. The remote access controller
communicates the power budget information to
the chassis management card that confirms the
availability of power from the system level, based
2 Server Equipment
Consider procurement criteria for
selecting energy efficient blade hardware
•Define the workloads and expected workload
levels to be run on the blade systems.
•Compare costs and energy efficiency of blade
systems from different vendors.
upon a total chassis power inventory. The CMC
can set power policies at the system level and
actual power consumption at each server module
is monitored ensuring that instantaneous power
consumption does not exceed the budgeted
The basic functions of the power management in
automatic mode are normally not visible for the
system administrator. However, priorities for each
server module can also be set manually, for example by selecting the lowest priority blades as the
first to enter any power saving mode.
In blade chassis, dynamic power capping can be
used even more effectively than for standard rack
servers, since the dynamic power cap can be specified across multiple servers. Power caps can be dynamically adjusted by the onboard administrator
and the service processor. Blades running lighter
workloads receive lower caps. Since workload
intensity and dynamics are normally different for
the different blades, power peaks occur at different times. Consequently, the overall cap for the
chassis can be set lower compared to the sum of
individual caps for single blades. HP has calculated
power savings and reduced TCO for a blade centre
where power supply design was based on power
capping. Maximum power and power provisioning
cost was reduced by about 20% as compared to
the approach without power capping [HP2011].
•Request product information from suppliers
■ Total Cost of Ownership (TCO) .
■ Overall energy efficiency (e.g.
SPEC-SERT as soon as available).
■ Energy efficient hardware components,
e.g. efficiency and right sizing of power
■ Management tools especially addressing
power management and optimization of
system design.
•Select equipment offering highest energy
efficiency for the workload types and levels
you are addressing and adequate power
management options.
2.3.2 Blade system – power and cooling
In practice, design of efficient blade server systems
is often an underestimated challenge, especially if
large high-density systems are implemented. The
main challenges are:
•Sufficient cooling capacity and appropriate
cooling design to cope with high heat densities
•Sufficient power capacity and distribution (local
PDU capacity, power wiring etc.)
Traditional cooling concepts often allow only
2-3kW/rack, which is 10 times less than the power
of a fully populated blade rack. This means that
standard cooling concepts of data centres and
server rooms are often not appropriate for larger
blade systems and have to be modified.
Use of management tools to optimize energy efficiency of blade systems
•Use management tools and intelligent network and power devices for monitoring of power consumption and load for your blade system.
•Analyse options to balance and manage loads and power consumption within and across blade
chassis and racks.
•Use power capping and power balancing features of blade chassis.
•Do a first order estimation on power/cooling capacity demand based on power calculators offered
by manufacturers.
• Assess real power demand with available management tools for complete duty cycles and set power caps according to peak load. Adjust power and cooling to fine-tune system based on power caps.
Table 2.6 shows typical options for the design of
different blade densities depending on business
requirements and constraints such as infrastructure and cooling capacity. Different blade density
levels allow the following options for cooling concepts [Rasmussen 2010]:
the heat load of blade chassis to
different racks: Individual blade chassis are
mounted to different racks to spread heat load.
For this concept, the percentage of blade chassis in the total system has to be very low.
•Dedicating cooling capacity: Excess cooling
­capacity is specifically dedicated to the blades.
For this approach the percentage of blades in
the system has to be relatively low, as only the
existing cooling capacity is used.
•Installing supplemental cooling: Supple­mental
cooling is provided for the blade racks. P­ ower
density per rack can be up to 10kW. The
­approach allows for good floor space utilisation
and high efficiency.
Tab. 2.6 Configuration
•Definition/design of high-density area: Specific
area in the data centre is dedicated to blades
(high density row or zone). High efficiency and
high floor space utilisation. Density up to 25kW.
Area has to be planned and re-designed.
•Design of high density centre: High density
blade racks throughout the data centre. An extreme and rather uncommon approach, which
for most situations leads to significant costs
and strong underutilisation of infrastructure.
In existing data centres, there are often certain
limits for the deployment of blade technology defined by the specific infrastructure. For example,
a standard raised floor system may not allow a
higher power density than 5kW per rack. Proper
specification of power and heat density is an important prerequisite to allow energy-, space- and
cost-efficient system design.
Another essential point regarding energy effi­ciency
at the system level is to avoid over-provisioning of
infrastructure and cooling. Density specification
should take into account both spatial and temporal variability, e.g. different local power densities in
data centres regarding blade racks and standard
racks and variation over time where density may
increase. Thus, power density has to be specified
either at rack or at row level. For larger systems
the row level is more appropriate, since cooling
and power distribution is mainly row-based. As
far as possible, it is recommended that density
specifications are defined for a rack or row. They
should be left unchanged for the time of operation
of the specific rack or row. Thus, implementation
of a new technology with different density level
should be done in a new rack or row. However,
there are also alternatives to this approach allowing some variation of power densities in installed
racks or rows:
•Adding hot pluggable UPS modules
•Using hot swappable rack PDUs
•Adding cooling capacity with rack-mounted
of blade systems at rack level and related requirements for cooling [after Rasmussen 2010]
Spreading load
Chassis/rack across racks
Dedicating cooling
Additional cooling
High density area
High density centre
Possible in most DCs
Possible in most DCs
Possible in most DCs
Not cost efficient
Not cost efficient
Rarely practical
Possible in most DCs
Possible in most DCs
Not cost efficient
Not cost efficient
Not possible
Possible in most DCs
Not possible
Rarely practical
Not possible
Not possible
Not possible
Not possible
Not possible
Maximum for optimised
efficient raised floor
Hot air scavenging
Hot air scavenging
Extreme costs
Not cost efficient
Possible in most DCs
depending on specific
Depending on specific
Not possible
Hot air scavenging,
room redesign
Hot air scavenging,
room redesign
Extreme costs
2 Server Equipment
For the definition of densities for rows, it has been
recommended to define a maximum ratio of peak
to average power of 2 for typical row designs.
Where double average power is exceeded by specific racks, IT loads should be redistributed within
the row or to other rows. Overall, it obviously
makes sense to distribute higher density racks in
the row. Power and cooling management systems
can be used to define rules for deploying installed
capacities e.g. allowing a rack to exceed average
power only if the power demand of a neighbour
rack is significantly below average.
An important issue is how to deal with future developments regarding future needs for IT extension. It is clearly not advisable to implement infrastructure covering maximum future capacity from
the beginning, as this would mean over-capacity
and high costs over a longer period of time. It is
generally recommended to install all piping and
wiring for full expansion of capacity but to install
the power and cooling equipment at later stages
based on specific demand. This approach allows to
prepare all the basic infrastructure of the building
but to implement the specific equipment according to power and cooling demand of the IT when
2.4 Server virtualization
Server virtualization offers great potential for
­energy savings. The technology allows for the consolidation of workloads on less physical hardware,
thereby strongly reducing power and cooling demand. Overall virtualization offers a number of
advantages for the effective design of IT systems
in server rooms and data centres , as for example:
•Reduction of hardware and space requirements
via deployment of virtual machines (VMs) that
can be run safely on shared hardware, increasing server utilisation from 5–15% to 60–80%.
•Test and Development Optimisation – Rapidly
provisioning test and development servers by
reusing pre-configured systems enhancing developer collaboration and standardizing development environments.
•Reducing the cost and complexity of business
continuity (high availability and disaster recovery solutions) by encapsulating entire systems
into single files that can be replicated and restored on any target server.
Established virtualization platforms like VMWare, Microsoft Hyper-V and Citrix XEN offer
many features like high availability, failover, distributed resource scheduling, load balancing,
automated backup functions, distributed power
management, server-, storage- and network
VMotion etc.
The primary technology options for server virtualization include:
•Physical partitioning
•Virtualization based on an underlying operating
•Application virtualization e.g. Microsoft Terminalserver, Citrix XenApp
•Hypervisor-based virtualization:
■ VMware ESX
■ Citrix /Open-Source: XENServer 5
■ Microsoft Hyper-V
Considering the market which is dominated by
only a few products, the following chapter focuses
on the hypervisor-based products: VMware ESX,
Microsoft Hyper-V and Citrix XEN Server.
The market leading virtualization platforms VMWare ESX/ESXi/Vsphere4, Microsoft HyperV and
Citrix XEN offer support for most common standard guest operating systems. They provide management consoles for administration of smaller
server environments as well as data centre level
VMware was the first product on the market in
2001. Its architecture predates virtualizationaware operating systems and processors such
as Intel VT and AMD-V. VMware ESX/VSphere4­
offers powerful administration tools like ­VMotion
of virtual machines across servers, storage
­VMotion, storage overprovisioning, desktop and
network virtualization, virtual security technology,
and it delivers a complete virtualization platform
from desktop through data centre up to cloud
Microsoft Hyper-V Server contains the Windows
Hypervisor, Windows Server driver model and virtualization components. It provides a small footprint and minimal overhead. It plugs into existing
IT environments, leveraging existing patching,
provisioning, management, support tools, and
processes. Some of the key features in Microsoft
Hyper-V Server 2008 R2 are live migration, cluster
shared volume support and expanded processor
and memory support for host systems. Live migration is integrated with Windows Server 2008® R2
Hyper-V™. Hyper-V™ live migration can move
running virtual machines without downtime.
Depending on user requirements, Citrix XENServer
may offer a cost effective way of implementing
virtualization, since basic elements like the bare
hypervisor, resilient distributed management ar-
chitecture, XENServer management and conversion tools come for free. Advanced management
and automation features like virtual provisioning
services, distributed virtual switching, XENMotion,
live migration, live memory snapshots and revert,
performance reporting and dynamic workload
balancing make the XENServer comparable to the
other two products. However, these features are
part of the advanced commercial editions.
