Understanding VirtualBox Snapshots

Question

I have a base Windows 10 image that I want to make changes to. I have already cloned it to a working copy.

I understand that the smart thing to do is to create snapshots so that I can roll back the changes. I am having some difficulty understanding the process.

I presume that I would need to do the following:

• Take a snapshot at the very start, before I make any changes to the image.
• After a few changes take a snapshot in case the next changes go wrong.
• Repeat ad lib and fade etc.

I have made a preliminary snapshot which I have named Before. It contains a Current State.

The part which I don’t understand is the next part. I can only create a new snapshot when I have a previous snapshot selected. The new snapshot appears nested inside the previous snapshots.

When I look at Oracle’s description (https://docs.oracle.com/en/virtualization/virtualbox/6.0/user/snapshots.html) there are some nested snapshots, and some not.

Nothing I have found on the Internet throws any light on multiple snapshots, nested or not nested.

All I really want to do is to keep a number of historical points I can roll back to if necessary.

I am using VirtualBox on a Rocky Linux host.

Answer 1

You are treating snapshots as backup copies, while they are very different from backup.

A snapshot keeps only the "what has changed" data, it does not contain the entire virtual machine. As such it's only meaningful in the context of the VM at the time that you took it.

In other words, the snapshots form a chain, where each builds on the previous. That's why they are nested.

To get an upper-level snapshot, you need to roll back to the initial state of the VM. The snapshot will then build on that initial state.

It's easy to make an error with snapshots. Remember that they are not backups and do not treat them as such.

Answer 2

Let's start with this situation:

Snapshot A
 |
 +-- Snapshot B  (current)

You can now make some changes create a Snapshot C:

Snapshot A
 |
 +-- Snapshot B
      |
      +-- Snapshot C  (current)

It was created under Snapshot B because B was the active snapshot at the time.

Now you can switch to Snapshot A, make some changes and create a new Snapshot D. It will be created under Snapshot A, because it's derived from A (A was active):

Snapshot A
 |
 +-- Snapshot B
 |    |
 |    +-- Snapshot C
 |
 +-- Snapshot D  (current)

You can still switch to other snapshots, including other branches of the hierarchy, without losing anything (as long as you don't have non-snapshotted changes).

It works a bit like folders. When you create a new folder, it will be created under the current folder.