Virtualization Explained: Virtualization definitions you need to know

Our Virtualization Explained series is a great reference when you need a server virtualization definition. This virtualization glossary explains many useful terms and technologies.

The Virtualization Explained series offers in-depth definitions of popular terms and technologies.

This virtualization glossary compiles all of our Virtualization Explained articles, from AMD-V to Xen. Now, whenever you need access to a certain server virtualization definition, it's only a few clicks away.

As we continually publish more Virtualization Explained articles, we'll add them to this virtualization glossary, so it will always have the most up-to-date information. If you need a server virtualization definition that the Virtualization Explained series hasn't covered yet, check out our full virtualization glossary, powered by

Virtualization Explained virtualization glossary
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AMD-V AMD Virtualization, commonly known as AMD-V, is processor technology designed to improve virtual machine performance on servers. Advanced Micro Devices (AMD) first announced AMD-V technology in 2004; processors that feature the technology today include the Turion 64 X2, Phenom, Phenom II and second- and third-generation Opteron series. One of the keys to AMD Virtualization is Rapid Virtualization Indexing, which lets virtual machines (VMs) manage memory directly and frees up processing power on the hypervisor.

See also: What is AMD-V?



Capacity planning A capacity planning strategy involves predicting future computing needs and the resources needed to handle them. It is especially important in virtual environments, where it is very easy to create new virtual machines. Without a strong capacity planning strategy, VM sprawl can occur. The basic approach is to monitor computing resources, evaluate utilization trends and then identify specific needs, such as hardware upgrades. A poor capacity planning strategy can lead to instability and application-availability problems.

See also: Virtualization capacity planning strategy guide

Clone A VM clone is an exact copy of a VM at a specific moment in time. The most common use of a VM clone is for mass deployment of standardized VMs, called VM templates. VM clones also come in handy for test and development, because they let staff use a real workload without affecting the production environment. A VM clone is not appropriate for backup, disaster recovery or other data protection methods.

See also: Cloning a VM on ESX Server

Clustered file system A clustered file system organizes files, stored data and access for multiple servers in a cluster. Clustered file systems are most useful when clusters work together and require shared access, which individual file systems do not provide. A Windows or Linux clustered file system can also identify and isolate defective nodes in a cluster; a Windows clustered file system will isolate the node logically, while a Linux clustered file system will use a utility to power down the node.

See also: What is VMware VMFS?



Guests and hosts A guest virtual machine and host server are two of virtualization's main building blocks. The guest virtual machine is a file that contains a virtualized operating system and application, and the host server is the hardware on which it runs. The other important component is the hypervisor -- the software that creates the guest virtual machine and lets it interact with the host server. The hypervisor also makes the host server able to run multiple guest virtual machines.

See also: What is a hypervisor?



Intel VT Intel Virtualization Technology, or Intel VT, provides hardware assistance to Intel processors and chipsets in virtual environments. The goal of Intel VT is to help systems meet the added demands of virtualization; the technology focuses on memory control and management to meet that goal. Intel VT appears in the company's Nehalem line of processors, but not all Intel processors support the technology, which is considered a high-end feature.

See also: What is Intel VT?



Live migration Virtual machine live migration is the process of moving a VM from one host server to another without shutting down the application. The benefits of virtual machine live migration are some of the biggest selling points for virtualization, affecting business continuity, disaster recovery and server consolidation. Virtual machine live migration is a feature in all of the major virtualization platforms, including VMware vSphere, Microsoft Hyper-V R2 and Citrix Systems XenServer.

See also: Live Migration in Hyper-V R2



Memory overcommit A hypervisor can let a guest VM use more memory space than the host server has available. This feature is called memory overcommit. Memory overcommit is possible because most VMs use only a little bit of their allocated physical memory. That frees up memory for the few VMs that do need more. Hypervisors with memory overcommit features can identify unused memory and reallocate it to more memory-intensive VMs as needed.

See also: What is memory overcommit?



Network virtualization Network virtualization lets you combine multiple networks into one, divide one network into many and even create software-only networks between VMs. The basis of network virtualization is virtual network software, to which there are two approaches: internal and external. Internal network virtualization uses virtual network software to emulate network connectivity among VMs inside a host server. With external network virtualization, virtual network software consolidates multiple physical networks or creates several virtual networks out of one physical network.