BMC Software, Eucalyptus Systems, HP, IBM, Intel,
Red Hat, Inc. and SUSE announced the formation
of an Open Virtualization Alliance, a consortium
committed to fostering the adoption of open virtualization technologies including Kernel-based
Virtual Machine (KVM). The consortium complements the existing open source communities
managing the development of the KVM hypervisor
and associated management capabilities, which
are rapidly driving technology innovations for
customers virtualizing both Linux and Windows®
applications. The consortium intends to accelerate
the expansion of third party solutions around KVM
and will provide technical advice and examples of
best practice.
fig 3.9
2.4.1 Energy saving potential of
Virtualization is one of the most powerful technologies for reducing energy demand in data centres
and server rooms. Consolidation of server hardware by concentrating workload on a lower number of physical servers often allows energy savings
of 40% to 80% and sometimes more, depending
on the specific case. Current technology provides
the possibility to implement virtualization with
consolidation factors of at least 10–20, ­depending
on the specific systems and requirements.
Figure 2.8 shows the example of a server con­­soli­dation by virtualization in the German Federal
Ministry for the Environment. The specific measures allowed ­energy savings of about 68%. The
case involved a reduction of hardware to 2 physical servers r­ unning VMware ESX [4].
Another example from IBM [5] for a virtualization
project involving blade server technology suggests
energy savings of more than 90% if all relevant
measures at hardware and infrastructure level are
Such examples illustrate that consolidation by
­virtualization is one of the major options to significantly increase energy efficiency in data centres.
However, as for the other IT based approaches the
full saving potential can only be accessed if the
infrastructure including power supply and cooling
is addressed in parallel.
Access Control
Help Desk
Inventory Server
IT Controlling
Certificate Server
Power [Watt]
File Server
Root DC
Office DC
Office DC
Software Packaging
Logging Server
Help Line
System Monitoring
Terminal Server
FC Switch
IT NSeries
Exchange FE
Conference Proxy
4 Rack Monitor
NSeries Storage
NSeries Controler
4 Rack Fan & KVM
ESX1 Server
Fig. 2.8
Reduction of energy demand by virtualization in a case study [4]
2 Server Equipment
2.4.2 Requirements and tools for
virtualization planning
Virtualization in data centres should be based on
a virtualization strategy that involves an evaluation and identification of appropriate server candidates.
For such an evaluation, data on performance,
­system utilisation, end-of-service timelines, business area and application specification is collected. Once the candidates for virtualization have
been identified, application specifications and
­machine load are analysed. Performance evaluation is conducted to assess among others the
following requirements as a basis for hardware
•CPU performance
•Required memory
•Disk I/O intensity
•Network requirements
•OS configuration
Fig. 2.9 Example
Several applications can typically be consolidated
to a single physical server, which is immune to
hardware failure and power interruptions while
possessing the ability to load-balance. To achieve
this goal, host servers may contain dual power
supplies, mirrored hard drives and teamed network
interface cards. For a centralized storage solution,
a Storage Area Network (SAN) with full faulttolerant capabilities can be used. Load ­balancing
can further be supported by virtual ­
migration between physical servers.
Depending on the type of workloads, a consolidation ratio between 10:1 and 20:1 can be considered. Regarding memory requirements, many
virtualization environments offer the feature
of memory over provisioning. By means of this
­feature the sum of memory allocated to all ­virtual
­machines can exceed the available physical memory by a factor of 2 to 3.
Virtualization is rarely done for energy saving purposes only. Thus although high energy savings are
normally guaranteed, successful virtualization projects typically require thorough planning, which
also involves ROI and TCO calculations.
Summing up the relevant cost factors, the TCO for
the new virtual server deployment is calculated. A
short-term and long-term ROI calculation can be
done to assess time-related costs.
The key to successful ROI calculation is to understand virtualization costs. The obvious expenses
for virtualization projects are hardware, software
(incl. licensing) and labour. Virtualization may involve buying of new, more powerful servers, upgrade of storage, network and security etc. Costs
for staff training and management are an additional issue. All these aspects have to be factured
into the ROI calculation.
Different software tools available on the market
support virtualization planning as well as ROI and
TCO calculation. For example, the Assessment
and Planning (MAP) Toolkit from Microsoft supports planning for migration including TCO and
ROI calculation. The MAP-Toolkit is an inventory,
assessment, and reporting tool that can assess IT
environments for various platform migrations and
virtualization without the use of software agents.
MAP’s inventory and readiness assessment re-
of a ROI/TCO Calculator from VMWare [6]
ports generate specific upgrade recommendations
for migration to Windows Vista and Windows
Server 2008 operating systems and also for virtualization. It provides recommendations on how
physical servers can be consolidated in a Microsoft Hyper-V virtualized environment. In addition
the Microsoft Integrated Virtualization ROI Tool
supports the calculation of potential power cost
savings with Hyper-V prior to deployment. The tool
provides support for examining current production
and development servers, desktop and application
virtualization opportunities by quantifying potential savings, service level benefits, investments and
The TCO/ROI methodology offered by VMware
(available as an online tool) allows to compare
TCO savings, required investments and business
benefits of virtualization solutions. It is based on
standard financial techniques, VMware field and
customer data and user metrics. Based on userspecific data, key figures like savings, investments,
ROI, NPV savings, TCO opportunities and payback
periods are calculated. Where specific user data is
not available, statistical data from industry is provided and may be used for calculations.
2.4.3 Power management in virtualized
environments – virtual server migration
Current software solutions for server virtualization
support the migration of virtual machines and a
temporary shut-down of hosts to reduce power
demand. One example providing such features
is VMwareVsphere4 (Distributed Power Management DPM). DPM monitors the resource use of the
running virtual machines in the cluster. If there is
excess capacity, DPM recommends to move some
virtual machines between hosts and to put some
hosts into standby mode to save power. In case
of insufficient capacity, DPM powers on standby
hosts again.
Power management can be operated in either
manual or automatic mode. In automatic mode
virtual machines are migrated and hosts are
moved into or out of standby mode automatically.
Automatic settings can be overridden on a perhost basis and power management can also be
enabled by a scheduled task.
The goal of VMware DPM is to keep the utilisation
of ESX hosts in the cluster within a target range.
DPM must meet the following requirements to be
an effective power saving solution:
•Accurate assessment of workload resource demands. Overestimating can lead to less than
ideal power savings. Underestimating can result in poor performance and violations of DRS
resource level SLAs.
•Avoiding powering servers on and off too frequently even if running workloads are highly
•Rapid reaction to sudden increase in workload
demands so that performance is not sacrificed
when saving power.
•Selection of the appropriate hosts to power on
or off. Powering off a larger host with numerous virtual machines might violate the target
utilisation range on one or more smaller hosts.
• Redistribution of virtual machines intelligently
after hosts are powered on and off by seamlessly leveraging DRS.
Effectively assessing and selecting virtualization solutions:
•Develop a virtualization strategy and assess servers to select good candidates for virtualization.
• Assess requirements regarding CPU performance, memory, Disk I/O intensity, Network requirements,
OS configuration.
• Consider the appropriate virtualization ratio and mix of workloads (1:6 to 1:20 depending on workload characteristics).
•Check products from different suppliers regarding required features for your specific purposes; consider licensing policies, power management features and price. The different main products on the
market have different advantages depending on the specific application needs.
•Do TCO and ROI calculations to identify the benefits of reduced cost for power supply and cooling.
Models provided by suppliers shall be refined according to the needs of the specific organisation.
•Consider power management options allowing VM migration and temporary shut-down of server
•Consider changed requirements for cooling and power supply (reduced and dynamically changing
power and cooling demand) and check options of some redesign for cooling.
2 Server Equipment
The basic way to use DPM is to power on and
shut down ESX hosts based on typical utilisation
patterns during a workday or week. For example,
services such as email, fax, intranet, and database
queries are used more intensively during typical
business hours from 9 a.m. to 5 p.m. At other
times, utilisation levels can dip considerably, leaving most of the hosts underutilised. Their main
work during these off hours might be performing
backup, archiving, servicing overseas requests etc.
In this case consolidating virtual machines and
shutting down unneeded hosts reduces ­
The following approaches may be used to manually adjust DPM activity:
•Increasing the Demand-Capacity-Ratio Target:
to save more power by increasing host utilisation (consolidating more virtual machines onto
few hosts) the value for the Demand-CapacityRatioTarget could be increased from default
(e.g. from 63% to 70%).
•Using VMware DPM to force the powering on of
all hosts before business hours and then selectively shut down hosts after the peak workload
period. This is a more proactive approach that
would avoid any performance impact of waiting
for VMware DPM to power on hosts in response
to sudden spikes in workload demand.
Each ESX host’s resource utilisation is calculated
as demand/capacity for each resource (CPU and
memory) where demand is the total amount of the
resource needed by the virtual machines currently
running and capacity is the total amount of the resource currently available on the host. Thus power
management of hosts is executed depending on
CPU and host’s memory resource utilisation compared to the defined utilisation range. For each
host evaluated for a power-off recommendation
DPM compares costs, taking into account an estimate of the associated risks with a conservative
projection of the power-savings benefit that can
be obtained.
Constant loads > Stable cooling
Fig. 2.10 Heat
2.4.4 Cooling and infrastructure for virtualized systems
While significantly reducing overall power demand, virtualization especially in larger systems
may cause increased rack power density. Power
management by migration of virtual machines
furthermore leads to dynamic spatial change of
power and heat density, thus locally increasing
the demand for power and cooling. Appropriate
power and cooling concepts have to be used to
meet the demand of virtualized environments and
to avoid hot spots.
If the total power and cooling capacity is not
adapted to the lower power demand, PUE will
worsen after virtualization. Virtualization can
reduce cooling load in a data centre to very low
levels which can cause negative effects. Thus rightsized power and cooling is crucial for exploiting
energy saving potentials. It is also essential to
reduce fixed losses by considering the following
•Scaling down power and cooling capacity to
match the load
•VFD fans and inverter pumps that are controlled
by cooling demand
•Using equipment with higher efficiency
•Cooling architecture involving shorter air paths
(e.g. row based)
•Capacity management system to adopt capacity
to demand
•Blanking panels to reduce in-rack air mixing
Migrating high-density loads > Unpredictable cooling
density before and after virtualization [5]
In a conventional environment involving tradi­
tional raised floor, room-based cooling can be
configured to adequately cool hot spots by rearranging vented floor tiles. Changing requirements
due to the dynamic migration of virtual servers,
however, also require dynamic cooling solutions.
