Introduction
Virtualization and emulation are inherently fascinating topics. Why should you need to have multiple
computers running multiple operating systems when one computer can run them all? Even better, if
you have a sufficiently powerful machine, why limit yourself to running only one operating system at
a time?
Savvy IBM mainframe users greet virtualization with a yawn and a statement like “That old thing?”
Thousands of IBM systems have been running critical business tasks in virtual machines on mainframes
for a few decades now. Originally introduced as a mechanism to simultaneously support legacy and
more modern operating systems on newer hardware without requiring software changes, virtualization
has been a part of enterprise computing since the 1960s.
The emergence of powerful, affordable, open source virtualization has taken a long time. As this book
illustrates, today’s virtualization technologies have an important place in today’s powerful, 24/7
enterprise computing environments. Virtualization can help you get the most out of your existing
hardware; reduce data center costs; help guarantee the seamless availability of critical systems; help
support “occasional-use” legacy applications without requiring aging, hard-to-replace hardware; and
much, much more.
I refer to today’s virtualization technologies as “commodity” virtualization technologies because they
run on standard, off-the-shelf hardware using a variety of readily available operating systems such as
Linux — no mainframe, special hardware, or proprietary service contract is necessary. The powerful,
open source virtualization technology of Xen is the perfect complement to the flexibility and reliability of
the Linux kernel and the enterprise and end-user application infrastructure that it powers. Just as Linux
has taken over most server and many desktop tasks, virtualization is the technology of the future, and
Xen is the most interesting and capable virtualization technology available today. (If you disagree with
any of the last few statements, feel free to send me an e-mail message from a UNIX-like system using the
command
cat > /dev/null .)
I wrote this book because virtualization is a powerful and cost-effective technology that deserves
explanation and promotion. Virtualization can save system administrators time and effort, increase
system and service availability, reduce ongoing infrastructure costs, and help minimize future hardware
expenses, all while maximizing the return on investment from your existing hardware. Not too shabby
for something that is free.
Commodity virtualization technology is still relatively young. As technologies such as Xen and
associated tools mature, the potential applications of virtualization in enterprise computing will continue
to expand. Xen’s approach to virtualization provides performance and capabilities that no other
commodity virtualization solution can match. Processor vendors such as Intel and AMD now provide
hardware support for virtualization in their latest processors, enabling virtual machines to make the
most of multiple cores, CPUs, and your systems’ other hardware resources. At the time of this writing,
the mainline Linux kernel (2.6.23.1) already includes built-in support for Xen virtual machines
(domainU), and support for Xen’s administrative interfaces (domain0). The Xen hypervisor is actively
being incorporated.
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