Abstract: Various techniques deduplicate common virtual disks during backups of virtual machines. For example, one method involves receiving a request to perform a backup operation to backup one or more virtual machine and then performing the backup operation in response to receiving the request. Performing the backup operation involves detecting whether a virtual disk associated with a virtual machine is already identified in a backup catalog for the backup operation, based upon a globally unique identifier associated with the virtual disk, and selecting to create a reference to a backup copy of the virtual disk instead of creating another copy of the virtual disk, if the backup catalog already identifies the virtual disk. Performing the backup operation can also involve accessing information identifying a hierarchy of virtual disks.