What is the difference between a virtual machine and a guest operating system?

VMware ESXi 4.1.x Installable
VMware ESXi 4.0.x Installable
VMware ESXi 4.0.x Embedded
VMware vSphere ESXi 5.1
VMware ESX Server 2.5.x
VMware ESX Server 3.0.x
VMware ESX Server 2.1.x
VMware ESX Server 2.0.x
VMware ESX 4.0.x
VMware ESX Server 3.5.x
VMware ESXi 3.5.x Installable
VMware ESX 4.1.x
VMware ESXi 4.1.x Embedded
VMware vSphere ESXi 5.0
VMware ESXi 3.5.x Embedded
VMware vSphere ESXi 5.5

The difference between a virtual machine and a guest operating system (or guest) is sometimes confusing, but is a critical point to understand when managing and monitoring a virtual infrastructure environment. This difference is especially relevant when you are monitoring CPU utilization.

The virtual machine component of the VMware virtual infrastructure represents virtualized hardware on which a guest operating system runs. The guest operating system, typically Windows or Linux, is installed into a virtual machine, much the same way that it is installed on a traditional physical machine. Each instance of a guest operating system runs in a separate virtual machine.

VMware offers several tools for managing and monitoring virtual infrastructure: VirtualCenter, the Management User Interface (MUI),vmkusage, and the esxtop service console utility. VMware virtual infrastructure management tools are designed to manage and monitor virtual machines -- not guest operating systems.

There is often a difference between CPU utilization measurements taken from within the guest operating system and those observed by virtual infrastructure monitoring tools. For example, consider an ESX Server host running five virtual machines, each with a CPU-intensive process running inside the guest operating system. Each of these guests report 100 percent CPU utilization, but they do not know they are sharing host CPU resources with other guests. VirtualCenter and other VMware virtual infrastructure monitoring tools, meanwhile, are aware of this distinction and report that each individual virtual machine is using 20 percent of the available CPU resources on the ESX Server host. The ESX Server host is 100 percent utilized and must divide available resources among all virtual machines.

For more information on virtual machine CPU resource monitoring issues, see CPU usage of an application differs in virtual machines compared to physical machines (2032)