About restoring common files
You can save disk space by creating a Hyper-V virtual hard disk that contains the files that a number of virtual machines require. Instead of copies of the same vhd or vhdx file existing in several places, multiple virtual machines can share a single file. Two or more virtual machines (each called a child) can access that virtual disk. The parent vhd or vhdx 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 or the vhdx files that are specific to the virtual machine.
Caution:
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:
Before you restore common files, make sure the virtual machines that use the common files are in the Off or Saved state. Otherwise, a virtual machine may have a lock on the common files and the restore fails.
To keep the common files that are currently on the server, deselect the common vhd files or vhdx files on the Restore Marked Files dialog box under List of backed up files. The restore does not replace the files on the destination Hyper-V server that are not selected in the Restore Marked Files dialog box.
To overwrite the existing common files, select all the vhd files or vhdx files from the List of backed up files. However, if the common files are in use on the destination server, the restore fails. For the restore to succeed, the virtual machines that use the common files must be in the Off or Saved state.
To restore common files without overwriting any common files that exist on the destination server: Specify a different location on the Restore Marked Files dialog box under Restore everything to different directory.