| 1 | <?xml version="1.0" encoding="UTF-8"?>
|
|---|
| 2 | <!DOCTYPE topic
|
|---|
| 3 | PUBLIC "-//OASIS//DTD DITA Topic//EN" "topic.dtd">
|
|---|
| 4 | <topic xml:lang="en-us" id="snapshots-contents">
|
|---|
| 5 | <title>Snapshot Contents</title>
|
|---|
| 6 |
|
|---|
| 7 | <body>
|
|---|
| 8 | <p>
|
|---|
| 9 | Think of a snapshot as a point in time that you have preserved.
|
|---|
| 10 | More formally, a snapshot consists of the following:
|
|---|
| 11 | </p>
|
|---|
| 12 | <ul>
|
|---|
| 13 | <li>
|
|---|
| 14 | <p>
|
|---|
| 15 | The snapshot contains a complete copy of the VM settings,
|
|---|
| 16 | including the hardware configuration, so that when you
|
|---|
| 17 | restore a snapshot, the VM settings are restored as well.
|
|---|
| 18 | For example, if you changed the hard disk configuration or
|
|---|
| 19 | the VM's system settings, that change is undone when you
|
|---|
| 20 | restore the snapshot.
|
|---|
| 21 | </p>
|
|---|
| 22 | <p>
|
|---|
| 23 | The copy of the settings is stored in the machine
|
|---|
| 24 | configuration, an XML text file, and thus occupies very
|
|---|
| 25 | little space.
|
|---|
| 26 | </p>
|
|---|
| 27 | </li>
|
|---|
| 28 | <li>
|
|---|
| 29 | <p>
|
|---|
| 30 | The complete state of all the virtual disks attached to the
|
|---|
| 31 | machine is preserved. Going back to a snapshot means that
|
|---|
| 32 | all changes that had been made to the machine's disks, file
|
|---|
| 33 | by file and bit by bit, will be undone as well. Files that
|
|---|
| 34 | were since created will disappear, files that were deleted
|
|---|
| 35 | will be restored, changes to files will be reverted.
|
|---|
| 36 | </p>
|
|---|
| 37 | <p>
|
|---|
| 38 | Strictly speaking, this is only true for virtual hard disks
|
|---|
| 39 | in "normal" mode. You can configure disks to behave
|
|---|
| 40 | differently with snapshots, see
|
|---|
| 41 | <xref href="hdimagewrites.dita#hdimagewrites"/>. In technical terms, it is
|
|---|
| 42 | not the virtual disk itself that is restored when a snapshot
|
|---|
| 43 | is restored. Instead, when a snapshot is taken,
|
|---|
| 44 | Oracle VM VirtualBox creates differencing images which contain
|
|---|
| 45 | only the changes since the snapshot were taken. When the
|
|---|
| 46 | snapshot is restored, Oracle VM VirtualBox throws away that
|
|---|
| 47 | differencing image, thus going back to the previous state.
|
|---|
| 48 | This is both faster and uses less disk space. For the
|
|---|
| 49 | details, which can be complex, see
|
|---|
| 50 | <xref href="diffimages.dita#diffimages"/>.
|
|---|
| 51 | </p>
|
|---|
| 52 | <p>
|
|---|
| 53 | Creating the differencing image as such does not occupy much
|
|---|
| 54 | space on the host disk initially, since the differencing
|
|---|
| 55 | image will initially be empty and grow dynamically later
|
|---|
| 56 | with each write operation to the disk. The longer you use
|
|---|
| 57 | the machine after having created the snapshot, however, the
|
|---|
| 58 | more the differencing image will grow in size.
|
|---|
| 59 | </p>
|
|---|
| 60 | </li>
|
|---|
| 61 | <li>
|
|---|
| 62 | <p>
|
|---|
| 63 | If you took a snapshot while the machine was running, the
|
|---|
| 64 | memory state of the machine is also saved in the snapshot.
|
|---|
| 65 | This is in the same way that memory can be saved when you
|
|---|
| 66 | close a VM window. When you restore such a snapshot,
|
|---|
| 67 | execution resumes at exactly the point when the snapshot was
|
|---|
| 68 | taken.
|
|---|
| 69 | </p>
|
|---|
| 70 | <p>
|
|---|
| 71 | The memory state file can be as large as the memory size of
|
|---|
| 72 | the VM and will therefore occupy considerable disk space.
|
|---|
| 73 | </p>
|
|---|
| 74 | </li>
|
|---|
| 75 | </ul>
|
|---|
| 76 | </body>
|
|---|
| 77 |
|
|---|
| 78 | </topic>
|
|---|
