One problem with using Windows guests on Vagrant is how big they are. You can be looking at 20GB for a stripped down disk with just the OS. This can easily become prohibitive if you want to use more than a few VMs in Vagrant (not to mention the time copying all this redundant data).

Having read about differencing disks, they seem like the perfect solution to this problem, by allowing VMs to share the same parent disk, without needing to copy it (as each VM only needs to keep track of differences to the base disk). From what I've read of differencing disks, this seems like a no-brainer for Vagrant (but unfortunately my experience extends no further than what I've read).

Is there a reason Vagrant doesn't use this approach? Any plans to put this on the roadmap? It seems like this feature is supported by both VirtualBox and Hyper-v.

(This page describes differencing disks, and the first diagram under "Using multiple differencing disks with one parent disk" accurately depicts the sort of thing I have in mind here http://technet.microsoft.com/en-us/library/cc720381(v=WS.10)... )