Sun Microsystems VIRTUALBOX VERSION 3.1.0_BETA2 - Manual (Page 84)

5 Virtual storage
noticeable however since the tables with sector information are always kept in memory
and can be looked up quickly.
Differencing images are used in the following situations:
1. Snapshots. When you create a snapshot, as explained in the previous section,
VirtualBox “freezes” the images of the virtual machine and creates differencing
images for each of them (to be precise: for those images which are not in “write-
through” mode). From the point of view of the virtual machine, the virtual
disks continue to operate before, but all write operations go into the differencing
images. Each time you create another snapshot, more differencing images are
created and attached forming a chain or tree.
If you now restore a snapshot – that is, if you want to go back to the exact
machine state that was stored in the snapshot –, the following happens:
a) VirtualBox copies the virtual machine settings that were copied into the
snapshot back to the virtual machine. As a result, if you have made changes
to the machine configuration since taking the snapshot, they are undone.
b) If the snapshot was taken while the machine was running, it contains a
saved machine state, and that state is restored as well; after restoring the
snapshot, the machine will then be in “Saved” state and resume execution
from there when it is next started. Otherwise it will be in “Powered Off”
state and do a full boot.
c) The differencing images holding all the write operations since the snapshot
was taken are thrown away, and the original parent images are made active
again. (If you restored the “root” snapshot, then this will be the root disk
images; otherwise, some other differencing image descended from it.) This
effectively restores the old machine state.
If you later delete a snapshot in order to free disk space, one of the differencing
images becomes obsolete. In this case, the differencing images cannot simply be
deleted. Instead, VirtualBox needs to look at each sector of the differencing im-
age and needs to copy it back into the parent; this is called “merging” images and
can be a potentially lengthy process, depending on how large the differencing
image is.
2. Immutable images. When an image is switched to “immutable” mode, a dif-
ferencing image is created as well. As with snapshots, the parent image then
becomes read-only, and the differencing image receives all the write operations.
Every time the virtual machine is started, all the immutable images which are
attached to it have their differencing images thrown away, effectively resetting
the virtual machine’s virtual disk with every restart.
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