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This book uses the term ''Virtualization'' in the context of running multiple operating systems on a single physical computer system. As such, the term does not refer to a single technology, but rather refers to a group of techniques and technologies which achieve operating system virtualization using a variety of different approaches.
When deciding on the best approach to implementing virtualization it is important to have a clear understanding of the different virtualization approaches that solutions which are currently available. The purpose of this chapter, therefore, is to describe in general terms the four virtualization techniques in common use today, namely guest operating system, shared kernel, hypervisor and kernel level.
== Guest Operating System Virtualization ==
Guest OS 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 or MacOS 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. Some examples of guest OS virtualization technologies include VMWare Server and VirtualBox.
The following figure provides an illustration of guest OS based virtualization.:
== Shared Kernel Virtualization ==
Shared kernel virtualization (also known as system level or operating system virtualization) takes advantage of the architectural design of Linux and UNIX based operating systems. In order to understand how shared kernel virtualization works it helps to first understand the two main components of Linux or UNIX operating systems. At the core of the operating system is the ''kernel''. The kernel (in simplistic terms) handles all the interactions between the operating system and the physical hardware. The second key component is the ''root filesystem'' which contains all the libraries, files and utilities necessary for the operating system to function. Under shared kernel virtualization the virtual operating systems each have their own ''root filesystem'' but share the kernel of the host operating system. This structure is illustrated in the following architectural diagram:
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. In simple termsEssentially, shared kernel virtualization is an extension of this capability which allows multiple instances of an operating system to run on a single physical host. Perhaps the biggest single draw back 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, therefore, possible to run 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.
Linux VServer, Solaris Zones and Containers, FreeVPS and OpenVZ are all examples shared kernel virtualization solutions.
Under hypervisor virtualization a program known as a hypervisor runs directly on the hardware of the host system. The task of the hypervisor is to handle protected and privileged CPU requests from the virtual machines and also to act as an intermediary between the hardware resources and the virtual machines.
In addition to the virtual machines, an administrative operating system and/or management console also runs on top of the hypervisor allowing the virtual machines to be managed by system administrators.
At the time of writing there are three different types of hypervisor based virtualization:
- '''Paravirtualization''' - Under paravirtualization the guest operating systems must be modified specifically to run on the hypervisor. This typically restricts limits support to open source operating systems such as Linux and excludes proprietary operating systems such as Microsoft Windowswhere the respective owners have agreed to target a specific hypervisor. The ability of the guest system to communicate directly with the hypervisor results in greater performance levels than other virtualization approaches.
- '''Full Virtualization''' - Full virtualization provides support for unmodifed guest operating systems. In this scenario, the hypervisor provides emulation to handle privileged and protected CPU operations made by unmodified guest operating systems. As a result of this emulation the performance levels are lower than those provided by paravirtualization.