A solution to this challenge is to position cooling
units within the rows and get them equipped to
sense and respond to temperature changes. Placing cooling units close to the servers allow short
air paths between cooling and the load. Dynamic
power variation in virtualized environments is a
major reason for moving towards row- or rackbased cooling.
Energy efficient management of virtualized systems:
• Implement a strict policy for implementing and managing virtualised servers. Avoid uncontrolled
server sprawl.
• Use virtual machine migration tools to shut down hardware at times of low loads. Use automatic
power management settings for the start and develop own customised settings in a subsequent
stage based on the typical operation patterns.
•Reduce cooling according to demand and implement equipment for dynamic local cooling if
needed. Address the demand for dynamic spatial changes.
•Adapt IT processes and workflows regarding deployment of virtual machines, data recovery/
backup processes, patch administration, availability considerations.
Accurate information about demand for power
and cooling capacities is crucial in order to respond to changing load profiles over time. Capacity management provides instrumentation for real
time monitoring and analysis of power-, coolingand physical space capacities and enables the effective and efficient use throughout the data centre. Areas of available or dangerously low capacity
can be identified. Capacity management systems
should be able to handle the following issues:
•Change of load density and location – Virtual-
ization can create hotspots e.g. by VM migration.
•Dynamic system changes – Maintaining system
stability may become a challenge if multiple
parties are making changes without centralized
•Interdependencies – Virtualization makes the
shared dependencies and secondary effects in
the relationship between power, cooling and
space capabilities more complex.
•Lean provisioning of power and cooling –
During virtualization the power and cooling
load goes down and rises again as new virtual
machines are created. This can be handled by
usage of scalable power and cooling systems.
2 Server Equipment
Further reading
HP (2011):HP Power capping and HP Dynamic power
capping for ProLiant servers. Hewlett Packard
­Development company.
SPEC (2011): Server Efficiency Rating Tool (SERT)
TM Design Document. 3rd draft. Standard Performance Evaluation Cooperation
Rasmussen, N. (2010): Strategies for deploying
blade servers in existing data centres. White paper
125. APC Schneider Electric
80 PLUS (2011): 80 PLUS power supplies.
[1] EPA (2010): Energy Star ENERGY STAR®
Program Requirements for Computer Servers (vers
[2] SPEC (2010): SPEC power and performance.
Benchmark methodology 2.0. Standard Performance Evaluation Cooperation
[3] 80 PLUS (2011): 80 PLUS power supplies.
Schäppi B. et al (2009) Energy and cost savings
by energy efficient servers. IEE E-Server best practice cases. Brochure 2009
IBM (2011) Server Management suite, Module
Active Energy Manager
HP (2011) Server managment suite «Systems Insight Manager»
VMware DPM: Information Guide: VMware Distributed Power Management Concepts and Use.
VMware TCO: VMware ROI TCO Calculator,
Overview and Analysis.
[4] Schäppi B. et al (2009): Energy and cost
savings by energy efficient servers. IEE E-Server
best practice cases. Brochure 2009
[5] BITKOM (2010): Bitkom/Beschaffungsamt
des Bundesministeriums des Innern, Leitfaden
Produktneutrale Leistungsbeschreibung x86-Server, 2010
[5] Comtec Power: Overcoming the Challenges
of Server Virtualization.
[6] VMware TCO: VMware ROI TCO Calculator,
Overview and Analysis.
[7] The Green Grid (2010): White paper Nr. 33
“A roadmap for the adoption of power-related
features in servers”, Pflueger, J., et al., The Green
Grid, 2010
3 Data Storage Equipment
Marcos Dias de Asuncao, Laurent Lefevre, INRIA
Information is at the core of any business, but storing and making available
all the information required to run today’s businesses has become a real
challenge. With the storage needs of organisations expected to grow by a
factor of 44 between 2010 and 2020 [1], strategies for high efficiency have
never been so popular. The constant fall in the price per MB of storage led
to a scenario where it is simpler and less costly to add extra capacity than
to look for alternatives to avoid data duplicates and other inefficiencies.
However, as the cost of powering and cooling storage resources becomes
more of an issue, inefficiencies are no longer accepted. Studies show that
large enterprises are currently faced with the difficult task of providing
sufficient power and cooling capacity, while midsize companies are challenged with finding enough floor space for their storage systems [2]. As
data storage accounts for a large part of the energy consumed by data
centres, it is crucial to make storage systems more energy efficient and to
choose the appropriate solutions when deploying storage infrastructure.
This chapter discusses a few technologies that support the energy efficiency of data
storage solutions. Moreover, it provides recommendations for best practice that, in
addition to the use of the discussed solutions, can improve the energy efficiency of
storage infrastructure in enterprises and data centres.
Storage solutions such as disk arrays include drives that provide the raw storage capability and additional components to interface with the raw storage and improve overall
reliability. We refer to the individual medium components that form the raw storage as
devices (e.g. tape loaders, hard disk drives and solid state drives). Composite storage
solutions such as network attached products are referred to as storage elements. When
discussing schemes for improving the energy efficiency of storage solutions, these are
mainly the two levels at which most techniques apply. Hence, we first present energy
efficient concepts for individual devices, and then analyse how these techniques are
used and combined to improve the energy efficiency of elements.
3.1 Storage Devices
3.1.1 Tape based systems
Tapes are often mentioned as one of the most
cost-efficient types of media for long-term data
storage. However, analyses [3][4] indicate that:
•Under given long-term storage scenarios, such
as backup and archival in mid-sized data centres, hard disk drives can be on average 23
times more expensive than tape solutions and
cost 290 times more than tapes to power and
•Data consolidation using tape-based archival
systems can considerably decrease the opera­
tional cost of storage centres. Tape libraries
with large storage capacity can replace islands
of data via consolidation of backup operations,
hence reducing costs with infrastructure and
possibly increasing its energy efficiency.
With an archival life of 30 years and large storage
capacity, tapes are an appealing solution for data
centres with large long-term backup and archival
requirements. Hence, for an environment with
multiple tiers of storage, tape-based systems are
still the most power-efficient solutions when considering long-term archival and low retrieval rate
of archived files. There are disk library solutions
that attempt to minimise the impact of the energy
consumption of disk drives by using techniques
such as disk spin-down. These technologies are
further discussed below.
Actuator Axis
Actuator Arm
Power Connector
Jumper Block
Fig. 3.1
HDD components
IDE Connector
3.1.2 Hard Disk Drives (HDDs)
HDDs have long been the preferred media for
non-volatile data storage that offers fast write
and retrieval times. Moving parts such as motors
and actuator arms account for most of the power
that HDDs consume (see Figure 3.1). To improve
data throughput of HDDs, manufacturers increase
the speed at which platters rotate, thus further
increasing their power consumption. Platters spinning at speeds of 15K RPMs are common for current high-throughput HDDs.
Several techniques are used to improve the energy efficiency of HDDs, including storing data
in certain regions of the platters to reduce the
Tab. 3.1 PowerChoice
­ echanical effort when retrieving data, controlm
ling the rotation speed of the platters, and reducing the power consumption during idle periods.
A common technique, termed as disk spin-down,
consists in spinning platters down and parking the
heads at the secure zone after a factory-set period
of inactivity. Moreover, instead of stopping platters completely, some drives spin the platters at
variable speed according to the read/write load.
platters are spun down). Seagate‘s PowerChoice
technology [5] is an example, where the number
of disabled components increases as the drive
reaches certain idleness thresholds. The intermediate idle states have recovery times that are generally shorter than recovering from a disk spundown situation.
Table 3.1 shows that the consumption in standby
is about 50% less than the idle consumption.
Such approaches may allow substantial savings
on RAID systems and Massive Arrays of Idle Disks
As spinning disks down can compromise performance, manufacturers explore additional
Some HDDs implement multiple idle and standby
states. Different actions are taken as the period
of inactivity increases (e.g. initially the servo system is disabled, then heads are parked, and later
technology profile for a Constellation 2.5-Inch drive
Power (W)
Power Savings* (%)
Recovery Time (sec.)
Default Timer to Enter
1 sec.
10 min.
30 min.
60 min.
* Power savings estimates and recovery times are preliminary; figures based on Seagate Constellation SAS 2.5-inch hard drive.
techniques such as larger cache sizes and read/
write command queuing. Furthermore, to benefit
from techniques such as spin-down and variable
spinning speed, schemes have been proposed at
the operating system and application levels to
increase the length of periods of disk inactivity.
Some of these approaches consist of rescheduling
data-access requests by modifying the application
code or data layouts. There are also less intrusive
techniques that provide compiler customisations
that re-schedule the data access requests at compilation without the need of modifying the application source code. Although these techniques
can reduce power consumption, it has also been
argued that frequent on-off cycles may reduce the
life time of HDDs.
As motors and actuators are responsible for most
power consumed by hard disk drives, an approach
for making drives more energy efficient is to use
Small Form Factors (SFFs). As 2.5-inch HDDs are
around a quarter the size of larger hard-disk drives
(3.5-inch, see Fig. 3.2), a chassis designed with
enough volume for 16 3.5-inch drives might be
redesigned to hold up to 48 2.5-inch hard-disk
drives without increasing the overall volume. Highperformance hard drives in 2.5-inch e­nclosures
show reduced power consumption as motors and
actuators are smaller and thus also emit less heat.
Manufacturers claim that for Tier-1 2.5-inch hard
disk drives, IOPS/W can be up to 2.5 times better than comparable 3.5-inch Tier-1 drives [6]. In
­addition, less power is required for cooling due
to smaller heat output and reduced floor space
Fig. 3.2 Picture
of a 2.5-inch HDD atop a 3.5-inch HDD (from wikipedia)
Table 3.2 shows the approximate power consumed by two models of high performance hard
disk drives produced by Seagate. It is evident that
the smaller form factor takes substantially less
power. When active, it consumes approximately
46% less power than its 3.5-inch counterpart,
whereas this difference can reach 53% when the
disk is idle. Considering the cost to power only
Tab. 3.2 Power
24 drives over a year, based on active power consumption and a price of 0.11€ per KWh, the difference between 3.5-inch drives and 2.5-inch HDDs
would be about 140€ per year. In data centres
with storage systems with hundreds or thousands
of disks, the savings can amount to thousands or
ten thousands of Euros.
consumption of two of Seagate‘s high performance HDDs
Form Factor
Cheetah 15K.7 300GB* Savvio 15K.2 146GB*
SAS 6Gb/s
SAS 6Gb/s
Spindle Speed (RPM)
Power Idle (W)
53% less
Power Active (W)
46.2% less
* Data obtained from the specification sheets available at the manufacturer’s website.
