and VMware estimates a hardware reduction ratio of up to 15:1. A virtual machine (VM) can be more easily controlled and inspected from a remote site than a physical machine, and the configuration of a VM is more flexible. This is very useful in kernel development and for teaching operating system courses, including running legacy operating systems that do not support modern hardware. A new virtual machine can be provisioned as required without the need for an up-front hardware purchase. A virtual machine can easily be relocated from one physical machine to another as needed. For example, a salesperson going to a customer can copy a virtual machine with the demonstration software to their laptop, without the need to transport the physical computer. Likewise, an error inside a virtual machine does not harm the host system, so there is no risk of the OS crashing on the laptop. Because of this ease of relocation, virtual machines can be readily used in disaster recovery scenarios without concerns with impact of refurbished and faulty energy sources. However, when multiple VMs are concurrently running on the same physical host, each VM may exhibit varying and unstable performance which highly depends on the workload imposed on the system by other VMs. This issue can be addressed by appropriate installation techniques for temporal isolation among virtual machines. There are several approaches to platform virtualization. Examples of virtualization use cases: Running one or more applications that are not supported by the host OS: A virtual machine running the required guest OS could permit the desired applications to run, without altering the host OS. Evaluating an alternate operating system: The new OS could be run within a VM, without altering the host OS. Server virtualization: Multiple virtual servers could be run on a single physical server, in order to more fully utilize the hardware resources of the physical server. Duplicating specific environments: A virtual machine could, depending on the virtualization software used, be duplicated and installed on multiple hosts, or restored to a previously backed-up system state. Creating a protected environment: If a guest OS running on a VM becomes damaged in a way that is not cost-effective to repair, such as may occur when studying malware or installing badly behaved software, the VM may simply be discarded without harm to the host system, and a clean copy used upon rebooting the guest . Full virtualization In