Private images are operating system and application stacks personalized to each user.  Although users enjoy a great deal of customization, each private image must be stored and managed individually, much like managing rich, distributed clients.  Current products do not allow private images to be patched or updated in their stored locations, but rather require them to be actively loaded and managed in-band, either on the server or the client.  The storage requirement of private images is much higher, since each user’s copy of the operating system and application must be stored.

Let’s now briefly touch on the essential characteristics of each of the client virtualization models.

* Server-based Virtualization Models: Terminal services represent the quintessential server-based model.  Here, the client is merely a display and input device.  All computation is done centrally on the server and all data is stored in a datacenter.  Nothing is persistent on the client.  It is the most proven, reliable server-side model, harkening back to the days of mainframe computing.  Remote Display Protocol (RDP) or Independent Computing Architecture (ICA) are used to deliver an image of the server-based application to a terminal viewer on the client, and return keystrokes and mouse clicks to the server.

Most enterprises of significant size use terminal services for some applications and users.  Bank tellers accessing the transaction system, call center workers entering orders in database and healthcare professionals working with text-based patient records are examples where terminal services may be a good solution.

The newest entry into server-side computing is virtual hosted desktops, more commonly known by VMware’s acronym, Virtual Desktop Initiative (VDI).  Given that additional vendors are creating similar products to VDI, we will use the generic term virtual hosted desktop (VHD) acronym for this discussion.