An Introduction to OS Virtualization (Part 2)

If you missed the first part in this article series please read An Introduction to OS Virtualization (Part 1).


Virtualization is hot, and more and more vendors are jumping into this market. There are already over 10 vendors delivering an application virtualization product. The next steps in virtualization have already been taken with the introduction of Operating System Virtualization. In this article series I will introduce you to the Operating System Virtualization space.

In the first article in this series, I explained the basics of Operating System Virtualization. In this second article I will continue with the introduction discussing the possibilities of this technique, the advantages and disadvantages and the current status of this technique.

What are the advantages and disadvantages of OS Virtualization?

I have already described the basics of how OS Virtualization works. Probably you will find this is a pretty cool technique, but why should you use this technique in your infrastructure? In other words; what are the advantages of OS Virtualization? Of course there are also some drawbacks which I will also mention, but let us start with the pros of OS Virtualization.


  • Flexible provisioning
    With OS Virtualization you can easily connect different VDisks to a system. The client can easily start another operating system or another role. This can be very useful for Terminal Server environments when using the silo principle (also called Application Load Managed Groups), one silo can be extended with additional capacity easily. Also, workstations or servers can be assigned with a specific role when needed by assigning another virtual disk (also known as on-demand capacity or hardware re-purposing).
  • Support of multiple images per system, including boot menu
    The extended flexible provisioning may be taken a step further with the possibility of assigning multiple images to one client simultaneously. When starting, a boot menu could also be presented to the client, so that the end-user can choose which operating system (with a different role) will be started. In this case a system could be used for several purposes within seconds.
  • Rapid Software (OS/Apps) Deployment
    Adding a new server or workstation to your infrastructure takes just a few minutes. Instead of installing and configuring the system manually or via a deployment tool, taking at least a few hours, with just a few steps the client is assigned to a virtual disk and can be used in production.
  • System are always 100% identical
    Often, troubleshooting issues are concentrated on one machine because something is different on that machine. Especially in Terminal Servers, it is crucial that all the servers hosting the same role (silo) are identical, as explained in the article Terminal Server Basics. Of course, this only applies to shared virtual disks because in private disk changes are stored.
  • Easy implementation of updates and hotfixes of the Operating System and Applications
    With OS Virtualization an update or hotfix should just be added to the Virtual Disk image instead of all servers. Therefore you can create a new virtual disk which includes the update or hotfix and test it easily by assigning this virtual disk to one machine. If the tests are successful, this virtual disk can be assigned to all the other clients. You need to create procedure/systems to manage the updates and different virtual disk versions.
  • Easy rollback scenarios
    Although the update or hotfix was tested thoroughly, during production the behavior of this hotfix/update is not as expected. By assigning the previous virtual disk to those clients and restarting them you can revert to the previous state easily and quickly.
  • After restarting, the system is back to a clean state
    This advantage only applies to shared images. If your system is behaving strangely, often a complete re-installation is the only possible solution. With OS Virtualization (in combination with the shared disk) a machine only needs to be restarted to load the default configuration again.


OS Virtualization is no “wonder of the world”, so there are also cons to this technique.

  • No work off-line capability
    At the moment OS Virtualization products must be connected with the OS Virtualization server to use the operating system on the virtual disk. When no network connection is available the system cannot be used.
  • High-speed LAN recommended (>100Mb)
    Because the virtual disk is connected from the OS Virtualization server via the network interface card a LAN is recommended. Over a WAN connection there is probably not enough bandwidth available and/or not reliable enough to use the system smoothly.
  • Not all operating systems are supported
    Although many operating systems are supported there are some Linux distributions which cannot run via the OS Virtualization technique.
  • Multiple PXE/BootP solutions in same network segment will cause issues
    When using OS Virtualization as an additional technique within your current infrastructure you should consider the PXE/Bootp Connection option. Multiple PXE servers in one subnet will not function as desired.
  • Imaging disadvantages apply to this technique
    As described earlier an image based technique is used to create the virtual disk. All disadvantages that apply to imaging techniques are also valid for the OS Virtualization component. Think of an application which adds the computer name in the registry/files or an application which creates a GUID/unique number during installation.

