Before you dive into desktop virtualization, it is important to understand the common methods of virtual and remote desktop delivery.
Desktop virtualization is more than just virtual desktop infrastructure (VDI). There are also client hypervisor systems, cloud-hosted desktops and virtual disk streaming from endpoints. Here are all the ways you can virtualize and deliver desktops and applications in your environment:
Server-hosted virtual desktops
This approach is commonly referred to as VDI. A server-hosted virtual desktop (SHVD) consists of a desktop operating system centrally hosted on a virtual machine in a data center. Users have their own desktops, so there is a 1:1 mapping of users to operating system instances.
SHVDs can be either persistent or nonpersistent. Persistent desktops keep all changes to the OS, app and user profile after logoff. These virtual desktops allow complete personalization by the user, including user-installed applications. Persistent virtual desktops usually lose some of the management benefits of desktop virtualization because they do not automatically revert back to a gold image.
Nonpersistent virtual desktops revert back to their gold images after user logoff or on an administrator-controlled schedule. Nonpersistent virtual desktops usually require a personalization system to maintain basic user preferences and data. Application-layering technologies are advancing the use of nonpersistent virtual desktops with the user-installed applications and personalization of persistent virtual desktops.
SHVD is accessed over the LAN or WAN using a remoting protocol such as Microsoft's Remote Desktop Protocol (RDP), Citrix Systems' High-Definition User Experience (HDX) and PC over Internet Protocol (PCoIP), developed by Teradici for VMware Inc. Poor network connections can greatly reduce performance and degrade the user experience.
Major vendors offering products in this space include Citrix, Dell (via its acquisition of Quest Software), Microsoft and VMware.
Client-hosted virtual desktops
Client-hosted virtual desktops (CHVD) involve a Type 1 (bare metal) or Type 2 (on top of an existing operating system) hypervisor. The hypervisor supports loading a Windows desktop operating system and masks the underlying hardware from the OS instance or instances.
In a CHVD environment, a central image is created and stored in the data center. This image can be checked out onto client devices, executed locally and then synced back to the data center to maintain user personalization, data and applications.
This technology is immature and still has significant challenges. Major vendors include Citrix Systems, VMware and, most recently, Microsoft. Microsoft Windows 8 includes a client hypervisor, but Remote Desktop Services doesn't yet take full advantage of it.
Server-based computing for app and desktop delivery
This type of server-based computing (SBC) commonly involves Microsoft Terminal Services or Remote Desktop Session Host. Multiple users share a single underlying operating system. Instead of remoting a full desktop to the remote client, only the application is shown. This technology augments an existing desktop environment by seamlessly appearing alongside locally installed applications.
SBC for application delivery is also accessed over a LAN or WAN using a remoting protocol such as RDP, HDX or PCoIP. Major vendors include Citrix, Dell and Microsoft.
SBC for desktop delivery uses the same technology as for application delivery, but instead of remotely delivering only individual applications, the entire server desktop is delivered. The server desktop is typically skinned to look and feel like a Windows 7 system using the Desktop Experience features built into Windows Server. Users access SBC for desktop delivery over the LAN or WAN using a remoting protocol, and Citrix, Dell and Microsoft are again the major vendors for SBC for desktops.
Desktop as a Service
Desktop as a Service (DaaS) is a delivery model in which businesses pay for their virtual desktops on a monthly basis from a service provider. The service provider is responsible for deploying and maintaining the customer desktop OS, theoretically reducing the operating expenses (Opex) a business would otherwise spend maintaining it with its own IT staff.
This market's biggest challenges to date have been the Microsoft desktop OS end-user licensing agreement -- which limits multi-tenancy -- and the lack of service-provider licensing agreements for Windows XP through Windows 8.
Often, this desktop delivery method is also referred to as a cloud desktop. DaaS today is more of a consumption and payment model than a technology, although this is changing as vendors begin to develop solutions specifically for the service provider market.
Virtual disk streaming
With this method, physical endpoints boot their operating systems over the network from an image stored on a server. Each endpoint uses local resources such as the CPU, graphics processing unit and memory, while the reads and/or writes to the disk drive are redirected to the central image. Since each endpoint uses local hardware resources, performance is like that of the native physical desktop.
Virtual disk streaming is a LAN-only technology and requires significant bandwidth of 100 Mbps to 1 Gbps. It must have low latency to each endpoint to provide access speeds similar to those for local disks. Major vendors' products in this space include Citrix and Dell (via its Wyse Technologies acquisition).
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