Using Undo Disks

Using Undo Disks

If an undo disk exists and you disable Undo Disks or assign the Virtual hard disk 1 setting to a different hard disk, Virtual Server deletes the undo disk.
Do not use Undo Disks on a virtual machine configured as a domain controller. Problems will occur with replication when you revert the virtual machine to an earlier state.

Undo Disks is a feature that saves changes to a virtual machine’s data and configuration in a separate undo disk file in case you want to reverse the changes. The feature provides you with a way to decide whether to permanently modify a virtual machine and its disks each time you end a virtual machine session. When you enable Undo Disks, it applies to all virtual hard disks installed on the virtual machine. For instructions, see Enable Undo Disks for a virtual machine.

When you run a virtual machine that is using Undo Disks, any changes to a virtual hard disk are temporarily stored in an undo disk (.vud) file, rather than in the original virtual hard disk file. Each time you end a session by shutting down or turning off the virtual machine, you decide what to do with the changes by selecting one of the following options:

  • Keep Undo Disks. This option saves the changes in the undo disk file and leaves the original virtual hard disk intact. It provides you with a way to leave the original virtual hard disk intact while also keeping all changes when you end a session.
  • Commit Undo Disks. This option updates the original virtual hard disk with all changes that were stored in the undo disk file.
  • Discard Undo Disks. This option deletes the undo disk file and leaves the original hard disk file unchanged. Virtual Server creates a new, empty undo disk file the next time you turn on the virtual machine.

When you discard or commit an undo disk, that action applies to all changes stored on the undo disk. In other words, you cannot selectively delete or commit changes on an undo disk.

The undo disk file is always created in the same folder as the virtual machine configuration (.vmc) file. If you want to change the location where the undo disk file is created, you can move the .vmc file to a new location and the undo disk file is created there.

Undo Disks is similar to differencing disks, with two notable exceptions. A differencing virtual hard disk is associated with one virtual hard disk rather than with the virtual machine, and you are not prompted to decide what to do with the changes when you shut down a virtual machine. For more information, see Using differencing disks. The Undo Disks feature is not compatible with clustering. The virtual machine must be shut down without being in a saved state before you can enable or disable Undo Disks. For more information about virtual machine states, see Managing virtual machine state. If you want to restrict a user's ability to commit changes from an undo disk to the original virtual hard disk, use the file system to deny Write access to the user on the original virtual hard disk file. If you defragment the physical disk on which an undo disk is stored, the undo disk file will be defragmented.