Windows implements the capability of bringing a volume to a known state
and then creating a shadow copy that can be used to back up a consistent
view of the volume. This technique is known as snapshots in some other file
systems. Making a shadow copy of a volume is a form of copy-on-write, where
blocks modified after the shadow copy is created are stored in their original
form in the copy. To achieve a consistent state for the volume requires the
cooperation of applications, since the system cannot know when the data used
by the application are in a stable state from which the application could be
safely restarted.