3 Data Storage Equipment
3.1.3 Solid State Drives (SSDs)
SSDs are equipped with, among other components, flash memory packages and a controller
responsible for various tasks. SSDs rely on NANDbased flash memory that employs one of two
types of memory cells according to the number of
bits a cell can store. Single-Level Cell (SLC) flash
stores one bit per cell and Multi-Level Cell (MLC)
memories can often store 2 or 4 bits per cell. Most
of the affordable SSDs rely on MLC while high-end
devices are often based on SLC.
Tab. 3.3 Comparison
of Seagate‘s Pulsar enterprise SSDs and Savvio 15K HDDs
Savvio 15K.2 73GB*
Pulsar SSD 50GB*
SSDs are more energy efficient and reliable due to
the lack of mechanical parts such as motors and
actuators. Moreover, they create less heat and can
be packed into smaller enclosures, thus decreasing the floor space and cooling requirements.
Table 3.3 presents a simple comparison between
a Seagate‘s Pulsar enterprise SSD and a high performance SAS 15k-RPM HDD. The SSD consumes
approximately 87% less power than the 15k-RPM
HDD in active mode, and around 82% less in idle
mode. In practice, however, the energy savings will
depend on how the storage solutions use the SSDs
and HDDs and the characteristics of the workload
applied to the storage equipments.
Form Factor
3.1.4 Hybrid Hard Drives (HHDs)
SAS 3Gb/s
SAS 6Gb/s
HHDs are HDDs equipped with large buffers made
of non-volatile flash memories that aim to minimise data writes or reads on the platters. Several
algorithms have been used for using the buffer
[7]. By providing a large buffer, the platters can
remain at rest for longer periods. This additional
flash memory can minimise the power consumed
by storage solutions by reducing the power consumed by the motors and mechanical arms. These
drives can present potentially lower power requirements when compared to HDDs, but the
­offerings for enterprise storage are limited.
Spindle Speed (RPM)
SATA 3Gb/s
53% less
Power Idle (W)
82.4% less
Power Active (W)
87% less
NAND Flash Type
* Data obtained from the specification sheets available at the manufacturer’s website.
Consider advantages of different storage technologies in procurement and system
•Tapes have the best energy efficiency for long-term storage.
• Current hard disk drives have platters that can rotate at various speeds thus saving energy at
lower speeds.
•The multiple idle states implemented by HDDs allow considerable energy savings when employed in composite storage solutions such as disk arrays and massive arrays of idle disks.
• Although more expensive, SSDs are much more efficient than HDDs.
•Consider using SSDs as a high performance storage layer.
3.2 Storage Elements
This section presents device-level techniques that
may be used and combined to improve the energy
efficiency of composite storage solutions such as
disk arrays, direct attached storage and network
storage (i.e. storage elements). Concepts specific
to the storage-element level are also analysed.
3.2.1 Large Capacity Drives and small form
For applications that do not demand high-performance storage it is usually more energy efficient
to use drives with larger capacity. Typical SATA
disk drives consume up to 50% less power per
terabyte of storage than Fibre Channel drives [8].
As discussed beforehand, SSF enclosures can
save floor space in data centres and decrease the
energy footprint by using more power-efficient
2.5-inch HDDs. As an example, using the industry-standard Storage Performance Council (SPC)
SPC-1C benchmark, Dell compared two of its disk
arrays, one with 3.5-inch HDDs and another with
2.5-inch HDDs [9]. Results showed that in addition
to providing 93% higher performance than the array with 3.5-inch drives, the array equipped with
2.5-inch drives consumed 40% less energy.
3.2.2 Massive Arrays of Idles Disks (MAIDs)
MAID is a technology that uses a combination of
cache memory and idle disks to service requests,
only spinning up disks as required. Stopping spindle rotation on less frequently accessed disk drives
can reduce power consumption (see Figure 3.3).
All disks spinning
full-speed; high
performance but
no power saving
How much power MAID features can save depends on the application that uses the disks and
how often the disks are accessed. The criteria used
to decide when drives are spun down (or put into
standby mode) or spun up, have an impact on
energy savings as well as on performance. When
initially conceived, MAID techniques enabled
HDDs to be either on or off, which could incur
considerable application performance penalties if
data on a spun-down drive was required. However
second generation MAID techniques shall allow
Intelligent Power Management (IPM) with different power saving modes and performance. MAID
2.0 as it is often called has multiple power saving
modes that align power consumption to different
QoS needs. The user can configure the trade-off
between response times and power savings. The
multiple power saving modes use for example the
different HDD idle states described beforehand.
Other power conservation techniques for disk
­arrays are the Popular Data Concentration (PDC)
[10] and other file allocation mechanisms [11].
The rationale behind this approach is to perform
consolidation by storing or migrating frequently
accessed data to a subset of the disks. By skewing
the load towards fewer disks, others can be transitioned to low-power consumption modes.
3.2.3 Efficient RAID Levels
25% disks spun
down; up to 25%
power saving but
some performance penalty
Different RAID levels provide different storage efficiency. When considering data protection, some
RAID levels such as RAID 6 present a significant
amount of overhead processing. However high
performance RAID 6 implementations can provide
the same performance as RAID 5 and up to 48%
reduction in disk capacity requirements compared
to RAID 10.
Fig. 3.3 Pictorial
Fujitsu, for example, allows customers to specify
schedules with periods during which the drives
should be spun down (or powered off) according
to the workload or backup policies.
view of MAIDs
3 Data Storage Equipment
Physical disk pool (2TB)
(Actual disk capacity available)
Virtual volume (10TB)
(Disk capacity recognised by server)
Data write
Fig. 3.4 Thin
provisioning (from Fujitsu ETERNUS solutions)
3.2.4 Horizontal storage tiering, storage
virtualization and thin provisioning
For efficient use of storage infrastructure, it is
important to design and enforce sound data
­management policies that use different tiers of
s­torage according to how often the data is accessed, whether it is reused and for how long it
has to be maintained (for business or regulatory
purposes). Manufacturers of data storage solutions have proposed software systems that allow
for seamless and automatic tiering by moving data
to the appropriate tier based on ongoing performance monitoring. Examples are EMC2’s Fully
Automated Storage Tiering (FAST), IBM’s System
Storage Easy Tier, Compellent‘s Data Progression
and SGI’s Data Migration Facility (DMF).
By combining server virtualization with storage
virtualization it is possible to create disk pools and
virtual volumes whose capacity can be increased
on demand according to the applications‘ needs.
Typical storage efficiency of traditional storage
arrays is between 30–40%. According to certain
reports [12], storage virtualization may increase
efficiency to 70% or higher reducing storage requirements and increasing energy savings.
Storage tier virtualization, also known as Hierarchical Storage Management (HSM), allows data
to be migrated automatically between different
types of storage without users being aware of it.
Software systems for automated tiering are used
for carrying out such data migration activities. This
approach can reduce cost and power consumption as it allows only data that is frequently accessed to be stored on high-performance storage,
while data less frequently accessed can be placed
on less expensive and more power efficient equipment that use techniques such as MAID and data
Thin provisioning, a technology that generally complements storage virtualization, aims to
maximise storage utilisation and eliminate preallocated but unused capacity. With thin provisioning, storage space is provisioned when data
is written. Reserve capacity is not defined by the
maximum storage required by applications but it is
generally set to zero. Volumes are expanded online
and capacity is added on the fly to accommodate
changes without disruption (see Figure 3.4). Thin
provisioning can lead to energy savings because
it reduces the need for over provisioning storage
capacity to applications.
3.2.5 Consolidation at the storage and
fabric layers
Storage consolidation is not a recent topic as Storage Area Networks (SANs) have been providing
some level of storage consolidation and improved
efficiency for several years by sharing arrays of
disks across multiple servers over a local private
network, avoiding islands of data. Moving direct
attached storage to network storage systems offers a range of benefits, which can increase the
energy efficiency. Consolidation of data storage
equipments can lead to both substantial savings
in floor space requirements and energy consumption. Some manufacturers argue that by providing
multi-protocol network equipment, the network
fabric can be consolidated on fewer resources,
hence also reducing floor space, power consumption and cooling requirements.
3.2.6 Data de-duplication
Storage infrastructures often store multiple copies of the same data. Several levels of data duplication are employed in storage centres, some
required to improve reliability and data throughput. However, there is also „waste“ that can be
minimised, hence recycling storage capacity. Current SAN solutions employ data de-duplication
(de-dupe) techniques with the aim to reduce data
duplicates. These techniques work mainly at the
data-block and file levels.
In addition to the level of data de-duplication, deduplication techniques also differ depending on
when the data de-duplication is performed: before
or after data is stored on disk. Both techniques
have advantages and shortcomings. Although
leading to reduced storage-media requirements,
de-duplication after the data is stored on disk
requires cache storage that is used for removing
duplicates. However, for backup applications, deduplication after storing the data usually leads to
shorter backup windows and smaller performance
degradation. Moreover, data de-duplication techniques differ on where data de-dupe is carried out:
at the source (client) side, target (server) side, or
by a de-duplication appliance connected to the
As data de-duplication solutions enable organisations to recycle storage capacity and reduce media
requirements, they are also considered a common
approach to reduce power consumption. The actual storage savings achieved by data de-duplication
solutions vary according to their granularity. Solutions that perform hashing and de-duplication
at the file-level tend to be less efficient. However,
they pose a smaller overhead. With the block-level
techniques, the efficiency is generally inversely
proportional to the block size.
