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The Greening of IT
90 How Companies Can Make a Difference for the Environment

application is inefficient and results in low utilization. Virtualization enables
“car pooling” of applications on servers. The physical car (server) might be
fixed, but the riders (applications) can change, be diverse (size and type), and
come and go as needed.
The example in Figure 6.1 shows how specific applications were moved to
another system with a better energy footprint. In the simple case illustrated,
we assume all systems are running at the same operating system level.
However, what if the applications require different operating system levels or
even completely different operating systems? That is where virtualization
comes into play.
The term “virtualization” is widely used and has several definitions:

■ Can create logical instances of a computer system consisting of CPU,
memory, and I/O capabilities
■ Can be put together from other virtual components
■ Can consist of a virtual CPU or virtual memory and disk
■ Can be a virtual network between a virtual computer and the outside
world ptg

To have real work done by a virtual system, the virtual system must run on
a real system. Obviously, additional intelligence is required to do this. There
are pure software solutions, or a system’s firmware might offer virtualization
features, or such features might be hardwired into the system. Many of the
current processor architectures have virtualization features integrated, which
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can be taken advantage of by software solutions such as the IBM System z
and p machines. In the field, various other solutions are available, such as
VMware Server, VMware ESX, Microsoft Virtual Server, and Xen.
To continue with our example, using virtualization gives a slightly differ-
ent picture, as shown in Figure 6.2. Instead of moving the applications to the
consolidated server, we now virtualize the existing systems 1 through 4 on
our consolidation target. The effect is clear: Not only is the application mov-
ing, but also its complete operating environment has moved with it. Taking
a closer look, we find other attractive features, as follows:

■ Consider the three separate systems. To communicate, they require a net-
work infrastructure such as NICs, cables, and switches. If our virtualiza-
tion system supports network virtualization, this infrastructure is no