Tuesday, September 16, 2008

Multicast File Transmission in WDS

In the environment detailed in my MS thesis and a recently accepted paper to SIGITE '08, we describe an environment that uses a standard set of virtual machine templates. This set of virtual machine templates is then distributed (and kept consistent) across a set of workstations. Then, users can create differential virtual machines (VMware's linked clones) based on the templates. Users store the linked clones on a file server, and can achieve virtual machine mobility between workstations. At RIT NSSA, this environment is semi-operational as I write this post. Since RIT NSSA teaches many different operating system technologies, there are many virtual machine templates that reach a summed size of 100GB. One of the issues that we discusssed in our paper and that I present in my thesis is the notion of updating template virtual machines across all workstations. This is a difficult subject because as the number of workstations increases, the copies of this template repository increase. Now, RIT NSSA has 80 workstations in the pilot-lab -- that means, right now, when they want to update or add a template to each machine, they have to inject the files into an image and re-deploy the OS and data on all 80 workstations. Another way they can achieve an update is through some differential robocopy script that copies the templates from the file server to the workstations -- this can be done in series or parallel (I've found that robocopies in series seem to work much better with the storage devices in that file server -- Adaptec 2820SA with 5 SATAII in RAID5). However, all of these approaches are inefficient because they either copy superflous data once as in deploying an install image, or they copy the same data 80 times as in differential robocopy. There has to be a better way!
Enter WDSMCAST.exe from the Server 2008 AIK. WDSMCAST enables multicast transmissions of custom data stores. So, I can create diretory and make a custom WIM with the directory's contents. Once I have a custom WIM, I can create a custom namespace on my WDS Transport Server using wdsutil /new-namespace with the /configstring parameter specified as the location of the custom WIM. Microsoft's documentation states that the custom WIM can be stored in any directory. This, however, caused a divide by zero in my test runs with WDSMCAST.exe:

So, I moved the custom WIM inside the RemoteInstall directory and then multicast transfer of the image works just fine.

WDSMCAST runs just fine inside Vista:

Therefore, we could create a differential version of the repository, generate a new WIM, create a multicast session based on the WIM, instruct each workstation to join the multicast session, and then have each workstation extract contents of the WIM ontop of the repository at the workstation.
While this is a nice solution, it is possible that a workstation require twice the size of the update in free disk space. For example, if we wanted to add 20GB of templates to all workstations, the workstations need at least 40GB of free space because 20GB is required for the WIM and 20GB is required for the extracted templates. In an environment where this is realistic, it would be neat to issue these differential updates in a multicast fashion with wdsmast.

Microsoft says, "You can create a custom content provider for cases where the default provider is not sufficient (for example when using Transport Server to deploy an operating system from inside a .vhd image). See the Windows Server 2008 SDK for guidelines and samples for authoring and registering the provider." I'm going to investigate custom content providers for the purposes of transmitting a template repository version and talk about custom content providers in future post.

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