Although data de-duplication is a promising technology for reducing waste and minimising energy
consumption, not all applications can benefit from
it. For example, performing data de-duplication
before the data is stored on disk, could lead to
serious performance degradation, which would be
unacceptable for database applications. Applications and services that retain large volumes of
data for long periods benefit more from data deduplication. The more data one organisation has
and the longer it needs to keep it, the better results data de-duplication technologies will yield. In
general, data de-duplication works best for data
backup, data replication and data retention.
3 Data Storage Equipment
Further Reading
McClure T. (2009): Driving Storage Efficiency
in SAN Environments, Enterprise Strategy Group White Paper, November 2009.
Craig B. and McCaffrey T. (2009): Optimizing
Nearline Storage in a 2.5-inch Environment Using
Seagate Constellation Drives, Dell Power Solutions, Jun. 2009.
SNIA (2010): Storage Power Efficiency Measurement Specification: Working Draft Version 0.2.10,
SNIA Green Storage Initiative, August 2010.
Storage Tiering with EMC Celerra FAST, EMC2
[1] IDC (2010): The Digital Universe Decade –
Are you ready? IDC, May, 2010.
[2] McClure T. (2009): Driving Storage Effi­
ciency in SAN Environments, Enterprise Strategy
Group - White Paper, November 2009.
[3] Reine D. and Kahn M. (2008): Disk and
Tape Square Off Again – Tape Remains King of the
Hill with LTO-4. Clipper Notes, February 2008.
[4] ORACLE (2010): Consolidate Storage Infrastructure and Create a Greener Datacentre. Oracle
White Paper, April 2010.
[5] Seagate (2011): PowerChoice Technology
Provides Unprecedented Hard Drive Power Savings and Flexibility - Technology Paper, Seagate,
[6] Seagate (2010): Seagate Savvio 15K.2
Data Sheet, Seagate, 2010.
[7] Bisson T., Brandt S., Long D. (2006):
NVCache: Increasing the Effectiveness of Disk
Spin-Down Algorithms with Caching, 14th IEEE
International Symposium on Modeling, Analysis,
and Simulation, pp. 422-432, 2006.
[8] Freeman L. (2009): Reducing Data Centre
Power Consumption Through Efficient Storage.
White Paper. NetApp, July 2009.
[9] Craig B. and McCaffrey T. (2009): Optimizing Nearline Storage in a 2.5-inch Environment
Using Seagate Constellation Drives, Dell Power
Solutions, Jun. 2009.
[10] Pinheiro E. and Bianchini R. (2004):
Energy Conservation Techniques for Disk ArrayBased Servers. 18th Annual International Conference on Supercomputing (ICS 2004), pp. 68-78.
Malo, France, 2004.
[11] Otoo E. D., Rotem D. and Tsao S.C.
(2009): Analysis of Trade-Off between Power Saving and Response Time in Disk Storage Systems,
IEEE International Symposium on Parallel Distributed Processing (IPDPS 2009), pp. 1-8, May 2009.
[12] Blade Network (2009): Storage Consolidation for Data Centre Efficiency, BLADE Network
Technologies White Paper, Jun. 2009.
Clark T. and Yoder A. (2008): Best Practices for
Energy Efficient Storage Operations Version 1.0,
SNIA Green Storage Initiative, October 2008.
Freeman L. (2009):Reducing Data Centre
Power Consumption Through Efficient Storage.
White Paper. NetApp, July 2009.
4 Network Equipment
Alexander Schlösser, TU Berlin, Lutz Stobbe, Fraunhofer IZM
According to current information, the energy consumption allocated to
switches, routers, and other networking equipment is approximately 8%
to 12% of the total data centre’s energy footprint. Due to this rather low
percentage of the total energy demand, networking equipment has not
been the focus of improvement measures. Nonetheless, this perception and
situation is changing, particularly in medium and large sized data centres.
There are a couple of reasons why the power consumption of networking
4.1 Technical and operational
equipment and the energy effects of the implemented network architec-
4.1.1 Functional model
ture are now becoming significant considerations in the design and operation of data centres.
With increasing quality-of-service (QoS) requirements in conjunction with
delay-critical applications, the functional importance of networking equipment and networks in data centres is growing. The power consumption
varies according to the selected technology and architecture including
­cabling, power supply, and cooling.
Figure 4.1 provides a simplified functional model
of energy-related aspects with respect to networks
and network equipment in data centres. The functional model helps to visualize the overlapping
aspects of the power and cooling infrastructure as
well as the interrelation of the network with the
main IT equipment including server and storage
systems. The model also outlines the main elements for improvement at the network level. This
includes the selected network architecture and
actual topology, the physical infrastructure, hardware components and cable as well as software
configuration and virtualization capability.
The energy efficiency of network infrastructure
and networking equipment is also influenced by
the applications, service level agreements, bandwidth and latency performance requirements that
have been defined by the operator of the data
centre. These performance-related aspects have to
be considered in the planning process to improve
energy efficiency.
Server Room / Data Center
Architecture & Topology
Power Supply & UPS
Virtualization & Configuration
Cooling & Air Flow
Components & Cabling
Monitoring & Control
Fig. 4.1: Data
centre networks functional model
4.1.2 Network attributes
The improvement of energy efficiency with respect
to the network infrastructure in data centres requires a structured approach. Planning should incorporate a strategic or long-term perspective due
to the fact that networking infrastructure is typically somewhat longer in place. It is assumed that
the basic network infrastructure is used for more
than 8 years. Changing the basic network architecture and actual topology including equipment
etc. is a considerable investment and risk factor.
Nevertheless, the improvement of the network
not only increases the performance characteristics
of the data centre but in many cases the energy
efficiency as well. The planning for improvement
starts with a strategic analysis.
The data centre operator needs to define network
attributes and performance requirements. This task
should include a market analysis. The IT world is
currently (2011) experiencing a tremendous shift
towards a centralized production of applications
resulting in new traffic volumes and patterns. In
other words, applications are not produced at the
end-user side with considerable computing power
and software packages. By utilizing Software-asa-Service (SaaS) and Cloud Computing, applications and traffic is produced in data centres and
data centre clouds. A necessary condition for this
trend is broadband connectivity and low latency.
This general trend leads not only to increased data
traffic between client and server, but also to an
increased server to server and storage to server
data flow. Enterasys [1] indicates in this respect
that the network architecture and configuration
will change in order to support the growing server
to server and storage to server traffic. In order to
increase performance (IT productivity), the technical trends are to aggregate networking (bottomup) and virtualized networking (top-down).
The network architecture will consist of fewer
tiers by merging access and aggregation as well
as aggregation and core network to some extent
(see also Figure 4.3). This trend has the potential
to reduce energy consumption due to unified networking. However, this is a balancing act. Little
information and data is available, and there is not
a single solution visible on the market.
The virtualization will also spread further incorporating network equipment and local area networks
(VLAN). Virtualization has the advantage of consolidating physical equipment and has therefore
also the potential of increased energy efficiency.
According to Enterasys [1], the common design
goals for data centres networks include:
•Bandwidth and low latency (selection of network technology)
•Scalability and agility (network architecture)
•Flexibility to support various services (this aspect is addressing consolidation)
•Security (increasingly important and influencing
availability and redundancy (quality of
­service requirements)
•Manageability and transparency (this aspect is
supported by virtualization solutions)
•Costs optimization (the objective is always
­lower CAPEX and OPEX)
4.1.3 Balancing network performance and
energy consumption
Bandwidth, high-speed, low latency, and lossless
traffic are important network performance ­criteria.
Customer satisfaction or what is called ­Quality
of Service (QoS) is an additional performance
require­ment. QoS is defined in terms of Service
Level Agreements (SLA) with characteristics such
as minimal throughput, maximal response time or
latency time. A responsive, converged, and intelligent network architecture capable of managing
traffic dynamically to agreed SLAs, is not only important for future competitiveness, it might also
define the basis for a systematic energy efficiency
However, implementing QoS can actually increase
the total network traffic and respective energy
consumption of the data centre. Individual network technologies and respective equipment fea-
Due to the fact that there is a large variety of products and network options available on the
market, it is recommended that data centre operators or IT administrators develop a priority list
with respect to network attributes, such as:
•network services,
•latency requirements,
•quality of service,
•virtualization support and
•other performance or interoperability aspects.
The best approach is a system optimization. It reflects the interaction of the network
infrastructure and performance with the other IT-equipment and support infrastructure.
ture advantages and disadvantages in this respect.
As a general trend, it has been observed that 10
Giga­bite Ethernet networking (10GbE) is becoming the technology of choice in data centres.
­Ethernet is not only linking servers (LAN), it is increasingly applied in storage networks (SAN).
Low latency and lossless networks are however
basic requirements for storage traffic. According
to Lippis (2011) [2] today’s 10GbE, switches produce 400 to 700ns of latency. By 2014, it is anticipated that 100GbE switching will reduce latency
to nearly 100ns. This shows that with increasing
bandwidth the latency improves. From an energy
consumption point of view, it is necessary to balance latency improvement (network technology)
with potentially higher power consumption of the
high bandwidth capacity (component). Component selection and I/O consolidation are aspects
that have to be addressed in that respect.
Similarly it is necessary to investigate lossless
networking (availability) versus bandwidth performance and subsequent energy efficiency. For
instance, lossless networking usually means more
complex protocols (overhead) and additional latency in conjunction with more processing power
and less bandwidth efficiency. Lossless networking
is however a necessary precondition for storage
area networks. In the past, (lossy) Ethernet was
the hindrance for its application in the storage
area network.
Fiber Channel (FC) and Infiniband (IB) were the
most common network technologies. However,
today multiple storage network options exist
based on available Ethernet such as Converged
Enhanced Ethernet (CEE), Fibre Channel over
Ethernet (FCoE), Internet Small Computer System
Interface (iSCSI) over Ethernet, ATA over Ethernet
(AoE) and Network-attached Storage (NAS). These
options help to unify the networking (and avoids
additional adapters), however create additional
overhead which results in less bandwidth efficiency. The energy efficiency tradeoffs (if they exist) are
currently unknown.
