Virtualisation 101: An Enterprise Guide to Virtualisation Basics

From benefits to implementation challenges, here's what you need to know about virtualisation.

What important terminology should I know?

What is a hypervisor?
The hypervisor is the most basic virtualisation component. It's the software that decouples the operating system and applications from their physical resources. A hypervisor has its own kernel and it's installed directly on the hardware, or “bare metal.” It is, almost literally, inserted between the hardware and the OS.

What is a virtual machine?
A virtual machine (VM) is a self-contained operating environment — software that works with, but is independent of, a host operating system. In other words, it's a platform-independent software implementation of a CPU that runs compiled code. A Java virtual machine, for example, will run any Java-based program (more or less). The VMs must be written specifically for the OSes on which they run. Virtualisation technologies are sometimes called dynamic virtual machine software.

What is paravirtualisation?
Paravirtualisation is a type of virtualisation in which the entire OS runs on top of the hypervisor and communicates with it directly, typically resulting in better performance. The kernels of both the OS and the hypervisor must be modified, however, to accommodate this close interaction. A paravirtualised Linux operating system, for example, is specifically optimised to run in a virtual environment. Full virtualisation, in contrast, presents an abstract layer that intercepts all calls to physical resources.

Paravirtualisation relies on a virtualised subset of the x86 architecture. Recent chip enhancement developments by both Intel and AMD are helping to support virtualisation schemes that do not require modified operating systems. Intel's “Vanderpool” chip-level virtualisation technology was one of the first of these innovations. AMD's “Pacifica” extension provides additional virtualisation support. Both are designed to allow simpler virtualisation code, and the potential for better performance of fully virtualised environments.

What is application virtualisation?
Virtualisation in the application layer isolates software programs from the hardware and the OS, essentially encapsulating them as independent, moveable objects that can be relocated without disturbing other systems. Application virtualisation technologies minimise app-related alterations to the OS, and mitigate compatibility challenges with other programs.

What is a virtual appliance?
A virtual appliance (VA) is not, as its name suggests, a piece of hardware. It is, rather, a prebuilt, preconfigured application bundled with an operating system inside a virtual machine. The VA is a software distribution vehicle, touted by VMware and others, as a better way of installing and configuring software. The VA targets the virtualisation layer, so it needs a destination with a hypervisor. VMware and others are touting the VA as a better way to package software demonstrations, proof-of-concept projects and evaluations.

What is Xen?
The Xen Project has developed and continues to evolve a free, open-source hypervisor for x86. Available since 2003 under the GNU General Public License, Xen runs on a host operating system, and so is considered paravirtualisation technology. The project originated as a research project at the University of Cambridge led by Ian Pratt, who later left the school to found XenSource, the first company to implement a commercial version of the Xen hypervisor. A number of large enterprise companies now support Xen, including Microsoft, Novell and IBM. XenSource (not surprisingly) and SAP-based startup Virtual Iron offer Xen-based virtualisation solutions.

What are the cost benefits of virtualisation?

IT departments everywhere are being asked to do more with less, and the name of the game today is resource utilisation. Virtualisation technologies offer a direct and readily quantifiable means of achieving that mandate by collecting disparate computing resources into shareable pools.

For example, analysts estimate that the average enterprise utilises somewhere between 5 percent and 25 percent of its server capacity. In those companies, most of the power consumed by their hardware is just heating the room in idle cycles. Employing virtualisation technology to consolidate underutilised x86 servers in the data centre yields both an immediate, one-time cost saving and potentially significant ongoing savings.

The most obvious immediate impact here comes from a reduction in the number of servers in the data centre. Fewer machines means less daily power consumption, both from the servers themselves and the cooling systems that companies must operate and maintain to keep them from overheating.

Turning a swarm of servers into a seamless computing pool can also lessen the scope of future hardware expenditures, while putting the economies of things like utility pricing models and pay-per-use plans on the table. Moreover, a server virtualisation strategy can open up valuable rack space, giving a company room to grow.

From a human resources standpoint, a sleeker server farm makes it possible to improve the deployment of administrators.

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