VHD (short for Virtual Hard Disk) is a file type spec that allows a hard drive system to be contained into an individual file. This file can be used as a boot partition on physical hosts and virtual machines. The VHD format is NOT proprietary to Citrix but is rather a Microsoft-maintained technology. We use it in PVS as a virtual disk image of an Operating System that a PVS target device can boot to from the network. For more information on VHD, visit this MSDN web resource.
In order to work properly in a read-only OS environment, PVS requires persistent storage for a write cache. All data about activities during a vDisk connection is written to this temporary file called write cache. WC (yes, looks awful lot like the European term for toilet) only grows in size and never decreases. Fortunately enough, it is flushed upon reboot. You have three places you can store the WC – locally on the target device hard drive, in the target RAM, or in a directory on the Provisioning Server itself. The new version of PVS 7.1 allows for a forth option – cache on RAM with overflow on HD.
Citrix Provisioning Services is a UDP-based streaming technology designed to deliver an operating system (vDisk) to client devices over the network. PVS uses PXE protocol specs (UNDI) to boot a target device (PXE client) and deliver a bootfile program that contains the instructions necessary to login to a Provisioning Server and start streaming the virtual disk over the network.
There are three really great things about PVS:
1. Single image management
Imagine you have a data center with 100 XenApp servers. Using traditional methods of server management, you would need to login to each and every one of them to make changes such as application updates, Windows patches, and lots of different things or maybe use GPOs to enforce certain modifications, etc.
With Citrix Provisioning Server (PVS) you can use a designated machine as a golden image, create a virtual disk from its hard drive, and assign it to hundreds or even thousands of servers for OS delivery. Since a vDisk has 2 modes – read/write and read-only, you can modify the image in read/write (Private mode) from one device and then stream to all your devices in read-only (Standard mode). That way all the changes made in Private mode update the VHD and can then be streamed to the rest of your devices in Standard mode propagating the changes you made instantaneously!
2. The Power of Read-Only
Read-only VHD is a truly powerful feature of PVS. Every time a machine is connected to a virtual disk from PVS, any changes made by users to the OS (outside of their roaming profiles) are flushed upon reboot! So, let’s say for instance, user John logs into a provisioned target device (e.g. XenApp server, XenDesktop, Windows endpoint, etc.) and messes with network adapter settings, clock, registry, etc., those changes are gone once the machine is shut down. Also, think about viruses! 🙂
PVS is fully enterprise-ready. Not only you have the option of adding existing machines to Device Collections in the PVS Console but in a virtual environment you can spin them yourself! XenDesktop Setup Wizard and Streamed VM Setup Wizard are at your disposal to quickly create new VMs on the fly when you need them.