In conclusion, the operator must consider the energy impacts of increased performance, scalability,
and adaptability of new consolidated network solutions. It is likely that positive energy tradeoffs result from new solutions. But proper dimensioning
is essential. It is recommended that operators who
purchase new equipment or complete network solutions ask for the overall energy impact / tradeoff
resulting from a new solution.
4.2 Improvement of energy efficiency
4.2.1 Merging traffic classes
(I/O consolidation)
Data centre networks have to transmit different
types of traffic within different types of application areas. This has led to specialized protocols
and network architectures. As a result, considerably complex networks often do not share their
resources. The basic improvement objective is the
physical reduction of components and the sharing
of network capacity by different functional units.
The general technical trend toward simpler, fewer
tiers, and I/O converged networking based on Ethernet is also driven by energy efficiency considerations. The overall topic is network consolidation. It
addresses server and storage networks as well as
the network distribution architecture. Convergent
Network Adapter (CAN) is merging former separate interfaces:
•Host Bus Adapter (HBA) in support of SAN
•Network Interface Controller (NIC) in support of
LAN traffic
•Host Channel Adapter (HCA) in support of IPC
I/0 consolidation is the capability of a switch or a
host adapter to use the same physical infrastructure to carry multiple types of traffic, each typically
having unique characteristics and specific h­ andling
requirements. From the network side, this equates
to having to install and operate a single network
instead of three as shown in Figure 4.2.
From the hosts and storage arrays side, this
equates to having to purchase fewer Converged
Network Adapters (CNA) instead of Ethernet
NICs, FC HBAs, and IB HCAs. A typical Fibre Channel HBA consumes about 12.5 W [3]. In terms of
network redundancy, an adequate consideration
of several options has to be regarded to design
­reliable networks.
4 Network Equipment
Inter Processor
Local Area
EN Switch
FC Switch
EN Switch
Fibre Channel/
10G / 40G / 100G Ethernet
Fig. 4.2
I/O consolidation and network convergence in data centre networks
I/O consolidation will enable the consolidation of different network types (LAN, SAN) at a higher
level as a preparatory measure for system virtualization. Furthermore, it will significantly reduce
the amount of physical infrastructure including switches, ports, connections and cables among
different networks. Converged networks will result in:
•Up to 80% decreased adapters and cables
•Up to 25% reduction in switches, adapters and rack space
•Up to 42% reduction of power and cooling costs [4]
4.2.2 Network consolidation
The main approach to optimize the power consumption of the data centre network includes
new network architectures and the convergence
of formerly separated networks (into a single technology). A typical architecture consists of a tree of
routing and switching equipments (multiple tiers/
layer) with more specialized and expensive equipment on the top of the network hierarchy. The goal
should be the consolidation of the network infrastructure by creating a flat network architecture
based on a functional network fabric.
Measures that can be taken include:
•Aggregate switches. Multiple physical switches
that operate in one logical device.
•Reduce tiers (layers). Use an aggregated switch
to do the work of multiple switch layers. Considered network services and security.
unified network fabric. This combines
the two approaches and allows operational
simplicity and high performance. Again, considered network services and security.
The convergence of server (LAN) and storage
(SAN) networks is a general trend with energy saving potential. Maintaining two separate networks
would increase the overall operation costs and
power consumption by multiplying the number
of adapters, cables, and switch ports required to
connect every server directly with supporting LANs
and SANs. To simplify or flatten the data centre
network structure, converged networking technologies such as iSCSI, Fibre Channel over Ethernet
(FCoE), and Data Centre Bridged (DCB) are currently being implemented in data centres.
Fig. 4.3 Network
4 Network Equipment
4.2.3 Network virtualization
Virtualization is a well established technology
to consolidate physical server with multiple virtual machines. Network virtualization follows the
same principle and describes various hardware
and software approaches to manage network
resources as logical units independent of their
physical topology. This results in reduced network
traffic, simplified security and improved network
control. Key elements for high efficient networks
are network level awareness and visibility of the
virtual machine (VM) lifecycle. The ability to configure network and port level capabilities at the
individual VM level as well as dynamically tracking
VMs as they move across the data centre are important for an efficient management of virtualized
environments. Energy efficiency is mainly achieved
by consolidation of routers, physical adapters for
I/O ports, and additional hardware for specific network services.
Extending system virtualization to the network
•Virtual router (software with routing functionality, multiple systems on 1 real machine)
•Virtual links (logical interconnection of virtual
•Virtual networks (Virtual routers connected by
virtual links)
The increase in server virtualization will result
in additional complexity and overhead for the
network. Obsolete networking switches are not
aware of Virtual Machines and this exposes the
risk of service outage and security breaches due
to incorrect network configuration. Networking
is a key area that also needs to be virtualized to
achieve the same level of agility, bandwidth and
Network service virtualization is a strategy to
simplify the network operations and consolidate
multiple appliances. The virtualization of a firewall
module or IPS by providing a software image to
different applications via single network hardware
would reduce the need of separate devices by utilizing the software in the same hardware.
Reduced power consumption is achieved by consolidating multiple services into a single physical
device without requiring deployment of dedicated
hardware for each instance. Eliminating the need
for additional physical devices effectively removes
the need for additional power supplies, cooling,
and rack space which would otherwise have been
Summarized benefits for network service virtualization:
•Management interfaces are more flexible
•Reduced acquisition cost by use of software
•Increased application performance by simplified
service extension and allocation
•Potential decreased power consumption by
equipment consolidation
A successful implementation of network virtualization depends on aspects like capital expenditure, the definition of precise objectives or the
compatibility with existing hardware. Therefore,
virtualization projects require a well balanced
cost-benefit analysis, a comprehensive project
management and a consequent consideration of
possible security risks.
For small to medium businesses, the choice between FCoE and iSCSI largely depends
upon application requirements and availability of personnel trained in Fibre Channel.
•When high capacity and performance oriented databases are the business critical applications, FCoE
and iSCSI are suitable solutions to improve the service level and reduce the power consumption.
•Centralized storage provisioning and disaster recovery require a common SAN > iSCSI is preferred
•Fibre Channel dominant networks adaptation of FCoE is recommended [5].
4.2.4 Components and equipment
The power consumption of network equipment is
generally influenced by the component selection
and actual configuration of the system. The main
influence has the supported network technology
standard (e.g. 10GbE). The chip-design and system integration level has the highest leverage.
The trend is driven by the performance improvement in semiconductor technology which still follows Moore’s Law. This also includes the thermal
performance of the chip and the interconnection
technology. Reliability is a growing issue in that
respect. Other factors are the system configuration
meaning the types and number of ports deployed
in the equipment. Finally, power consumption of
network equipment is influenced by the efficiency
of the power supply unit and power management
Power Management
The magnitude of the network equipment’s energy
consumption is related to active use and periods
of idling. The difference in power consumption
between active (100% load) and idle (with estab­
lished link) is typically about factor 1.1 (less than
10% difference). If the link is deactivated, the
­power consumption drops by factor 2 (50% of
However, it is expected that in smaller installations
(e.g. server room, small DC) idle phases could especially occur during night time. Advanced power
management including a type of “networked
standby” is not yet common. The term networked
standby has been coined by the European EuP/
ErP framework directive preparatory study for
ENER Lot 26. This study argued that “resumetime-to-application” is the key criteria for the
implementation of networked standby. The power
management of network equipment is closely related to the server and storage systems which they
Consider procurement criteria for selecting energy efficient network
hardware and particularly power supply units.
•Choose equipment with power management functionalities and compare power
consumption of different devices in idle and standby states.
•Compare costs and energy efficiency of network systems from different vendors
•Request product information from suppliers regarding:
Ioverall energy efficiency (e.g. ECR, TEER as soon as available)
Iefficiency and modularity of power supply units
Iefficiency and scalability of blower units (variable fan speeds, etc.)
In case a cluster of server or storage devices
would be powered down into a standby (sleep)
mode, it would be possible to also power down
parts of the access switches. Again, the critical
factor is ­latency and reliability of the system wakeup. With the introduction of the standards IEEE
802.3az “Energy Efficient Ethernet” and Standard ­ECMA-393 “proxZzzyTM for sleeping hosts”
­specific ­approaches for low power management
are underway.
4.2.5 Floor-level switching
Power Supply Unit
The reliability and conversion efficiency of Power
Supply Units (PSU) influences the overall energy
consumption. The conversion efficiency of larger
PSU (>500W output) has been improved in past
years to typical levels of over 85% and in excess of
90%. Due to the fact that larger core switches and
routers consume up to a few KW, even the smallest improvements in conversion efficiency (even if
only 1%) will result in noticeable energy savings.
Further, product specifications do not necessarily disclose information on the PSUs conversion
There are two basic types of switch distribution
on the floor or application level: End-of-Row and
Top-of-Rack. End-of-Row (EoR) switching is a
conventional networking approach, featuring a
single large chassis-based switch support of one
or more racks. From an energy efficiency point of
view, there are two considerations in respect to
Centralized switching with good
scalability and energy savings compared to
suboptimal ToR solution
•Disadvantage: Considerable cabling effort with
inefficiency in dense systems
Top-of-Rack (ToR) switching defines a system with
a switch integrated in each rack. This concept ensures short latency and high data transmission.
The advantage and disadvantage of ToR regarding
energy efficiency are:
4 Network Equipment
48 Port
48 Port
all ports used
48 Port
48 Port
48 Port
48 Port
10 Server
14 Server
16 Server
8 Server
simple cabling
Rack #1
Rack #2
Rack #1
Optimal ToR switching
Rack #2
Rack #3
Rack #4
suboptimal server
unused ports
Suboptimal ToR utilization
Fig. 4.4: Floor
•Advantage: Decentralized
switching for dense
server environments (I/O consolidation) which
reduces cabling effort. The shorter cabling
distance between server and switch improves
transmission speed and reduces energy consumption for this transmission.
•Disadvantage: If ToR is utilised in less dense
computing (few servers in a rack), the system is
over-dimensioned. Energy efficiency is low due
to suboptimal utilisation of available ports.
In conclusion, ToR has eco-advantages when
­applied in properly dimensioned systems.
