Difference Between Hyper-V Saved State and a Snapshot

On October 11, 2011

When working with Microsoft Hyper-V, it is helpful to understand the difference between the Saved State and a Snapshot. Here is a brief explanation of both.

Snapshot

A snapshot can be taken whether the virtual machine (VM) is running or not. It is a point in time of the state of a VM. You can revert back to a previous point in time whenever there is a need. For example, if you plan to install a service pack, you can take a snapshot before you install the service pack and if things go wrong, you can revert to the point in time before you installed the service pack. It is similar to the System Restore concept in Microsoft operating systems. A snapshot consists of three components:

A single xml file for the VM configuration. This includes all the settings that you configured for the VM, such as memory, number of processors, description of the VM, etc.
A memory save state file.
A difference disk, which is a file that ends with the extension .avhd. There will be a separate .avhd file for each snapshot.

Saved State

A Saved state is a point in time of a running VM, however, unlike a Snapshot, a Saved state can only take place on a VM that is running. Also, a Saved state can be restored only once (as long as you have not applied a Snapshot since the system state was saved). This is different than Snapshot because a Snapshot can be restored multiple times. In fact, you can go back and forth between different Snapshots to the exact point in time when the Snapshots were taken.

Taking a Snapshot

To take a Snapshot, simply right-click a VM in the Hyper-V console and select Snapshot. The VM may be running or shutdown. You may also delete a Snapshot, whether the VM is running or not.

Saving the State

To save the system state, right-click the VM while it is running and select Save. Your VM will appear to be shutdown. To resume, you simply right-click the Saved VM and select Start.

As great as Snapshots are, keep in mind that according to Microsoft if you have more than three Snapshots for a VM, you may experience performance degradation. However, the degradation will depend on the amount of RAM and processors that are available and how you have configured the system resources.

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