Editing Virtual machine (section)

== Snapshots ==
{{Main|Snapshot (computer storage)}}
A ''snapshot'' is a state of a virtual machine, and generally its storage devices, at an exact point in time. A snapshot enables the virtual machine's state at the time of the snapshot to be restored later, effectively undoing any changes that occurred afterwards. This capability is useful as a [[backup]] technique, for example, prior to performing a risky operation.{{Citation needed|date=June 2023}}

Virtual machines frequently use [[Disk image#Virtualization|virtual disks]] for their storage; in a very simple example, a 10-[[gigabyte]] [[hard disk drive]] is simulated with a 10-gigabyte [[flat file]]. Any requests by the VM for a location on its physical disk are transparently translated into an operation on the corresponding file. Once such a translation layer is present, however, it is possible to intercept the operations and send them to different files, depending on various criteria. Every time a snapshot is taken, a new file is created, and used as an overlay for its predecessors. New data is written to the topmost overlay; reading existing data, however, needs the overlay hierarchy to be scanned, resulting in accessing the most recent version. Thus, the entire stack of snapshots is virtually a single coherent disk; in that sense, creating snapshots works similarly to the [[incremental backup]] technique.{{Citation needed|date=June 2023}}

Other components of a virtual machine can also be included in a snapshot, such as the contents of its [[random-access memory]] (RAM), [[BIOS]] settings, or its configuration settings.  "[[Save state]]" feature in [[Video game console emulator|video game console emulators]] is an example of such snapshots.{{Citation needed|date=June 2023}}

Restoring a snapshot consists of discarding or disregarding all overlay layers that are added after that snapshot, and directing all new changes to a new overlay.{{Citation needed|date=June 2023}}