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vTerminology: A Guide to Key Virtualization Terminology

April 24, 2013
John A. Davis


One of the most important steps in mastering a new technology is learning the associated terminology or vocabulary. In the IT field, this can be a very challenging step, as much of the terminology is often used inconsistently. This white paper defines the terminology associated with IT virtualization. It is mainly vendor-agnostic, but it does provide some vendor-specific terms, product names, and feature names used by VMware, Citrix, and Microsoft.


Core Definitions

This section defines many of the most commonly used terms in the virtualization vocabulary. These are considered core, high-level terms. These are straight forward, commonly accepted definitions.

Virtual Machine (VM) - A set of virtual hardware devices, including virtual CPU, virtual RAM, virtual I/O devices, and other virtual hardware devices. Software that resembles and behaves like a traditional, physical server and runs a traditional operating system (OS), such as Windows or Linux.

Many products and technologies today provide a platform on which VMs can be built and run. Although these technologies may have many fundamental differences, they tend to share these characteristics:

- Many VMs can run on each physical host concurrently.
- VMs running on the same host are isolated from one another.
- The OS installed in the VM is unaware that it is running in a VM.
- Administrators and users in one VM cannot access the underlying host OS or the guest OS of other VMs running on the same host.

Virtual Server - A VM running a server OS such as a Windows Server or a Red Hat Enterprise Linux Server. A virtual server typically runs one server-based application.

Virtual Desktop - A VM that is running a desktop OS, such as Windows 7 or Red Hat Enterprise Desktop. A virtual desktop typically has one direct, concurrent user.

VM Template - An object that represents the "golden image" of a particular virtual server build or virtual desktop build, typically including a well-configured OS and applications. Administrators can quickly deploy new VMs by automatically copying the template to create the new VM.

VM Guest OS - The OS that runs in a VM.

Virtual Hardware Device (Virtual Device) - A software component that resembles and behaves like a specific hardware device. The guest OS and software applications in the VM behave as though the virtual hardware device is actually a physical hardware device. A VM is a set of virtual hardware devices that correspond to the set of devices found in traditional, physical servers, such as virtual CPUs, virtual RAM, virtual storage adapters, and virtual Ethernet adapters.

Virtual Network Interface Card (vNIC) - Software that resembles and behaves like a traditional Ethernet Adapter. It has a MAC address, and it receives and sends Ethernet packets.

Virtual SCSI Adapter - Software that resembles and behaves like a traditional SCSI adapter. It can generate SCSI commands and attach multiple virtual disks.

Virtual CPU (vCPU) - Software that resembles and behaves like a traditional, physical CPU. Depending on the underlying technology, vCPUs could be software-emulated or software-modified:

Software Emulated - A process that resembles and behaves like a specific model of a physical CPU that, in some cases, could be different than the model of underlying physical CPU in the host hardware.
 Software Modified - A process that provides a filtered, indirect connection to the underlying host CPU. Typically, the vCPU provides subsets of the instruction and feature sets that are available on the host CPU. The vCPU traps and modifies privileged commands but sends other commands directly to the hardware.

Virtual Disk - Resembles and behaves like a physical disk. It may be a file, a set of files, software, or some other entity, but to a VM, it appears to be a SCSI disk. For example, in Microsoft Hyper-V, virtual disks are referred to as VHD files with the file extension .vhd.

Virtual Ethernet Switch (vSwitch) - Software that resembles and behaves like a physical Ethernet switch. It allows vNICS from multiple VMs to connect to virtual ports. It allows physical NICs to connect to virtual ports and serve as uplinks to the physical network. A vSwitch maintains a MAC address table and routes traffic to specific ports, rather than repeating traffic to all ports. It may include other features commonly found in physical Ethernet switches, such as VLANs.

Virtual Network - A network provided by virtual switches. It may be an extension of a traditional network that is built on physical switches and VLANs, or, it may be an isolated network formed strictly from virtual switches.

Virtual Infrastructure - A collection of VMs, virtual networks and storage, and other virtual items that can deploy and run business applications, as an alternative to running applications directly on physical infrastructure. It allows IT personnel to install software applications in traditional OSs, such as Windows and Linux, without needing to know details of the underlying physical infrastructure. The OSs and applications run in VMs, in virtual networks, and on virtual storage.

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