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The variety of controllers is only supplied for Virtual Box for compatibility with existing hardware and other hypervisors.
Disk image files reside on the host system and are seen by the guest systems as hard disks of a certain geometry.
As a result, the differencing images can form a complex tree with parents, "siblings" and children, depending on how complex your machine configuration is.
Write operations always go to the one "active" differencing image that is attached to the machine, and for read operations, Virtual Box may need to look up all the parents in the chain until the sector in question is found.
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.
Still, you might find write-through images useful if you want to preserve critical data irrespective of snapshots, and since you can attach more than one image to a VM, you may want to have one immutable for the operating system and one write-through for your data files.
Irrespective of the disk capacity and format, as briefly mentioned in Section1.7, “Creating your first virtual machine”, there are two options of how to create a disk image: fixed-size or dynamically allocated.When a differencing image is active, it receives all write operations from the virtual machine instead of its parent.The differencing image only contains the sectors of the virtual hard disk that have changed since the differencing image was created.Imagine you have accidentally infected your VM with a virus and 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).
When a guest operating system reads from or writes to a hard disk, Virtual Box redirects the request to the image file.