Hyper-V 3.0 part of Windows Server 8 promises lots of new features. One of the many new features will be Live VHD merge. This new feature will prevent having to reboot a virtual machine after deletion of the snapshot as is currently needed for virtual machines running on Hyper-V 2.0.
The ability to create a point in time state of a virtual machine (also known as a snapshot or checkpoint ) is one of the advantages of virtual machines. After creation of a snapshot, the original virtual disk is made read only and all addional writes are written to a snapshot file (or delta file).
Snapshots are NOT a way to make backups for virtual machine. Snapshots are usefull to temporary save the state of the virtual machine before applying a patch or other software change. After the update has bee applied do not wait too long before deletion of the snapshot.
Hyper-V 2.0 part of Windows Server 2008 has a nasty issue/design flaw. When a snapshot file was deleted by the administrator, it was actually not immediately merged with the original virtual disk and then deleted from disk.
It was merged and deleted only after a reboot was performed. Not everyone was aware of the ‘by design’ behaviour. It could easily lead to a volume running out of free disk space and a paused state of virtual machines. Read more about this at vcritical.com .
Virtual machine snapshots of Hyper-V 2.0 virtual machines are supported in production environments, but not recommended according to this posting on msdn.com.
Microsoft fixed this issue in Hyper-V 3.0. The feature called Live VHD Merge will make sure the snapshot file is merged and deleted from disk without the need for a reboot!