Virtual desktop infrastructure, sometimes referred to as virtual desktop interface (VDI) is the server computing model enabling desktop virtualization, encompassing the hardware and software systems required to support the virtualized environment.
Many enterprise-level implementations of this technology store the resulting "virtualized" desktop on a remote central server, instead of on the local storage of a remote client; thus, when users work from their local machine, all of the programs, applications, processes, and data used are kept on the server and run centrally. This allows users to run operating system and execute applications from a smartphone or thin client which exceed the user hardware's ability to run.
Some virtualization platforms allow the user to simultaneously run multiple virtual machines on local hardware, such as a laptop, using hypervisor technology. Virtual machine images are created and maintained on a central server, and changes to the desktop VMs are propagated to all user machines through the network, thus combining both the advantages of portability afforded by local hypervisor execution and of central image management. This approach requires more capable user hardware capable of running the local VM images, such as a personal computer or notebook computer, and thus is not as portable as the pure client-server model.
This latter model can also be implemented without the server component, allowing smaller organizations and individuals to take advantage of the flexibility of multiple desktop VMs on a single hardware platform without additional network and server resources.
 Technical definition
Desktop virtualization involves encapsulating and delivering either access to an entire information system environment or the environment itself to a remote client device. The client device may use an entirely different hardware architecture from that used by the projected desktop environment, and may also be based upon an entirely different operating system.
The desktop virtualization model allows the use of virtual machines to let multiple network subscribers maintain individualized desktops on a single, centrally located computer or server. The central machine may operate at a residence, business, or data center. Users may be geographically scattered, but all must be connected to the central machine by a local area network, a wide area network, or the public Internet.
Virtual desktop infrastructure (VDI) is the practice of hosting a desktop operating system within a virtual machine (VM) running on a hosted, centralized or remote server. The term was coined by VMware Inc.
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. 
A simple use for desktop virtualization involves remote administration—where the controlling computer will work almost the same as on a duplicate desktop, except that the actions of the controlling computer may be almost unnoticeable on the remote computer display. This differs from simple remote desktop software in that several people can use the same controlling computer at once, without disturbing each others' work. This could be useful for several administrators doing different tasks on the same server. It can also be used for using hardware attached to the controlled computer, without disturbing a person who may already be using the computer.
However, a major use spreads the resources of one machine to several users. In some cases one can buy one large computer (or server) and several thin clients or dumb terminals, rather than purchasing a complete computer for each physical workstation. The controlling thin-client computers need only enough resources to run the remote controlling software, therefore virtualization can provide a very simple and cheap computing system. Users of such a "thin client" or "dumb terminal" may not even know that "their" software actually runs on another computer. If one already has enough computers, but they are not powerful enough, only one new computer may be needed, with the old ones re-usable as thin clients
 Advantages and disadvantages
The shared resources model inherent in desktop virtualization offers advantages over the traditional model, in which every computer operates as a completely self-contained unit with its own operating system, peripherals, and application programs. Overall hardware expenses may diminish as users can share resources allocated to them on an as-needed basis. Virtualization potentially improves the data integrity of user information because all data can be maintained and backed-up in the data center.
Potential advantages include:
- simpler provisioning of new desktops
- reduced downtime in the event of server or client hardware-failures
- lower cost of deploying new applications
- desktop image-management capabilities
- longer refresh cycle for client desktop infrastructure
- secure remote access to an enterprise desktop environment
Limitations of desktop virtualization include:
- potential security risks if the network is not properly managed
- challenges in setting up and maintaining drivers for printers and other peripherals
- difficulty in running certain complex applications (such as multimedia)
- increased downtime in the event of network failures, which can be prevented by the use of a clustered file system
- reliance on connectivity to corporate or public network
- complexity and high costs of VDI deployment and management
 VDI Modes of Operation
There are essentially four models for VDI operation.
- Hosted (delivered as a service)
- Remote Synchronization
Both Hosted and Centralized modes rely upon a constant network or internet connection to the server where the VDI instance is running. This model is similar in concept to thin clients, in that the client device only displays the virtual desktop. For this reason, a constant network connection is required.
The Remote Synchronization model allows users to copy a VDI instance to a system, and then run the virtual desktop without a connection. In this model, users normally use virtual machines that are running on a centralized server, but can copy an image to be used locally when traveling. This dis-connected or untethered mode of operation has its own set of advantages and disadvantages compared to traditional desktops and centralized VDI desktops.
The Client-hosted model only uses centralized servers to manage virtual machine images, always running virtual machines on laptops or desktops. Local execution eliminates the infrastructure required for VDI execution servers in the data center and also reduces network bandwidth consumption since the virtual machines are executing locally and not over a remote network.