See also: Network virtualization news, help and research



P2V A physical-to-virtual server migration, also known as a P2V server migration, is the process of converting a physical workload into a VM. To perform a physical-to-virtual server migration, copy bits from the physical disk to the VM, inject drivers, then modify other bits to support the drivers. Some operating systems and virtual server migration tools let you perform a P2V server migration while the host is running, but others require a shutdown.

See also: Using P2V backup in a disaster recovery strategy

Provisioning Virtual server provisioning is the process of creating a VM on a host server. The preferred method of virtual server provisioning is to use VM templates -- generic VMs designed for specific uses and applications -- stored on the SAN, but you can provision VMs manually as well. One downside is that the ease of virtual server provisioning can leave organizations susceptible to VM sprawl.

See also: Virtual machine provisioning and configuration



Server consolidation Server consolidation is virtualization's biggest selling point. Server virtualization lets one physical server run multiple VMs, which reduces the amount of physical hardware required. Other server consolidation benefits include lower power and cooling costs and less reliance on expensive capital projects. Traditional physical servers have a utilization rate of 5% to 10%, but with virtualization and server consolidation, utilization rates can increase to as high as 80%.

See also: What is server consolidation?

Snapshot A VM snapshot backup is the most common way to protect a virtual machine. A VM snapshot is a copy of the state of a VM -- and any virtual disks assigned to it -- as it exists in server memory at a specific moment. The snapshot is usually saved to the SAN, where it can be recovered in case of a failure. Regular VM snapshot backups can significantly reduce recovery point objectives.

See also: When to use VM backups versus snapshots in Hyper-V

Storage virtualization Storage virtualization separates the operating system from physical disks used for storage, making storage location-independent. The benefits of storage virtualization include more efficient storage use and better management. Dynamic provisioning is similar to storage virtualization, but it still requires more traditional storage management.

See also: Virtualization storage tutorial



Virtual hardware The computing resources that the host server assigns to a guest VM make up the virtual hardware platform. The hypervisor controls the virtual hardware platform and allows the VM to run on any host server, regardless of the physical hardware. The virtual hardware platform includes memory, processor cores, optical drives, network adapters, I/O ports, a disk controller and virtual hard disks. Virtualization lets a user adjust the levels of these resources on each VM as needed.

See also: Understanding VMware physical and virtual hardware

Virtual machine A virtual machine (VM) is a file that includes an application and an underlying operating system. A virtual machine combines with a physical host server and a hypervisor to make server virtualization possible. Some of the most important benefits of a virtual machine include server consolidation, more efficient management and improved storage. Capacity planning and workload balancing help administrators get the most out of their virtual machines.

See also: What is a virtual machine?

Virtual machine monitoring Virtual machine monitoring can mean a lot of different things, but when most people say "virtual machine monitoring," they really mean "virtual machine performance monitoring." Virtual machine performance monitoring tools keep tabs on the state of VMs in an environment -- if they're up, if they're experiencing downtime, or if they've failed. It is possible to monitor VM performance from within, but it's recommended to monitor from outside the VM instead.

See also: Virtual machine monitoring and security guide

Virtual switch A virtual switch is the key to network virtualization. It connects physical switches to VMs through physical network interface cards and ports. A virtual switch is similar to a virtual bridge, which many virtualization platforms use, but it is more advanced. Virtual LANs, EtherChannel and additional virtual networking tools are only available in a virtual switch. Some virtual switches even offer their own security features.

See also: Virtual switch standards

VMDK The Virtual Machine Disk (VMDK) file format is used to identify VMware virtual machines. (In virtualization, the hypervisor creates a VM file that consists of an operating system instance, an application and other associated components.) Other platforms that support the VMDK file format include Sun Microsystems xVM, Oracle VirtualBox and QEMU. It competes with Microsoft's Virtual Hard Disk format, which is used in Virtual Server and Hyper-V.

See also: Importing VMDK disk files into VirtualBox



Xen The open source Xen hypervisor is popular in cloud computing infrastructures and as the basis for several commercial hypervisors, most notably Citrix XenServer. The University of Cambridge Computer Laboratory created Xen, XenSource further developed the code, and Citrix acquired XenSource in 2007.

See also: Xen vs. KVM Linux virtualization hypervisors

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