5.4. Special Image Write Modes

For each virtual disk image supported by Oracle VM VirtualBox, you can determine separately how it should be affected by write operations from a virtual machine and snapshot operations. This applies to all of the aforementioned image formats (VDI, VMDK, VHD, or HDD) and irrespective of whether an image is fixed-size or dynamically allocated.

By default, images are in normal mode. To mark an existing image with one of the non-standard modes listed below, use VBoxManage modifyhd. See Section 7.24, “VBoxManage modifymedium”. Alternatively, use VBoxManage to attach the image to a VM and use the --mtype argument. See Section 7.19, “VBoxManage storageattach”.

The available virtual disk image modes are as follows:

  • Normal images have no restrictions on how guests can read from and write to the disk. This is the default image mode.

    When you take a snapshot of your virtual machine as described in Section 1.11, “Snapshots”, the state of a normal hard disk is recorded together with the snapshot, and when reverting to the snapshot, its state will be fully reset.

    The image file itself is not reset. Instead, when a snapshot is taken, Oracle VM VirtualBox "freezes" the image file and no longer writes to it. For the write operations from the VM, a second, differencing image file is created which receives only the changes to the original image. See Section 5.5, “Differencing Images”.

    While you can attach the same normal image to more than one virtual machine, only one of these virtual machines attached to the same image file can be executed simultaneously, as otherwise there would be conflicts if several machines write to the same image file.

  • Write-through hard disks are completely unaffected by snapshots. Their state is not saved when a snapshot is taken, and not restored when a snapshot is restored.

  • Shareable hard disks are a variant of write-through hard disks. In principle they behave exactly the same. Their state is not saved when a snapshot is taken, and not restored when a snapshot is restored. The difference only shows if you attach such disks to several VMs. Shareable disks may be attached to several VMs which may run concurrently. This makes them suitable for use by cluster filesystems between VMs and similar applications which are explicitly prepared to access a disk concurrently. Only fixed size images can be used in this way, and dynamically allocated images are rejected.

    Warning

    This is an expert feature, and misuse can lead to data loss, as regular filesystems are not prepared to handle simultaneous changes by several parties.

  • Immutable images only remember write accesses temporarily while the virtual machine is running. All changes are lost when the virtual machine is powered on the next time. As a result, as opposed to Normal images, the same immutable image can be used with several virtual machines without restrictions.

    Creating an immutable image makes little sense since it would be initially empty and lose its contents with every machine restart. You would have a disk that is always unformatted when the machine starts up. Instead, you can first create a normal image and then later mark it as immutable when you decide that the contents are useful.

    If you take a snapshot of a machine with immutable images, then on every machine power-up, those images are reset to the state of the last (current) snapshot, instead of the state of the original immutable image.

    Note

    As a special exception, immutable images are not reset if they are attached to a machine in a saved state or whose last snapshot was taken while the machine was running. This is called an online snapshot. As a result, if the machine's current snapshot is an online snapshot, its immutable images behave exactly like the a normal image. To reenable the automatic resetting of such images, delete the current snapshot of the machine.

    Oracle VM VirtualBox never writes to an immutable image directly at all. All write operations from the machine are directed to a differencing image. The next time the VM is powered on, the differencing image is reset so that every time the VM starts, its immutable images have exactly the same content.

    The differencing image is only reset when the machine is powered on from within Oracle VM VirtualBox, not when you reboot by requesting a reboot from within the machine. This is also why immutable images behave as described above when snapshots are also present, which use differencing images as well.

    If the automatic discarding of the differencing image on VM startup does not fit your needs, you can turn it off using the autoreset parameter of VBoxManage modifyhd. See Section 7.24, “VBoxManage modifymedium”.

  • Multiattach mode images can be attached to more than one virtual machine at the same time, even if these machines are running simultaneously. For each virtual machine to which such an image is attached, a differencing image is created. As a result, data that is written to such a virtual disk by one machine is not seen by the other machines to which the image is attached. Each machine creates its own write history of the multiattach image.

    Technically, a multiattach image behaves identically to an immutable image except the differencing image is not reset every time the machine starts.

    This mode is useful for sharing files which are almost never written, for instance picture galleries, where every guest changes only a small amount of data and the majority of the disk content remains unchanged. The modified blocks are stored in differencing images which remain relatively small and the shared content is stored only once at the host.

  • Read-only images are used automatically for CD/DVD images, since CDs/DVDs can never be written to.

The following scenario illustrates the differences between the various image modes, with respect to snapshots.

Assume you have installed your guest OS in your VM, and you have taken a snapshot. Later, your VM is infected with a virus and you would like to go back to the snapshot. With a normal hard disk image, you simply restore the snapshot, and the earlier state of your hard disk image will be restored as well and your virus infection will be undone. With an immutable hard disk, all it takes is to shut down and power on your VM, and the virus infection will be discarded. With a write-through image however, you cannot easily undo the virus infection by means of virtualization, but will have to disinfect your virtual machine like a real computer.

You might find write-through images useful if you want to preserve critical data irrespective of snapshots. As you can attach more than one image to a VM, you may want to have one immutable image for the OS and one write-through image for your data files.