 Hosted virtual desktops
Hosted virtual desktops result from desktop virtualization services provided through an outsourced, hosted subscription model. Hosted virtual desktop services generally include a managed desktop client operating-system configuration. Security may be physical, through a local storage-area network, or virtual through data-center policies. Transferring information technology infrastructure to an outsourced model can shift accounting for the associated costs from capital expenses to operating expenses.
According to a report by Gartner, hosted services accounted for more than 500,000 desktop units as of March 2009, but will grow to 49 million desktop units by 2013, and may make up as much as 40% of the worldwide "professional PC market" by revenue.
 Centralized virtual desktops
With this model, all VDI instances are hosted on one or more centralized servers. Data is maintained on storage systems attached to the centralized servers.
For this model, there are two modes for VDI, static (also called persistent) and dynamic (also called non-persistent).
In static mode, there is a one-to-one mapping of a desktop to a user. That is, each user gets a unique desktop, and the desktop image must be managed and maintained.
In a dynamic mode, there is often a master image of the desktop stored, all user data is stored separately from the desktop. When a user logs in and requests a desktop, a VM that is “cloned” from the master image is combined with his personal data and applications.
 Remote Synchronized virtual desktops
Remote VDI instances takes the concept of a centralized VDI image that is maintained by an IT department, and adds the ability to work while disconnected from a central server or internet.
An image is copied to a local system, where it may run without requiring a network connection. Images are "checked out" for a period of time, and typically need to be refreshed periodically. The local system typically runs an operating system, along with a hypervisor, which in turn runs the VDI instance. Thus, this model requires more local CPU, memory and disk space than alternative forms of VDI. Additionally, security and other issues can be of greater concern for this model. However, this model enables traveling and other highly mobile users to utilize virtual desktops without network connectivity restraints.
 Client-hosted Virtual Desktops
Client-hosted VDI uses centralized image management but runs the desktop locally on a client hypervisor on a conventional desktop or laptop. Some client-hosted virtualization solutions also offer options for centrally storing the data on a SAN, which still executes locally on a desktop PC – taking advantage of local CPU and graphics processing capabilities.
Because this model is designed for local execution as the primary mode of operation, it usually builds in additional security capabilities such as full disk encryption, time-based lockout, and remote kill capabilities. Some solutions have been optimized to use a type-1 client hypervisor, which eliminates the need for an additional operating system on the client, thereby reducing system requirements and increasing performance and security.
 IT Requirements for VDI
One of the primary concerns for companies implementing desktop virtualization, or VDI projects is understanding the Information Technology requirements. Virtualization software vendors typically provide sizing guidelines and reference architectures, along with total cost of ownership calculators and other tools to help decide what the cost implications will be for deploying virtual desktop infrastructure. The needs for infrastructure vary widely depending on whether a server-hosted or client-hosted approach.
 VDI Server Requirements
The system CPU and memory requirements may be calculated using a specific vendor's reference architecture or sizing guideline. Typically it is possible to over provision the CPU resources, thereby allocating 20 or 30 virtual desktop instances for a system with only 8 or 16 cores.
Memory (RAM) requirements is another item that is typically straightforward to calculate utilizing vendor supplied sizing information. A common amount of RAM required for a virtual desktop instance is 1 or 2 GB of physical memory, although this varies based on workload, applications, operating system and other factors.
Client-hosted virtualization does not require VDI execution servers, because the virtual machines are executed on end-point machines.
 VDI Network Requirements
The networking requirements, both between servers and clients running VDI images, along with internal server to server networks and storage networks are all considerations which also must be configured properly. Networking requirements can vary significantly, depending primarily upon the size of the server used to host the VDI sessions. For very large systems it is common to utilize multiple 1 Gb/s Ethernet LAN connections, with additional multiple 8 Gb/s FC SAN storage connections.
Incremental network investments typically are not required for client-hosted VDI, because the network load is similar to the network consumption for traditional PC management operations.
 VDI Storage Requirements
The impact of VDI on storage can be significant. Additionally, it can be difficult to understand the performance impact that a VDI deployment will have on storage systems. The primary focus of deployment guidelines regarding storage are how to calculate storage capacity requirements.
Storage can play a significant role in the overall performance of VDI. There are several storage technologies which can have a significant beneficial impact on VDI performance. Client-hosted VDI typically reduces storage requirements by an order of magnitude because storage is only used for offline backups. 
 See also
- Dynamic desktop
- Remote desktop software
- Virtual machine
- Comparison of remote desktop software
- Application virtualization
- Workspace virtualization
- Multiseat desktop virtualization
- User environment management
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