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Though the concept of virtualization is far from new, recent requirements such as the need to maximize hardware utilization, decrease hardware costs, reduce power consumption and simplify system management and security have led to a significant increase in both the deployment of virtualization and the number of available virtualization solutions. In fact virtualization solutions are now available to meet the needs of the global enterprise all the way down to the home user.

This book is about one such virtualization solution known as Xen. Xen is a feature rich, open source, hypervisor-based virtualization solution which, in spite of its relatively recent origins, has gained both wide acceptance and an enviable reputation throughout the IT industry.

The objective of this book is to provide the reader with an understanding of the basic approaches to virtualization together with detailed information on deploying virtualization using Xen technology. Whilst many books tend to focus on the theory of virtualization, this book emphasizes the practical aspects of working with Xen, including detailed step by step tutorials designed to show exactly how to create and manage Xen based guest domains.

== What is Virtualization? ==

In a traditional computing model, a computer system typically runs a single operating system. For example, a desktop computer might run a copy of Windows XP or Windows Vista, while a server might run Linux or Windows Server 2008. The concept of virtualization, as it pertains to this book, involves the use of a variety of different technologies to allow multiple and potentially varied operating system instances to run concurrently on a single physical computer system, each sharing the physical resources of the host computer system (such as memory, network connectivity, CPU and storage). Within a virtualized infrastructure, a single physical computer server might, for example, run two instances of Windows Server 2008 and one instance of Linux. This, in effect, allows a single computer to provide an IT infrastructure that would ordinarily require three computer systems.

== Why is Virtualization Important? ==

Virtualization has gained a considerable amount of coverage in the trade media in recent years. Given this sudden surge of attention it would be easy to make the assumption that the concept of virtualization is new. In fact, virtualization has been around in one form or another since it was first introduced on IBM mainframe operating systems in the 1960s. The reason for the sudden popularity of virtualization can be attributed to a number of largely unconnected trends:
Green computing - So called green computing refers to the recent trend to reduce the power consumption of computer systems. Whilst not a primary concern for individual users or small businesses, companies with significant server operations can save considerable power usage levels by reducing the number of physical servers required using virtualization. An additional advantage involves the reduction in power used for cooling purposes, since fewer servers generate less heat.
Increased computing power - The overall power of computer systems has increased exponentially in recent decades to the extent that many computers, by running a single operating system instance, are using a fraction of the available memory and CPU power.

Virtualization allows companies to maximize utilization of hardware by running multiple operating systems concurrently on single physical systems.

* '''Financial constraints''' - Large enterprises are under increasing pressure to reduce overheads and maximize shareholder returns. A key technique for reducing IT overheads is to use virtualization to gain maximum return on investment of computer hardware.

* '''Web 2.0''' - The term Web 2.0 has primarily come to represent the gradual shift away from hosting applications and data on local computer systems to a web based approach. For example, many users and companies now use Google Apps for spreadsheet and word processing instead of installing office suite software on local desktop computers. Web services such as these require the creation of vast server farms running hundreds or even thousands of servers, consuming vast amounts of power and generating significant amounts of heat. Virtualization allows web services providers to consolidate physical server hardware, thereby cutting costs and reducing power usage.

* '''Operating system market fragmentation''' - In recent years the operating system market has increasingly fragmented with Microsoft ceding territory to offerings such as Linux and Apple's Mac OS. Enterprises are now finding themselves managing heterogeneous environments where, for example, Linux is used for hosting web sites whilst Windows Server is used to email and file serving functions. In such environments, virtualization allows different operating systems to run side by side on the same computer systems. A similar trend is developing on the desktop, with many users considering Linux as an alternative Microsoft Windows. Desktop based virtualization allows users to run both Linux and Windows in parallel, a key requirement given that many users looking at Linux still need access to applications that are currently only available on Windows.


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<table border="0" cellspacing="0" width="100%">
<tr>
<td width="20%">[[Xen Virtualization Essentials|Previous]]<td align="center">[[Xen Virtualization Essentials|Table of Contents]]<td width="20%" align="right">[[An Overview of Virtualization Techniques|Next]]</td>
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<td width="20%">Table of Contents<td align="center"><td width="20%" align="right">An Overview of Virtualization Techniques</td>
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