Server administrators using a software program to divide a single physical server into multiple isolated virtual environments in order to mask or hide server resources is server virtualization. Resources such as processors, operating systems and numbers and identities of individual servers, are all instances of server virtualization in information technology. Occasionally called virtual private servers, these virtual environments can also be known as instances, guests, emulations or containers. There are three ways to approach this: the paravirtual machine model, the virtual machine model, and virtualization of the operating system layer (OS).
A virtual machine is based on the guest/host paradigm. Every individual guest runs on the hardware layers virtual imitation, allowing the guest operating system to be able to be run without any modifications or adaptations. By allowing the administrator to create guests that use a different OS the guest has no concept about the hosts OS as it is unaware that it is not being run on real hardware. It uses a hypervisor to send instructions to the CPU as it requires a real computing source in order to run. This hypervisor is a virtual machine monitor (VMM), and it will validate any guest-issued directions and instruction while it manages any further executed code required for additional privileges. Microsoft Virtual server and VMware are two instances where the virtual machine model is used with server virtualization.
Also based around the guest/host paradigm, the paravirtual machine model (PVM) uses a virtual machine monitor as well. In this instance however, the virtual machine model will actually modify the code of the guests’ operating system. This is called porting, and it supports the virtual machine model so that it can make best use of privileged systems by acting sparingly. Paravirtual machines are also capable of running with multiple operating systems just as virtual machines are. UML and Xen are both instances that use and utilize the paravirtual machine model in server virtualization.
The third way in which server virtualization can be implemented is at the operating systems (OS) level and it works slightly differently to the other two ways in so much as it is not based on the guest/host paradigm. Operating system functionality is exported to the guest when the host runs a single operating system kernel as its core. While all guests must use the same OS as the host, varied distributions of the same system in this case are allowed. By distributing in this way CPU usage overheads are reduced as system calls are eliminated between layers. If there is a security breach or failure in a single partition it will not affect other partitions because a further requirement is that each partition is isolated from each other. Common binaries and libraries can be shared on the same physical machine which enables the OS virtual server to easily host thousands of guests simultaneously.
Efficiently using server resources, improving server reliability and helping with disaster recovery and development are just samples of the extent at which server virtualization can be used.
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