Merging snapshots in Microsoft Hyper-V R1 and R2

When you create a snapshot in Hyper-V, it freezes the original VHD files and creates a new file with a .avhd extension that is a ‘differencing disk’.  All changes are written to the AVHD file and the old VHD is only used as read-only.

When you delete the snapshot in Hyper-V, the AVHD file is not removed.  For that, you have to shut the VM down, at which point Hyper-V will automatically begin merging the AVHD file with the VHD.  Depending on the configuration of your disks, where the snapshot files are stored, and the size of the snapshot files, the merge process can be very quick or take a long time.

You should use snapshots very sparingly in a production environment anyway, but you might need to do one before a patch/software install.

By the way, VMware merges snapshots while the VM is running, without requiring any downtime.