Using Virtual Machine Snapshots

Virtual machine snapshots allow you to capture the state of the VM at a particular point in time. VM snapshots can be taken when the VM is powered on or when the VM is shut down. VM snapshots include not only the current state of the VM, but the current VM's configuration. For example, if you take a snapshot, add an additional hard disk, and then revert back to that snap shot, the VM will be configured as though you'd never added the hard-disk drive in the first place. By viewing the settings of a snapshot, you can check how the VM was configured at the time the snapshot was taken, but you will be unable to make any modifications to this configuration.

Snapshots taken when a computer is powered down require less disk space, because they don't have to store the contents of the system's RAM. They are also easier to work with because the VM is in a consistent state.

Creating Snapshots

To create a VM snapshot, right-click on the VM in the Hyper-V Manager console, and then click Snapshot. This creates a new snapshot, visible in the console, which has the VMs name and the current date and time. In the Actions pane, you can click Rename to give the snapshot another, more descriptive, name. You can create a maximum of 50 snapshots for each virtual machine hosted by Hyper-V.

If you have a lot of snapshots and none of them have meaningful names, it can be difficult to figure out precisely which existing snapshot you wish to restore at a future point in time.

Applying Snapshots

You can apply an existing snapshot. When you do this, all changes since the previously taken snapshot are lost. If you want to keep those changes, take an additional snapshot. Applying a snapshot doesn't delete other snapshots, it just reverts the VM to a known pre-existing state. If you apply an earlier snapshot, make changes to the VM, and then take another snapshot, you'll create a new branch on the snapshot tree.

Reverting to Snapshots

Clicking Revert from the Hyper-V Manager console when a VM is selected applies the most recently taken snapshot, discarding any changes that have been made to the VM since then. Reverting always returns the VM to the most recent snapshot. You can revert to a VM only if a snapshot currently exists. You use reversion when you want to> return a VM to a previously known configuration.

Deleting Snapshots

When you delete a snapshot, you have the option of deleting just that snapshot or the entire snapshot tree. When you delete a link in a snapshot chain, the changes represented in that link are merged back in such a way that the integrity of the remaining snapshots is preserved. For example, if you create a snapshot of a server at 9 am on Monday, Tuesday, and Wednesday, you can delete the snapshot taken on Tuesday without losing the other two snapshots. The changes that were recorded in the deleted Tuesday snapshot are merged in a way that allows you to maintain the integrity of the Monday and Wednesday snapshots, freeing up disk space. You can use the Delete Snapshot Subtree option to delete all snapshots in a branch.