Virtualization is a broad term of computer systems that refers to anabstraction mechanism which hides the physical characteristics of certain computational resources in order to simplify the way in which other systems,applications or end users interact with them. Thus, virtualization enables sharing the resources of a computer system in multiple execution environments.
The concept of virtualization is not new. It has its roots in the mid 1960's, when it was used by IBM as a method for logical partitioning of large centralized systems (mainframes) into separate virtual machines. The virtual computers distributed to users of the system, allowing each user to work in an isolated environment without affecting other users. For this sharing to be possible IBM introduced a new feature called Virtual Machine Monitor (VMM).
The Virtual Machine Monitor is a software layer that is placed on top of the hardware layer and has direct access to hardware resources. The main objective of Virtual Machine Monitor is to manage and allocate system resources to one or more virtual machines in order to make virtualization possible.
The role of the Virtual Machine Monitor is analogous to the role of an Operating System. On the one hand, the Operating System provides a virtual execution environment for processes, while the Virtual Machine Monitor provides a virtual environment for running Operating Systems.
The quest for virtualization at full hardware speed has not been an easy one, and many different routes have been taken. It takes a huge amount of power to support virtual infrastructure and VMware is the software leader in this area. With a VMware virtual machine you can virtualize any Operating System that would run natively on the hardware.
User Mode Linux (UML) is another approach: it enables you to run a Linux kernel as a process on top of another kernel. This works well enough for many organizations and individuals. But UML has its flaws: running the Operating System with the same security as its applications is insecure. Furthermore, individual Virtual Machines are unable to take advantage of multiple CPUs and you rely on the host Linux kernel to give quality of service when the VMs come under load.
In 2003 a team of researchers at the University of Cambridge released the world's most advanced hypervisor: Xen. At first, the team developed a multimedia operating system called 'Nemesis'. This introduced the seed of what would become Xen: it had a domain switcher, which was a very low-level, very thin virtual machine layer that provided resource guarantees to the domains above it. Later, the developers needed a VMM that would provide strong resource isolation between users running VMs on the physical machine, and most importantly, would provide isolation without a noticeable drop in performance. Having failed to find anything that came close to their criteria, they built their own.
- VMware: http://www.vmware.com
- UML: http://user-mode-linux.sourceforge.net/
- Xen: http://xen.org
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