Cloning VirtualBox VM Snapshots

This is another “how to” tech article, anyone who is not interested in such things may stop reading now.

VirtualBox is a great virtualization software (hypervisor as the lingo currently goes) – I believe it matches up nicely against the current VMWare Workstation line and they offer both an open source version which is free for any use as well as a commercial version (with some added features such as SATA support) that is free for personal use.

VirtualBox allows you to take snapshots of the current VM state so that you can safely return to a previous state of your VM if you messed something up (for example – installed too much software on your Windows XP VM). Unfortunately, unlike what the VirtualBox UI will have you think, the snapshot features allows you to take progressive snapshots but you can’t fork your snapshot tree – you can’t create branches off old snapshots. Snapshots which are not current can be either discarded (have their state merged into another state) or reverted too (discard all the newest data and return to the old state).
old state

Additionally, you can’t copy (clone) VMs with snapshots except copying the oldest state(1).

Still, if you want to keep your old state, and maybe fork some previous snapshots and create something of a real tree structure, there is a round about way to achieve something similar:

Suppose you have created a set of snapshots similar to the screenshot above, and now you want to use “Old State B” as a basis for a new set of snapshots (that will not include the changes in “Old State C”).

In the screenshot below you can see the VirtualBox Media Manager lists the VMs virtual HD with all its snapshots – “Arbel.vdi” is the base snapshot “Old State A”, the next down is “Old State B”, then “C” and finally “Current state”.
Virtual Media Manager

Theoretically what you would want to do is to use VirtualBox’s command line management interface to clone the virtual HD state of “Old State B” (UUID 4e1f756e-f158-4c07-bc9a-20261f574730) into a new base HD, like this:

$ VBoxManage clonehd 4e1f756e-f158-4c07-bc9a-20261f574730 
 .VirtualBox/HardDisks/new-old-state-b.vdi -format VDI -remember


And then create a new VM that you can use for the new set of snapshots (as we said, you can’t actually see a tree of snapshots, but you can represent each branch using its own VM. Not really space efficient, but works).

The problem is that VirtualBox’s clonehd will not let you clone a snapshot other then the base. This is due to the fact that snapshots are simply “difference lists” between the previous snapshot and the current state. Whenever you take a snapshot VirtualBox will “freeze” the current state as a read-only VDI file and create a new VDI where it will save only the blocks that the VM has written too after the snapshot. In order to clone a snapshot VirtualBox will need to first consolidate all the changes between all the previous snapshots. This functionality apparently is not implemented yet.

What you can do is to clone the base snapshot – this is a normal VDI file that all the snapshot VDIs refer to (and it is read-only with regard to the VM so you can clone it even if the VM is currently running). So I can clone “Old State A”, but not “Old State B”. But what I can do is to force VirtualBox to consolidate “Old State A” and “Old State B” back into the base snapshot (“Arbel.vdi” in the above screenshot) by discarding the base snapshot: open the “Snapshots” tab for the VM whose state you want to clone, select the first snapshot “Old State A” and choose “discard” from the toolbar. What really happens is that VirtualBox pretends to remove “Old State A” but it actually removes “Old State B” after copying all the changes in “B” back into “A”. You now have a base snapshot which contains the state in “Old State B” which you can clone. Do note though that in VDI file terms, the VDI file for “Old State B” (4e1f756e-f158-4c07-bc9a-20261f574730 above) was removed and you need to clone the base VDI, like this:

$ VBoxManage clonehd .VirtualBox/HardDisks/Arbel.vdi 
 .VirtualBox/HardDisks/new-old-state-b.vdi -format VDI -remember


Now you can create a new VM with the “new-old-state-b” VDI as the boot hard disk.

What happened to “Old State A”? Its gone – not there any more. If you want to keep it in case you want to base a new set of snapshots on that, you have to clone it first before discarding it (for the cost of the whole disk space it took, of course).

I do wish though that VirtualBox would offer the ability to manage a tree of snapshots it would make the whole situation much more convenient to work with for QA purposes, and I believe they will offer this capability in the near future as most of the infrastructure seems to be already in place (including the UI to display it 🙂 ). Another tiny feature that I’m missing from VMWare Workstation is “VM groups” to make it easier to manage many VMs – but that is a different story altogether.

Update:
Starting with version 3.1, VirtualBox does support a tree of snapshots – it will let you boot to any intermediary snapshot and take new snapshots from that. Which is more or less exactly what I wanted.

