Tech Line

Deep Freezing Virtual Server VMs

Scripting simplifies legacy VM backups.

• By Chris Wolf
• 06/14/2005

Chris: I'm thinking of consolidating two MS-DOS servers to a single Windows 2003 server running Microsoft Virtual Server 2005. Are there any good scripting solutions that would allow me to back up all of the VMs on the server in one operation?
— Anonymous

Anonymous: Whenever I hear of MS-DOS, I almost always find myself clicking the remote to turn on my HDTV to make sure I'm still in 2005. Perhaps when you migrate your MS-DOS systems to VMs, you can get someone to do some carbon dating on your old DOS server hardware to determine its original age.

All kidding aside, your problem is not that uncommon. In the past two years, I've run into three other organizations that have had to keep MS-DOS boxes around in order to support a needed application that is not compatible with any other operating system. In these situations, migrating the DOS systems to VMs is a logical choice.

For backups, both Microsoft and VMware recommend running back-up agent software on your virtual machines to reliably back them up. Hot backup of virtual machine virtual disk files from the host system is not supported because some VM write operations wind up in physical memory before making their way to the virtual disk file. So even if you could back up a virtual disk file by using an open file manager, it may be corrupted and thus not worth restoring.

If you want to run a single back-up job from the host that allows all VMs to be backed up in one operation, you can suspend all VMs on the host prior to the backup, and then resume the VMs once the backup completes. Of course, this can cause the VMs to be temporarily unavailable while the backup runs, but I think that this won't be much of a problem in your situation. I'm assuming that your DOS application is not mission-critical and thus you can suspend your server VMs at 3 a.m. in order to back them up from the host.

Virtual Server 2005 offers a feature that allows for VMs to be automatically suspended, known as save state, when the Virtual Server service is stopped. So, to solve your back-up dilemma, you'll first need to configure each VM to automatically save its state when the Virtual Server service stops. These settings are configured on the VM's General Properties page in the Virtual Server Management Web UI.

From the General Properties page of the VM you want to back up, you'll need to click the "Run the Virtual Machine Under the Following User Account" checkbox. Then enter a user name and password of an account with Administrative rights under which to start the virtual machine. In the "Action When Virtual Server Starts" menu, select "Always Automatically Turn on Virtual Machine." In the "Action When Virtual Server Stops" menu, select Save State.

With these settings configured, your VMs will automatically pause and save their state when the Virtual Server service stops, and will then automatically start when the Virtual Server service restarts. Now to back up the VMs, you need to follow these three general steps:

1. Stop the Virtual Server service
2. Back up the VM files
3. Start the Virtual Server service

Here's a batch file that uses ntbackup.exe to back up the VMs:

:: Stop Virtual Server Service - Saves state of all VMs
net stop "Virtual Server"

:: Back up VMs
ntbackup backup "%VMRootFolder%" /J "%JobName%" _
  /D "%Description%" /F "%BackupFile%"

:: Start Virtual Server Service - Resumes all saved VMs
net start "Virtual Server"

Note that if you're running enterprise back-up software, you could run the "net stop" command as a pre-backup script and then run the net start command as a post-backup script. This allows all of the VMs to be successfully backed up when the host system is backed up by your backup software.

Like the great Han Solo, your VMs will become frozen in time, only to live again!

[Chris Wolf has just released Virtualization: From the Desktop to the Enterprise (Apress) and welcomes your virtualization questions for this column. —Editors]