The transient nature of VDI requires some method of delivering per-user profiles to the desktops, which is generally handled via roaming profiles, much like in the traditional terminal server world. Windows administrators have typically weathered bristly relationships with roaming profiles, but the reality is that they're not going away, and their benefits outweigh their detriments, at least for now.
However, Citrix is heading toward a better world after licensing code from Sepago to address profile management issues. Citrix will be leveraging sepagoPROFILE to hopefully ease this particular burden in the future.
XenDesktop's management methods also allow for quick updating of VMs, as it's possible to modify the baseline VM image that will then be used to boot all new VMs. You can update that image in the middle of the day, and every VM will pick up those changes the next time it reboots.
As with any enterprise-scale virtualization infrastructure, shared storage is a must. In order to migrate running VMs from one host to another, especially with the write-cache nature of XenDesktop, all the hypervisor hosts need to be playing in the same sandbox.
Apps in the stream
But what about the applications? This is where Citrix brings its app streaming tech to the table. A baseline VM image can be built that links to any number of streamed applications, such as Microsoft Office apps. The user who logs into that VM sees normal application launch icons, but these icons link to an application stream from a Citrix XenApp server. Thus, the application isn't installed on the VM at all, but is pulled into the VM when needed from the network. This reduces the footprint of the VDI infrastructure significantly, since you only need a single installation of Office 2007, rather than one installation per desktop. The apps run like they were natively installed, and users won't notice a difference.
Streaming apps are different than hosted applications. Streaming apps execute on the VM itself, while hosted apps run from a terminal server. Terminal services are useful to VDI in a variety of ways. For instance, heavy apps that require more RAM are better off running as hosted applications, while most other apps, like the Office suite, perform better when deployed as streaming applications.
The downside of hosted apps in a VDI environment is that these apps are essentially double-hopped, since they're displayed via terminal services within an existing terminal services session.
By bringing VDI into this mix, the issue of applications that do not function in terminal-services infrastructures is essentially out of the picture. Every user is given an actual desktop system running in a VM, and recalcitrant apps can be locally installed on those VMs, rather than delivered via streaming or hosting.
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