About restoring common files

You can save disk space by creating a Hyper-V virtual hard disk that contains the files that are required by a number of virtual machines. Instead of copies of the same .vhd file existing in several places, multiple virtual machines can share a single .vhd file. That virtual disk (the parent .vhd file) can be accessed by two or more virtual machines (each called a child). The parent .vhd file is called a common file, because more than one virtual machine uses it.

The files unique to each virtual machine are maintained on differencing virtual disks. These virtual disks are in a child relationship to the parent disk. The parent and child virtual disks may be on the same physical drive or on different physical drives.

For example, the base version of Windows XP can be installed as a read-only image on a virtual hard disk (parent). Two or more virtual machines can run the same XP system files from the base image on the parent virtual hard disk. Applications that are unique to a virtual machine are not included in the common files. Instead they are on the .vhd files that are specific to the virtual machine.


Use care when restoring common files. If you restore an earlier version of the common files (overwriting the current version), the virtual machines that rely on those files may experience problems.

When restoring common files, note the following: