Virtual Machine

A virtual machine (VM) is a software implementation of a computing environment in which an operating system (OS) or program can be installed and run.



The virtual machine typically emulates a physical computing environment, but requests for CPU, memory, hard disk, network and other hardware resources are managed by a virtualization layer which translates these requests to the underlying physical hardware.



VMs are created within a virtualization layer, such as a hypervisor or a virtualization platform that runs on top of a client or server operating system. This operating system is known as the host OS. The virtualization layer can be used to create many individual, isolated VM environments.



Typically, guest operating systems and programs are not aware that they are running on a virtual platform and, as long as the VM's virtual platform is supported, this software can be installed in the same way it would be deployed to physical server hardware. For example, the guest OS might appear to have a physical hard disk attached to it, but actual I/O requests are translated by the virtualization layer so they actually occur against a file that is accessible by the host OS.



A virtual machine can also be a virtual environment, which is also known as a virtual private server. A virtual environment is used for running programs at the user level. Therefore, it is used solely for applications and not for drivers or operating system kernels.



A virtual machine may also be a group of computers that work together to create a more powerful machine. In this type of machine, the software makes it possible for one environment to be formed throughout several computers. This makes it appear to the end user as if he or she is using a single computer, when there are actually numerous computers at work.



Virtual machines can provide numerous advantages over the installation of OS's and software directly on physical hardware. Isolation ensures that applications and services that run within a VM cannot interfere with the host OS or other VMs. VMs can also be easily moved, copied, and reassigned between host servers to optimize hardware resource utilization. Administrators can also take advantage of virtual environments to simply backups, disaster recovery, new deployments and basic system administration tasks. The use of virtual machines also comes with several important management considerations, many of which can be addressed through general systems administration best practices and tools that are designed to manage VMs.



References:



• http://searchservervirtualization.techtarget.com/definition/virtual-machine

• http://www.wisegeek.com/what-is-a-virtual-machine.htm