What are the possibilities/scenarios for using OS Virtualization?

Now that we know how OS Virtualization works and which pros and cons are applicable to this technique, it is time to take look at which scenarios OS Virtualization may be used.

  • Citrix XenApp / Terminal Servers
    OS Virtualization is a nice complementary solution for Citrix XenApp/Terminal Server Based infrastructures. One of the greatest challenges of these infrastructures is to keep the server 100% identical for a consistent user experience. By using the shared virtual disks after every reboot the Terminal Server is back in his default state and changes are applied to all servers.
  • VDI / DDI solutions
    Virtual Desktop Infrastructure solutions are also becoming increasingly popular. One of the biggest disadvantages of most VDI products is the need for expensive SAN storage to host the virtual machines. With the OS Virtualization shared disk mechanism the virtual machines can use the same virtual disk and no expensive disk space is needed on the SAN.
  • Web Servers
    Most Web Servers are pretty static and do not save data locally. Also the need for Web resources can be very different during the day. With flexible provisioning the needed resources can be assigned during the day, for example sharing the hardware with another role.
  • Back-up Servers
    Back-up Servers are normally used for a few hours a day (during non business hours). So the hardware is relatively unused for most of the time. Using flexible provisioning, the hardware may be used for other roles during business hours and after that “assigned” to the back-up server role virtual disk for the back-up process.
  • Development/Test environments
    When using the DTAP principle, OS Virtualization can easily provide you with machines to run development and test tasks on, where the hardware can also be used efficiently with flexible provisioning.
  • Lab environments
    An attribute of Lab environments is the need to restore the status of the environment to the default state quickly so the students can successfully run the exercises. OS Virtualization can provide that with the shared disk option and it can also “flexible provision” the system with the correct Lab environment.
  • Educational environments
    In educational environments it is always a challenge to offer fully functioning workplaces for all the students. Also, depending on the training given there can be the need for several operating systems. Using OS Virtualization with multiple virtual disks assigned, the student can use the boot menu to start the relevant environment for his training. Also, after restarting, the machine is in its default state again (when using shared virtual disks).
  • Public Workstations
    Just like educational environments, public workstations should always offer functionality and no personal information should be stored.
  • (Very) Secure environments
    OS Virtualization is also a technique that can be used for (highly) secure environments. For example, if a machine is stolen (which uses OS Virtualization) no data is available on this machine.

What is the Current Status of OS Virtualization in the market?

At the moment there are two players in the OS Virtualization market. The first is Citrix, with their Provisioning Server product, which has been acquired via Ardence OS Streaming. Version 5 is the current version and it uses Microsoft VHD disk format for the virtual disks. All the new features and a review of this product can be found here. A relatively new player is XTreaming Technology which I have not tried out yet. The product seems to have many similarities to the Citrix Provisioning Server product.

At the moment, implementations of OS Virtualization are taking place within different environments. Mainly these are organizations which are already using Citrix products or are implementing VDI using Citrix XenDesktop (which has a version which includes Provisioning Server) or have a specific scenario (as described in the scenario section) for why they are using OS Virtualization.

In my opinion, this technique will not be used to host all clients and servers in the near future. For the next few years this product will be used as an additional component for a specific scenario or to solve some issues in the current infrastructure. When some disadvantages are solved, like offline usage, this technique will have a very promising future.


With this article series I have introduced you to the world of operating system virtualization. First I explained what OS Virtualization is and how it works in part one. In this article I described the advantages and disadvantages, the situations in which OS Virtualization has an added value and my opinion on the current status of the technique.

If you missed the first part in this article series please read An Introduction to OS Virtualization (Part 1).

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