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In computing, virtualization refers to the act of creating a virtual (rather than actual) version of something, including virtual computer hardware platforms, storage devices, and computer network resources.
Virtualization began in the 1960s, as a method of logically dividing the system resources provided by mainframe computers between different applications. Since then, the meaning of the term has broadened.
Hardware virtualization or platform virtualization refers to the creation of a virtual machine that acts like a real computer with an operating system. Software executed on these virtual machines is separated from the underlying hardware resources. For example, a computer that is running Microsoft Windows may host a virtual machine that looks like a computer with the Ubuntu Linux operating system; Ubuntu-based software can be run on the virtual machine.
Nested virtualization becomes more necessary as widespread operating systems gain built-in hypervisor functionality, which in a virtualized environment can be used only if the surrounding hypervisor supports nested virtualization. For example, Windows 7 can run Windows XP applications inside a built-in virtual machine. Furthermore, moving existing virtualized environments to a cloud, following the infrastructure as a service (IaaS) approach, is much more complicated if the destination IaaS platform does not support nested virtualization.
The way nested virtualization can be implemented on a particular computer architecture depends on supported hardware-assisted virtualization capabilities. If a particular architecture does not provide hardware support required for nested virtualization, various software techniques are employed to enable it. Over time, more architectures gain required hardware support; for example, since the Haswell microarchitecture (announced in 2013), Intel started to include VMCS shadowing as a technology that accelerates nested virtualization.
Virtual machines running proprietary operating systems require licensing, regardless of the host machine's operating system. For example, installing Microsoft Windows into a VM guest requires its licensing requirements to be satisfied.
Desktop virtualization is the concept of separating the logical desktop from the physical machine.
One form of desktop virtualization, virtual desktop infrastructure (VDI), can be thought of as a more advanced form of hardware virtualization. Rather than interacting with a host computer directly via a keyboard, mouse, and monitor, the user interacts with the host computer using another desktop computer or a mobile device by means of a network connection, such as a LAN, Wireless LAN or even the Internet. In addition, the host computer in this scenario becomes a server computer capable of hosting multiple virtual machines at the same time for multiple users.
As organizations continue to virtualize and converge their data center environment, client architectures also continue to evolve in order to take advantage of the predictability, continuity, and quality of service delivered by their converged infrastructure. For example, companies like HP and IBM provide a hybrid VDI model with a range of virtualization software and delivery models to improve upon the limitations of distributed client computing. Selected client environments move workloads from PCs and other devices to data center servers, creating well-managed virtual clients, with applications and client operating environments hosted on servers and storage in the data center. For users, this means they can access their desktop from any location, without being tied to a single client device. Since the resources are centralized, users moving between work locations can still access the same client environment with their applications and data. For IT administrators, this means a more centralized, efficient client environment that is easier to maintain and able to more quickly respond to the changing needs of the user and business.
Another form, session virtualization, allows multiple users to connect and log into a shared but powerful computer over the network and use it simultaneously. Each is given a desktop and a personal folder in which they store their files. With multiseat configuration, session virtualization can be accomplished using a single PC with multiple monitors, keyboards, and mice connected.
Thin clients, which are seen in desktop virtualization, are simple and/or cheap computers that are primarily designed to connect to the network. They may lack significant hard disk storage space, RAM or even processing power, but many organizations are beginning to look at the cost benefits of eliminating “thick client” desktops that are packed with software (and require software licensing fees) and making more strategic investments. Desktop virtualization simplifies software versioning and patch management, where the new image is simply updated on the server, and the desktop gets the updated version when it reboots. It also enables centralized control over what applications the user is allowed to have access to on the workstation.
Moving virtualized desktops into the cloud creates hosted virtual desktops (HVDs), in which the desktop images are centrally managed and maintained by a specialist hosting firm. Benefits include scalability and the reduction of capital expenditure, which is replaced by a monthly operational cost.
Operating-system-level virtualization, also known as containerization, refers to an operating system feature in which the kernel allows the existence of multiple isolated user-space instances. Such instances, called containers, partitions, virtual environments (VEs) or jails (FreeBSD jail or chroot jail), may look like real computers from the point of view of programs running in them. A computer program running on an ordinary operating system can see all resources (connected devices, files and folders, network shares, CPU power, quantifiable hardware capabilities) of that computer. However, programs running inside a container can only see the container's contents and devices assigned to the container.
- Application virtualization and workspace virtualization: isolating individual apps from the underlying OS and other apps; closely associated with the concept of portable applications
- Service virtualization: emulating the behavior of specific components in heterogeneous component-based applications such as API-driven applications, cloud-based applications and service-oriented architectures
- Memory virtualization: aggregating random-access memory (RAM) resources from networked systems into a single memory pool
- Virtual memory: giving an app the impression that it has contiguous working memory, isolating it from the underlying physical memory implementation
- Storage virtualization: the process of completely abstracting logical storage from physical storage
- Distributed file system: any file system that allows access to files from multiple hosts sharing via a computer network
- Virtual file system: an abstraction layer on top of a more concrete file system, allowing client applications to access different types of concrete file systems in a uniform way
- Storage hypervisor: the software that manages storage virtualization and combines physical storage resources into one or more flexible pools of logical storage
- Virtual disk: a computer program that emulates a disk drive such as a hard disk drive or optical disk drive (see comparison of disc image software)
- Data virtualization: the presentation of data as an abstract layer, independent of underlying database systems, structures and storage
- Database virtualization: the decoupling of the database layer, which lies between the storage and application layers within the application stack over all
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Finn explained that Standard covers 2 CPUs in a host, and goes from one VOSE (virtual operating system environment - 1 free Std install in a VM on that host) to two, and 'now has all the features and scalability of Datacenter.' He noted there will be a small price increase, but said he thought that wouldn't matter, as it 'should be virtualised anyway and the VOSE rights doubling will compensate. Windows Server Datacenter was a minimum of two 1-CPU licenses with unlimited VOSEs. 'Now it is a simpler SKU that covers two CPUs in a host with unlimited VOSEs,' Finn said.
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