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VDI architecture and platforms
VDI comprises several joined components that enable clients to run their desktops remotely from within the data center. One of the key elements is a hypervisor, such as Citrix XenServer, Microsoft Hyper-V or VMware ESX Server, which runs on a server and executes the desktop virtual machines. A second ingredient is a management platform offered by the hypervisor vendors for their respective products. This manages the hypervisor servers as well as the pool of VMs used by existing connections and requested by new connections, and facilitates provisioning desktop virtual machines to VDI clients. The third element is a session broker, which is responsible for distributing sessions from clients to VMs and redirecting users of disconnected sessions back to their original virtual machines. For a VDI client to start up, these three components work hand-in-hand: On receiving a request for a new session and after successful authentication -- typically via Active Directory -- the session broker asks the management platform to start up a virtual machine -- provisioned with the OS, applications and profiles assigned to the connecting user -- that establishes a remote desktop connection between the client and desktop VM on the hypervisor server. While all VDI platforms are architected in this fashion, vendor implementations vary significantly, each with its pros and cons.
Citrix XenDesktop's Provisioning Server acts as a central proxy
VMware View is on par with Citrix XenDesktop in most areas, but lags in user experience, at least until VMware releases its version of PC-over-IP to match or exceed Citrix's ICA protocol and HDX extensions. VMware View includes VMware ThinApp for application virtualization, but unlike Citrix XenApp's flexible concurrent online and offline application support, its application delivery functionality is focused mostly on isolating applications from the operating system and other applications.
Microsoft has so far played a role in virtual desktop infrastructure with its Hyper-V hypervisor and System Center Virtual Machine Manager (SCVMM) management platform, but has relied on its partnership with Citrix Systems for a session broker. Microsoft will include a session broker in Windows 7, but it's intended for smaller deployments. On the app virtualization front, Microsoft is offering Application Virtualization (App-V), formerly known as SoftGrid. Like VMware ThinApp, it's more limited than Citrix XenApp and focused primarily on isolating apps from each other and the OS.
Leadership in the VDI market is currently shared by VMware and Citrix Systems. While VMware boasts the widest deployed hypervisor platform, Citrix Systems, with a 90+% market share, according to Gartner's Gammage, dominates server-based desktop and app computing. Luckily, and mostly the result of an interoperability agreement Microsoft and Citrix Systems signed in 2006, customers can mix and match virtual desktop platforms and hypervisors. "Many of our customers run XenDesktop with Microsoft or VMware hypervisors," confirmed Calvin Hsu, director of XenDesktop product marketing at Citrix Systems.
Parallels has a VDI product that's based on its Parallels Virtuozzo Containers virtualization offering. Contrary to competing hypervisors that require a full OS image per VM, virtual containers -- the equivalent of virtual machines -- leverage a single OS. Instead of having resources assigned to each container, all containers share the resources of the host operating system. On the downside, Parallels VDI product doesn't have the clear-cut OS isolation, and is less flexible and more proprietary than traditional hypervisors. Similar to Microsoft, Parallels doesn't have its own session broker and relies on third-party brokers such as Quest Software Inc.'s vWorkspace and Ericom Software's PowerTerm WebConnect.
This was first published in August 2009