Figure 4.4 illustrates the ToR switching concept
and its proper utilisation
Level Top-of-Rack Switching
Further Reading
Hintemann R. (2008): Energy Efficiency in the
Data centre, A Guide to the Planning, Modernization and Operation of Data centres, BITKOM, Berlin, online available:
[1] Enterasys (2011): Data centre Networking
– Connectivity and Topology Design Guide; Inc Enterasys Networks, Andover.
[2] Lippis (2011): Open Industry Network Performance & Power Test Industry Network Performance & Power Test for Private and Public Data
centre Clouds Ethernet Fabrics Evaluating 10 GbE
Switches; Lippis Enterprises, Inc, Santa Clara.
[3] Cisco (2008): Converging SAN and LAN Infrastructure with Fibre Channel over Ethernet for
Efficient, Cost-Effective Data centres; Intel, Santa
[4] Emulex (2008): Sheraton Case Study. Virtual
Fabric for IBM BladeCentre Increases Server Bandwidth, Reduces Footprint and Enables Virtualization for High-performance Casino Applications;
Emulex, Costa Mesa 2010.
[5] (2008): Blade Platforms and Network Convergence;,White Paper 2008.
EC JRC ISPRA (2011): Best Practices for the EU
Code of Conduct on Data centres
European Commission (2011), EC Joint Research Centre, Ispra, online available:
Juniper (2010): Government Data centre Network Reference Architecture, Using a High-Performance Network Backbone to Meet the Requirements of the Modern Government Data centre
Juniper (2010), Juniper Networks, Inc., Sunnyvale, available online:
5 Cooling and power supply in data centres
and server rooms
Andrea Roscetti, Politecnico di Milano, Thibault Faninger, Bio Intelligence Service
Cooling can be responsible for up to 50% of the total energy consumption
in server rooms and data centres. Concepts for energy efficient cooling are
therefore essential in both small and larger IT facilities.
The following section shows a number of general options to reduce energy
5.1 Cooling in server rooms
Server closets or small server rooms are usually
equipped with comfort cooling systems (typically
office HVAC1 systems). Small data centres are
­typically equipped with 1-5 racks of servers, with a
total IT power of maximum 20 kW.
5.1.1 Split systems and portable systems
Fig. 5.1: Split
cooling system room unit
Split cooling systems are commonly used in small
server rooms. The cooling power range of this
­family of systems is 1–100 kW. Generally split/DX2
cooling systems have several advantages:
•Investment costs are typically low.
•Design and installation are quite simple.
•Floor space required for the installation is small
(units are typically wall mounted).
•Installation is possible in almost all situations.
•Maintenance and replacement of the systems is
quite simple and fast.
On the other hand the following drawbacks have
to be considered:
•Overall efficiency is quite low for small, older or
oversized systems.
•Comfort cooling has poor humidity control.
•Piping between external and internal units has
limitations in length and height.
external unit (Source: Daikin)
Portable systems may be installed for example to
prevent hot spots. The technology provides the following advantages:
•Investment costs are very low.
•Installation is simple.
•Floor space required for the installation is small.
•Maintenance and substitution of the system is
simple and fast.
1) HVAC: Heating, ventilation and air conditioning
2) DX: direct expansion
The following disadvantages are to be considered:
•Overall efficiency is quite low: A-class mobile
systems are less efficient than D-class split
•Cooling has poor humidity and temperature
•Installation is only possible if air can be vented
to the outside.
5.1.2 Measures to optimize energy
efficiency in server rooms
Oversizing of cooling is common practice for small
server rooms. To avoid over-sizing of cooling for
well-insulated server rooms, a rule of thumb suggests that the cooling power should not exceed
120% of the IT installed power.
When buying new appliances up to 12 kW cooling power, the EU Energy Label can be considered
to support the selection of energy efficient equipment. A high EER3 /SEER4 and A-class level efficiency or above are the right choices. SEER and
the kW/h annum estimated from the label are the
most important criteria for comparison. Table 5.1
shows the efficiency for the current best available
The label is implemented with a transition period
until 1 January 2013. Before that time the label
can be used by manufacturers already but is not
mandatory. During the transition also the old label
for air conditioners may still be used (2002/31/
Fig. 5.2: Energy
label for cooling only air conditioners (Source: regulation
supplementing Directive 2010/30/EU of the European Parliament and of the
Council with regard to energy labelling of air conditioners)
Tab. 5.1: Best
available technology efficiency values for small cooling systems <12 kW
(source: Ecodesign regulation requirements for air conditioners and comfort fans)
Benchmarks for air conditioners
Air conditioners, excluding double and single duct
Double duct air conditioner
Single duct air conditioner
3) Energy Efficiency Ratio: ratio of output cooling to input electrical power at a given operating
point (indoor and outdoor temperature and humidity conditions)
4) Seasonal EER: represents the expected overall performance in a given location (test method)
Existing server rooms
•Eliminate solar gain, heat transmission and ventilation losses to other rooms/outside space.
•Control and manage the environmental conditions (set points): inlet air to IT (not setpoint) must be 18–27°C however suggested range is
24 to 27°C.
•Verify the ducts/pipes insulation (cold and hot air/water/liquid).
• Evaluate the substitution of obsolete or less efficient components of the cooling system (compare the efficiency class of existing systems with
the most efficient ones available on the market).
•Control and verify the layout of the installed cooling system (e.g. distance between cooling systems and loads).
•Turn off lights and remove other mechanical/electrical loads and sources of heat if possible.
New server rooms
•Evaluate the use of precision cooling systems (in order to remove sensible heat from IT and avoid over-dehumidification).
•Define and assess the room and the IT characteristics, taking into account space restrictions and distance between load and external units.
•Avoid the use of mobile units or ducted units with low EER (note: A-class mobile systems are less efficient than D-class split systems!).
•Compare different systems:
■ Opt for the higher energy label class (mandatory for small systems).
■ Maximise the cooling efficiency (SEER), see BAT table.
•Consider the use of free cooling.
5.2 Cooling for medium to
large data centres
5.2.1 General aspects
The traditional approach for cooling in medium
and large data centres has been based on air cooling. A standard data centre is designed to cool on
average 7.5–10 kW/m2, which translates to 1–3
kW/rack. Newer data centres are designed to cool
an average 20 kW/m2, which still limits the power
density per rack to 4–5 kW (recall that full rack
capacity in consolidated systems or blade server
systems may be higher than 25 kW/rack).
IT equipment is arranged in rows with air intakes
facing the cold aisle. Cool air is supplied to the
cold aisle, passes through the equipment and then
is discharged to the hot aisle.
Important elements to consider are the airflow
characteristics. The recommended air flow directions are front to rear, front to top, or front+top
to rear (see reference). If different equipment with
different operating conditions or airflow directions is installed in the same room, a separate
area should be created. In case the equipment has
different environmental requirements, it is preferable to provide separate environmental controls in
order to avoid inefficiencies due to the lower set
point or poor air flow control. For further details
see reference [1].
5 Cooling and power supply in data centres and server rooms
5.2.2 Temperature and humidity settings
Data centres should be designed and operated
at their highest efficiency possible under given
climate conditions (dry bulb5). Recommended
temperature is between 18 and 27 °C and relative
­humidity lower than 60% (inlet air to IT equipment). Respectively the dew point should be
between 5.5 and 15 °C. Studies on inlet air temperatures suggest 24–27 °C as the optimal range.
At higher temperatures the energy consumption
of internal fans in servers and other IT equipment
will prevail over the improved ­efficiency of the
data centre cooling system (see references). Lower
temperature settings waste energy through overcooling.
In addition to temperature settings, airflow optimization (e.g. hot aisle/cold aisle, blanking plates
and sealing leaks) is essential to ensure high efficiency. See reference [2] for hot-aisle cold-aisle
optimization. Especially higher temperature settings require optimised air-flow to avoid hot spots.
At very high power densities (e.g. 25 kW per
rack), traditional room cooling based on CRAC/
CRAH systems is no longer sufficient to prevent
hot-spots. See references [3], [4] and [5] for more
detailed information. In this case special rack- and
row-based cooling may be appropriate.
Management of cooling systems:
•Control and manage the environmental conditions (set point, schedule, position and number of
•Replace obsolete or less efficient components of the cooling system (compare the efficiency class of
existing systems) with more efficient ones available on the market.
•Verify the ducts/pipes insulation (cold and hot air/water/liquid).
•Locate CRAC at the end of the hot aisle (units are to be placed perpendicularly to the hot aisles).
•Segregate equipment with different airflow/temperature requirements.
•Air flows:
■ Place air supplies (perforated floor tiles or diffusers) in cold aisles only, near the active IT
■ Install airflow barriers as hot aisle and/or cold aisle containment to reduce mixing of hot exhaust
air with cooler room air.
■ Install blanking panels at all open rack locations and within racks to prevent recirculation of hot air.
•Cable order:
■ Use overhead cable tray.
■ Control the positioning and the sealing of cable openings and floor tiles.
Criteria for selecting new energy efficient cooling systems:
•Compare efficiency of chiller units, (see references for cooling requirements).
•Compare the different air flow design options (cold/warm aisles, raised floor/return plenum concepts).
•Evaluate the use of:
■ rack based cooling (for high density systems)
■ free cooling (direct/indirect)
■ free water cooling
■ installation of liquid cooling (direct/indirect)
■ waste heat recovery
•Set up a modular cooling system (linked to the IT design concept and management).
•Use Computational Fluid Dynamics (CFD) simulation software for optimisation of the cooling process.
5) the value measured by a thermometer freely exposed to the air but shielded from radiation
and moisture level, typically the air temperature
5.2.3 Component efficiency – chillers, fans,
air handling units
Air cooled and liquid cooled chillers differ regarding their EER (Energy Efficiency Ratio6) which is
typically around 3.5 for water systems and around
2.5 for air systems. The “rated energy efficiency
ratio“ (EERrated) expresses the declared capacity
for cooling [kW] divided by the rated power input
for cooling [kW] of a unit when providing cooling
at standard rating conditions. Eurovent provides
data which allows a comparison of the characteristic efficiency of several cooling and ventilation
systems and components ( Water-cooled chillers are a first choice
over air-cooled and DX, thanks to the higher thermodynamic efficiency. The opportunity to decrease
condensing temperature or increase evaporat-
5.2.4 Free cooling
ing temperature should be evaluated. Reducing
­delta-T between these temperatures means that
less work is required in the cooling cycle, hence
improving efficiency. The temperatures are dependent upon the required internal air temperatures (see Temperature and humidity settings).