Its not entirely trivial to use: to start a VM from an old snapshot, choose the snapshot to use and select “Restore Snapshot” – VirtualBox will then rebase the “current state” on that snapshot by discarding the old “current state” and “forking” a new “current state” from the snapshot you’ve chosen. You can then start the VM based on this new state.

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  1. which is most often not what you want. If you don’t mind discarding all of the old snapshots, you can clone the current VMs state using the method described in this VirtualBox forum thread []

9 Responses to “Cloning VirtualBox VM Snapshots”

  1. Robert Lancer:

    You can also export a virtual appliance of the latest snapshot and that consolidates it very well into one VDI

  2. Oded:

    Yes, but that would export what is called the HEAD in SCM terms. For my purpose though, which is creating different VMs with different configurations: For example on one VM I’d install Flash Player 9 and on another I’d install Flash Player 10.

    What I can do is take a snapshot after I got base working and before installing my target configuration, then install the target configuration and clone the HEAD (latest configuration). Then I’d revert back to base and install a new configuration.

    Good idea – it’d probably be easier then the process I described above.

  3. David:

    If you “fork” as per your “Update” at the end of your article, is the old ‘head’ retained or do you lose it?

    What I want to do is create a ‘base OS install’ and then fork different VMs from that, so I only install the OS once. I don’t want to damage my existing VMs if I fork from this base state at a later time.

  4. Oded:

    With VirtualBox 3.1 It will be as you plan. The way to do it is:
    1. install your base OS, then shut it down and get a snapshot.
    2. Run your VM and install whatever extra stuff you need, then shut it down and take a snapshot.
    3. Choose the first snapshot from VirtualBoxe’s Snapshots tab and click “Restore”. It will warn about losing the current state so just OK that (the current state is identical to the last snapshot).
    4. Now run the VM and install other extra stuff that you need, shut it down and take a snapshot.

    What you will have then is 3 snapshots:
    – Snapshot 1: Just your base OS ready to install other stuff
    – Snapshot 2: Base OS with software setup A – you can always restore to it and boot your VM and you’ll get that setup.
    – Snapshot 3 (from step 4 above): Base OS with software setup B – you can always restore to it and boot your VM and you’ll get that setup.

    Also – when you’re done with one setup and want to switch to the otherside of the fork, don’t bother shutting down properly – just close the VM and tell it to “restore to snapshot” and then it will revert to how it was just after the snapshot – ready to be run again with no messy shutdown process, and no “current state” to worry about. You can also get the snapshot while the VM is running, and then when you restore to the snapshot and power on the VM it will immediately be running!

    I personally am still using my original setup (before VirtualBox 3.1) because I want the ability to run both copies of the setup at the same time 🙂 .

  5. David:

    Thank you Oded – just what I wanted to know 🙂

  6. Jonathan Dickinson:

    FYI: VBox is not a hypervisor – things like ESXi and Xen are hypervisors (because they don’t run inside your primary operating system a.k.a. domain0, your primary operating system runs in them) – funny thing is Microsoft HyperV fails to qualify as a hypervisor because it runs partially inside domain0 :).

    At any rate thanks for the article, most helpful.

    • Oded:

      Thanks for the comment, but I don’t think you are right in this – a hypervisor is not an operating system: both ESXi and Xen are software stacks that run on top of an operating system – the commercial products actually package a small Linux distribution and install that “underneath” the hypervisor.

      Technically term “hypervisor” is currently used to mean the software that interacts with the CPU to run another operating system in “protected mode” that is protected from the host operating system (whether it is a prepackaged light weight Linux or an operating system that is used to run other things such as a graphical desktop), and in that sense both VirtualBox and HyperV are fully qualified hypervisor. You can say that commercially you prefer to use the term “hypervisor” to only mean products that run on top of a packaged operating system and do not let you access the host operating system as a general purpose computing platform, but I don’t think that is technically correct.

      • Jonathan Dickinson:

        Ah I just read up on it on Wikipedia and it looks like my previous literature was incorrect – Type 1 hypervisors are the ones I was thinking about (where Type 2 are user-space applications such as VBox).

  7. Liviu Balan:

    If you want to learn more about Snapshot and Clone in VirtualBox you can checkout VirtualBox Snapshot and Clone video tutorial.

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