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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE topic
PUBLIC "-//OASIS//DTD DITA Topic//EN" "topic.dtd">
<topic xml:lang="en-us" id="snapshots-contents">
<title>Snapshot Contents</title>
<body>
<p>Think of a snapshot as a point in time that you have preserved. More formally, a snapshot consists
of the following: </p>
<ul>
<li>
<p>The snapshot contains a complete copy of the VM settings, including the hardware
configuration, so that when you restore a snapshot, the VM settings are restored as well. For example, if you
changed the hard disk configuration or the VM's system settings, that change is undone when you restore the
snapshot. </p>
<p>The copy of the settings is stored in the machine configuration, an XML text file, and thus
occupies very little space. </p>
</li>
<li>
<p>The complete state of all the virtual disks attached to the machine is preserved. Going back
to a snapshot means that all changes that had been made to the machine's disks, file by file and bit by bit,
will be undone. Files that were since created will disappear, files that were deleted will be restored,
changes to files will be reverted. </p>
<p>Strictly speaking, this is only true for virtual hard disks in "normal" mode. You can
configure disks to behave differently with snapshots, see <xref href="hdimagewrites.dita#hdimagewrites"/>. In
technical terms, it is not the virtual disk itself that is restored when a snapshot is restored. Instead, when
a snapshot is taken, <ph conkeyref="vbox-conkeyref-phrases/product-name"/> creates differencing images which
contain only the changes since the snapshot were taken. When the snapshot is restored, <ph
conkeyref="vbox-conkeyref-phrases/product-name"/> throws away that differencing image, thus going back to
the previous state. This is both faster and uses less disk space. For the details, which can be complex, see
<xref href="diffimages.dita#diffimages"/>. </p>
<p>Creating the differencing image as such does not occupy much space on the host disk
initially, since the differencing image will initially be empty and grow dynamically later with each write
operation to the disk. The longer you use the machine after having created the snapshot, however, the more the
differencing image will grow in size. </p>
</li>
<li>
<p>If you took a snapshot while the machine was running, the memory state of the machine is also
saved in the snapshot. This is in the same way that memory can be saved when you close a VM window. When you
restore such a snapshot, execution resumes at exactly the point when the snapshot was taken. </p>
<p>The memory state file can be as large as the memory size of the VM and will therefore occupy
considerable disk space. </p>
</li>
</ul>
</body>
</topic>
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