Disk space required for Hyper-V 2008 R2 snapshot delete and merge

In Server 2008 R2 how much free disk space is required to delete a Hyper-V snapshot and merge it into the VHD file? Do I need free space equal to current VHD + the snapshot files or just free space equal to the snapshot files? For example if my VHD is 100 GB and the snapshot folder is 25 GB do I need 125 GB of free space or would 25 GB of space be sufficient? If the VHD is completely re-written it makes sense that I would need 125 GB of free space. If the VHD file is simply appended to I would assume that 25 GB is all that would be needed.



I know that this has been asked before but I wanted to get a little bit more clarification. Thanks for your help.