Before you start actually configuring virtual machines, you should have some idea of what you're doing. Virtualization is a technique where an instance of the operating system isn't necessarily attached to one physical machine but runs on a virtualized hardware layer that is separate from the actual physical hardware. The operating system doesn't access hardware directly but instead talks to the virtualization layer, which in Xen terminology is called the hypervisor. The hypervisor manages access to the real hardware for the virtual machines. This makes it possible to run several instances of an operating system on one physical machine. In a Xen environment, another advantage is that you can allocate hardware resources dynamically. That's to say, if one of your virtual machines is out of physical memory, you just allocate some more memory. It will depend on the operating system in your guest domain whether it will use the added memory and need to reboot.
Companies are adapting virtualization for many reasons; two of them are seen more often than others. First, by using virtualization, you can use hardware more efficiently. Instead of having a server that has a workload no higher than 20 percent of its capacity because only one instance of an operating system is used on it, virtualization allows you to run more instances of an operating system on the server, thus allowing you to use hardware more efficiently. This is especially beneficial for large data centers, since it saves expensive rack space because you just don't need as many servers if you're using virtual machines. Second, by using virtualization, you can increase availability. In a virtual environment, it is possible to replace a virtual machine from one server to another server. This allows you to do hardware maintenance on a server without it causing any downtime for your users.
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