I. Background of Virtualisation
Virtualisation is the separation of resource or request for a service from the underlying physical delivery of the service. It can dramatically improve the efficiency and availability of resources and applications in your organisation. A common example is computer software gaining access to more memory than physically installed, which is achieved by the partitioning of memory space and background swapping of data to disk storage.
In view of the underutilisation under the old "one server, one application" model, the explosion of the data size, high administration costs for the servers and the incompatibility of different operating system (OS), this gives rise to the need for the virtualisation technology.
Virtualisation technology can be applied to different IT infrastructure layers. The current trend of virtualisation includes server / hardware virtualisation, desktop virtualisation, application virtualisation and virtual infrastructure
- Server / Hardware Virtualisation
Server / hardware virtualisation allows a single physical machine to run multiple virtual machines on top of a host operating system or a virtualisation layer. The resources of the single computer are shared across the virtual machines. Each virtual machine emulates a physical computer and has its own CPU, memory, disks and network interface card. In other word, a single physical machine is able to install multiple different OS such as Window, Linux and Unix.
One of the most common approaches to server virtualisation is to use hypervisor technology. Hypervisors use a thin layer of code in software to achieve fine-grained, dynamic resource sharing. Hypervisor can be further classified into Type 1 and Type 2. Type 1 hypervisors run directly on the system hardware which are typically the preferred approach for server consolidation because they can achieve higher virtualisation efficiency whereas Type 2 hypervisors run on a host operating system that provides virtualisation services such as I/O device support and memory management. Virtualisation solutions that use a Type 2 hypervisor are also referred to as operating system (OS) virtualisation, and in some environments are called containers.