5.4. Special image write modes

With normal images (the default setting), there are no restrictions on how guests can read from and write to the disk.

When you take a snapshot of your virtual machine as described in Section 1.10, “Snapshots”, the state of such a "normal hard disk" will be recorded together with the snapshot, and when reverting to the snapshot, its state will be fully reset.

(Technically, strictly speaking, the image file itself is not "reset". Instead, when a snapshot is taken, 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 the next section for details.)

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.^[28]

By contrast, 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, i.e. 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.

Next, 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 (unless you really want to have a disk that is always unformatted when the machine starts up). As a result, normally, you would first create a "normal" image and then, when you deem its contents useful, later mark it immutable.

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).

Again, technically, VirtualBox never writes to an immutable image directly at all. All write operations from the machine will be 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.^[29] The differencing image is only reset when the machine is powered on from within 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 8.23, “VBoxManage modifymedium” for details.

An image in multiattach mode 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.

Finally, the read-only image is used automatically for CD/DVD images, since CDs/DVDs can never be written to.