Efficiency of fans primarily depends on the motor
efficiency. The use of fixed speed fans consumes
substantial power and makes management of data
floor temperature difficult. Variable speed fans are
particularly effective in case of high redun­dancy in
the cooling system or highly variable IT load. Fans
may be controlled by the return air temperature or
the chilled air plenum pressure.
6) Energy Efficiency Ratio: ratio of output cooling to input electrical power at a given operating
point (indoor and outdoor temperature and humidity conditions)
Oversized transformers
Inefficient, higher losses
Electrical Distribution
Utility Transformer
Power Supply (UPS)
Low load capacity
Inefficient UPS topology
Low input power factor
High input current factor
Generator Backup
Excessive redundancy
Excessive heater loads
Fig. 5.3: Electrical
“Free cooling” is a technique providing cooling by
use of the lower level of external air/water temperatures compared to the indoor required conditions. The lower the average external temperature
is over a year, the higher the opportunity for free
cooling and the efficiency level. Waterside and airside economisers may provide an alternative for
supplemental cooling. Climatic conditions define
the economic efficiency and payback of investments. Full free cooling operating mode can be
used if the difference between the cooling water’s
return temperature and the ambient temperature
is bigger than about 11 K. Consequently, the higher the designed inlet temperature, the higher the
energy savings. If a higher server room temperature is chosen for a cooling system’s design, free
cooling can be used for a longer period of time
per year. Free cooling implementation requires a
feasibility check and an economic evaluation. For
estimated savings also see the evaluation tool for
free cooling developed by The Green Grid.
Recommendations regarding sources with specific
information on free cooling are provided in the
section on further reading.
Power Distribution
Unit (PDU)
Inefficient transformers
Excessive use
IT Load
Low power factor
High current
Harmonic THD
Low utilization
Other Loads (Cooling, Lighting, etc.)
Cable losses (typ.)
Unused floorspace
No lighting controls
High server area
infrastructure components and inefficiency in a data centre (ASHRAE: Save Energy Now Presentation Series, 2009).
5 Cooling and power supply in data centres and server rooms
5.2.5 Rack based cooling / in row cooling
5.3 Power supply and UPS in
data centres
If the power density of modern IT equipment is
over 25 kW per rack, traditional room cooling
based on CRAC/CRAH systems is no longer sufficient to prevent hot-spots. See references for more
detailed information.
The power supply system in a data centre primarily transforms the current from alternate (AC)
to direct (DC). Losses due to transforming vary
­depending on the load level. The highest efficiency
is typically reached between 80 and 90% of the
total load while for levels below 50% energy
­efficiency ­decreases significantly.
Figure 5.3 shows the typical power chain in data
centres. Typical sources of inefficiency are indicated for all components.
Uninterruptible Power Supply (UPS) systems often
provide a large potential for energy savings. UPS
is continuously operated to provide standby power
and power conditioning for IT equipment and
parts of the infrastructure.
Besides their primary function, which is to provide
short-term power when the input power source
fails, UPS also provide different features to correct
utility power issues. Three main system topologies are available, depending on the application
•Passive standby, also called Voltage and Frequency Dependent (VFD), is solely capable of
protecting the load from power disruptions
(power failures, voltage dips, surge voltages). In
a normal electric supply situation the UPS has
no interaction with the utility power. When the
input supply is outside UPS design load tolerances, an inverter engages the energy storage
mechanism to provide power to the load, bypassing utility electrical supply. This topology is
more common in low-power applications.
•Line interactive, also called Voltage Inde­pendent
(VI), is capable of protecting the load like a VFD
UPS and in addition provides protection to the
load by regulating frequency within optimal
limits. In particular, it protects from undervoltage applied continuously to the input, or overvoltage applied continuously to the input. This
topology is not commonly used above 5,000 VA
•Double conversion, also called Voltage and Frequency Independent (VFI), is capable of protecting the load against adverse effects from voltage (like a VI) or frequency variations without
depleting the stored energy source, as it continuously supplies total load power by regulating
utility electricity before it reaches the load. This
topology is rare for loads below 750 VA.
Each topology has its advantages and drawbacks.
In the range 750 VA–5,000 VA line-interactive UPS
tend to have longer operating lives and increased
reliability with a lower total cost of ownership,
while double conversion on-line UPS occupy less
space and can regulate output frequency. UPS can
also offer different energy storage mecha-nisms to
supply power to the attached load in the event of
power disruption:
•Electrochemical batteries, storing and discharging electrical energy through the conversion of
chemical energy;
•rotary (flywheel), providing short term energy
storage in the form of a spinning massive disk.
Tab. 5.2: Characteristic
UPS Topology
efficiencies of UPS topologies
Efficiency at
25% load
Efficiency at
50% load
Efficiency at
75% load
Efficiency at
100% load
Tab. 5.3: Minimum
average efficiency requirements for AC-Output UPS proposed in
EnergyStar UPS (P is the Real Power in Watts (W), ln is the natural logarithm)
Minimum Average Efficiency Requirement (EffAVG_MIN),
UPS Class
Output Power
Data centre
P > 10 kW
Input Dependency, as specified in the ENERGY STAR
Test Method Product Class
0.0058 x ln (P) + 0.86
Two options are available for delivering the energy
to the load:
•Static UPS: no moving parts in the power path
(except the fans for cooling). It converts AC
power into DC (rectifier for storage in batteries to provide continuity in case of mains loss)
and then into AC again for power supply units
installed in servers.
•Rotary UPS: transfers power via a motor/generator and is used for applications requiring
ride-through of short-duration power system
outages, voltage dips, etc.
The UPS energy losses are due to electrical power
conversion inefficiencies (in the charger and inverter) and battery charging losses or energy
losses in inertial systems (flywheels). The electric
losses (and the heat generation) are more important in double conversion UPS (rectifier, inverter,
filter, and interconnection losses), than in lineinteractive and standby UPS (filter, transformer,
and interconnection losses). DC-output UPS (also
known as rectifiers) and combined AC-DC-output
UPS can be used for some applications and may
avoid losses in the inverter and rectifier.
Criteria for new installations
•Assess your needs and size the UPS systems correctly (evaluate multiple or modular UPS,
scalable and expandable solutions): battery back-up time, cost, size, number of outlets, etc.
• Analyse the UPS technology and efficiency. Take into account the partial load efficiency of UPS.
•Select correct topology of the power supply systems.
•Select UPS systems compliant with the EU Code of Conduct for UPS or Energy Star.
Criteria for optimisation
•Analyse the UPS technology and efficiency.
• Evaluate options and benefits of replacement of old equipment.
•Evaluate costs and benefits of redundancy.
5 Cooling and power supply in data centres and server rooms
Most UPS manufacturers quote UPS-efficiency at
100% load. However efficiency drops off significantly at partial load conditions (see Most UPS run
at 80%, and in case of redundancy, the load may
drop to 50% and below. At loads of 50% or lower
both modern and legacy UPS systems run less efficiently with significant dips occurring at loads
below 20%. For best practice UPS loads shall be
matched as closely as possible to the data centre
IT loads. Scalable UPS solutions are available for
efficient sizing of UPS capacity.
Minimum efficiency requirements for UPS are
specified in the EU Code of Conduct for UPS (new
edition 2011) and in the Energy Star programme
requirements (draft version 2011). New Energy
Star energy efficiency requirements for AC-Output
and DC-Output UPS are currently under development (see Table 5.3).
Further Reading
ASHRAE (2011): Thermal Guidelines for Data
Processing Environments – Expanded Data centre
Classes and Usage Guidance – ASHRAE, 2011,
online available at:
[1]ASHRAE: Save Energy Now Presentation
­Series, 2009.
[2]Niemann, J. et al. (2010). Hot-Aisle vs.
Cold-Aisle Containment for Data centres; APC by
Schneider Electric White Paper 135, Revision 1.
[3]Rasmussen, N. (2010). An improved architecture for High-efficiency High-density data centres; APC by Schneider Electric White Paper 126,
Revision 1.
[4]Blough, B. (2011). Qualitative analysis of
cooling architectures for data centres; The Green
Grid White Paper #30.
[5]Bouley, D. and Brey, T. (2009). Fundamentals of data centre power and cooling efficiency
zones; The Green Grid White Paper #21.
[6]Rasmussen, N. (2011). Calculating Total
Cooling Requirements for Data centres; APC by
Schneider Electric White Paper 25, Revision 3.
[7]ENERGY STAR Uninterruptible Power
Supply Specification Framework (2010).
Available at:
EU Code of conduct for data centres (2009):
Full list of identified best practice options for data
centre operators as referenced in the EU Code of
The Green Grid (2011): Evaluation tool for free
ENERGY STAR (2011): UPS efficiency
The programme also considers to include requirements for multi-modes UPS. This type of UPS operates with more than one set of input dependency
characteristics (e.g. can function as either VFI of
VFD). Multi-mode UPS can run in more efficient
less protective modes and switch to less efficient
higher protective modes, when necessary. Thus
significant energy savings are possible.
The Green Grid (2011): Evaluation tool for
power supply systems
High Performance Buildings: Data centres
Uninterruptible Power Supplies (UPS)
EU CODE of CONDUCT (2011): EU code of
conduct on Energy Efficiency and Quality of AC
Uninterruptible Power Systems (UPS):
[8]Ton, M. and Fortenbury B. (2008). High
Performance Buildings: Data centres - Uninterruptible Power Supplies. Available at
[9]Samstad, J. and Hoff M.; Technical Comparison of On-line vs. Line-interactive UPS designs;
APC White Paper 79. Available at
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Contact: Austrian Energy Agency | Dr. Bernd Schäppi | Mariahilferstrasse 136 | A-1150 Vienna |
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