One of the great features of desktop virtualization (VDI) being touted by the industry is the ability to manage and update all of your desktops from a single central master image.
Citrix’s solution to the single image process is accomplished by a product called Provisioning Services (PVS). This software is the result of their purchase of a company called Ardence back in 2007. Provisioning Services is an often misunderstood piece of software, and its great benefits and potential are not necessarily apparent to everyone.
PVS works by streaming a master (read-only) image from the server to a target server or workstation. Any subsequent writes are then sent back to the PVS server and written to a cache file. The reads and writes are sent back and forth between the PVS server and target in a constant stream over the network. The easiest way to grasp this is to imagine that the cable connecting the hard disk inside of the server to the motherboard (and thus the CPU and RAM) is replaced by a network cable running back to the PVS server. The operating system sees the PVS disk as though it were a normal hard disk, and everything is done entirely transparent to the OS. The magic happens when the server is powered up; instead of booting from a local disk it is instead set to boot to the network card (PXE, BOOTP) which talks to a service on the PVS server, which streams the assigned operating system image to the target. The target device starts up immediately, as though it was booting from a local disk.
The beauty here is that this single read-only image can be simultaneously streamed to multiple diskless targets, both physical and virtual. This central image can now be maintained in one place. This makes tasks such as installing updates or new software quick and easy. After installing an update into the master image, all machines running that image will boot up into the updated image on next restart. To put that in perspective, think of the time and effort required to push out something such as a service pack to Windows or Microsoft Office to your entire firm. Now imagine simply installing that update once and having every machine in your environment receive that update on next reboot, without any additional effort.
Look for a follow-up post discussing the benefits that Provisioning Services can bring to a XenApp implementation.