Changes

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 instance.

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 required three computer systems.

Virtualization has gain 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, the virtualization has been around in one form or another since the introduction of ???? by it first introduced on IBM mainframe operating systems in the 1960's1960s.

== Guest Operating System Software Virtualization ==

Guest OS Software virtualization is perhaps the easiest concept to understand. In this scenario the physical host computer system runs a standard unmodified operating system such as Windows, Linux, Unix UNIX or MacOS Mac OS X. Running on this operating system is a virtualization application which executes in much the same way as any other application such as a word processor or spreadsheet would run on the system. It is within this virtualization application that one or more virtual machines are created to run the guest operating systems on the host computer. The virtualization application is responsible for starting, stopping and managing each virtual machine and essentially controlling access to physical hardware resources on behalf of the individual virtual machines. The virtualization application also engages in a process known as ''binary rewriting'' which involves scanning the instruction stream of the executing guest system and replacing any privileged instructions with safe emulations. This has the effect of making the guest system think it is running directly on the system hardware, rather than in a virtual machine within an application.

Some examples of guest OS software virtualization technologies include VMware Server , VirtualPC and VirtualBox.

The following figure provides an illustration of guest OS software based virtualization:

This type of virtualization is made possible by the ability of the kernel to dynamically change the current root filesystem (a concept known as ''chroot'') to a different root filesystem without having to reboot the entire system. Essentially, shared kernel virtualization is an extension of this capability. Perhaps the biggest single draw back drawback of this form of virtualization is the fact that the guest operating systems must be compatible with the version of the kernel which is being shared. It is not, for example, possible to run Microsoft Windows as a guest on a Linux system using the shared kernel approach. Nor is it possible for a Linux guest system designed for the 2.6 version of the kernel to share a 2.4 version kernel.

Under paravirtualization the kernel of the guest operating system is modified specifically to run on the hypervisor. This typically involves replacing any privileged operations that will only run in ring 0 of the CPU with calls to the hypervisor (known as ''hypercalls''). The hypervisor in turn performs the task on behalf of the guest kernel. This typically limits support to open source operating systems such as Linux which may be freely altered and proprietary operating systems where the owners have agreed to make the necessary code modifications to target a specific hypervisor. These issues not withstandingnotwithstanding, the ability of the guest kernel to communicate directly with the hypervisor results in greater performance levels than other virtualization approaches.