| 1 | <?xml version='1.0' encoding='UTF-8'?>
|
|---|
| 2 | <!DOCTYPE topic PUBLIC "-//OASIS//DTD DITA Topic//EN" "topic.dtd">
|
|---|
| 3 | <topic xml:lang="en-us" id="diffimages">
|
|---|
| 4 | <title>Differencing Images</title>
|
|---|
| 5 |
|
|---|
| 6 | <body>
|
|---|
| 7 | <p>The previous section mentioned differencing images and how they are used with snapshots, immutable images, and
|
|---|
| 8 | multiple disk attachments. This section describes in more detail how differencing images work. </p>
|
|---|
| 9 | <p>A differencing image is a special disk image that only holds the differences to another image. A differencing
|
|---|
| 10 | image by itself is useless, it must always refer to another image. The differencing image is then typically
|
|---|
| 11 | referred to as a <i>child</i>, which holds the differences to its <i>parent</i>. </p>
|
|---|
| 12 | <p>When a differencing image is active, it receives all write operations from the virtual machine instead of its
|
|---|
| 13 | parent. The differencing image only contains the sectors of the virtual hard disk that have changed since the
|
|---|
| 14 | differencing image was created. When the machine reads a sector from such a virtual hard disk, it looks into the
|
|---|
| 15 | differencing image first. If the sector is present, it is returned from there. If not, <ph
|
|---|
| 16 | conkeyref="vbox-conkeyref-phrases/product-name"/> looks into the parent. In other words, the parent becomes
|
|---|
| 17 | <i>read-only</i>. It is never written to again, but it is read from if a sector has not changed. </p>
|
|---|
| 18 | <p>Differencing images can be chained. If another differencing image is created for a virtual disk that already has
|
|---|
| 19 | a differencing image, then it becomes a <i>grandchild</i> of the original parent. The first differencing image
|
|---|
| 20 | then becomes read-only as well, and write operations only go to the second-level differencing image. When reading
|
|---|
| 21 | from the virtual disk, <ph conkeyref="vbox-conkeyref-phrases/product-name"/> needs to look into the second
|
|---|
| 22 | differencing image first, then into the first if the sector was not found, and then into the original image. </p>
|
|---|
| 23 | <p>There can be an unlimited number of differencing images, and each image can have more than one child. As a
|
|---|
| 24 | result, the differencing images can form a complex tree with parents, siblings, and children, depending on how
|
|---|
| 25 | complex your machine configuration is. Write operations always go to the one <i>active</i> differencing image that
|
|---|
| 26 | is attached to the machine, and for read operations, <ph conkeyref="vbox-conkeyref-phrases/product-name"/> may
|
|---|
| 27 | need to look up all the parents in the chain until the sector in question is found. You can view such a tree in
|
|---|
| 28 | the Virtual Media Manager. </p>
|
|---|
| 29 | <fig id="fig-diff-images">
|
|---|
| 30 | <title>Differencing Images, Shown in Virtual Media Manager</title>
|
|---|
| 31 | <xref href="images/virtual-disk-manager-2.png" format="png" platform="htmlhelp">
|
|---|
| 32 | <image href="images/virtual-disk-manager-2.png" width="12cm" placement="break">
|
|---|
| 33 | <alt>Differencing Images, Shown in Virtual Media Manager</alt>
|
|---|
| 34 | </image>
|
|---|
| 35 | </xref>
|
|---|
| 36 | <image platform="ohc" href="images/virtual-disk-manager-2.png" width="12cm" placement="break">
|
|---|
| 37 | <alt>Differencing Images, Shown in Virtual Media Manager</alt>
|
|---|
| 38 | </image>
|
|---|
| 39 | </fig>
|
|---|
| 40 | <p>In all of these situations, from the point of view of the virtual machine, the virtual hard disk behaves like any
|
|---|
| 41 | other disk. While the virtual machine is running, there is a slight runtime I/O overhead because <ph
|
|---|
| 42 | conkeyref="vbox-conkeyref-phrases/product-name"/> might need to look up sectors several times. This is not
|
|---|
| 43 | noticeable however since the tables with sector information are always kept in memory and can be looked up
|
|---|
| 44 | quickly. </p>
|
|---|
| 45 | <p>Differencing images are used in the following situations: </p>
|
|---|
| 46 | <ul>
|
|---|
| 47 | <li>
|
|---|
| 48 | <p><b outputclass="bold">Snapshots.</b> When you create a
|
|---|
| 49 | snapshot, as explained in the previous section, <ph conkeyref="vbox-conkeyref-phrases/product-name"/>
|
|---|
| 50 | <i>freezes</i> the images attached to the
|
|---|
| 51 | virtual machine and creates differencing images for each image
|
|---|
| 52 | that is not in <i>write-through</i> mode. From
|
|---|
| 53 | the point of view of the virtual machine, the virtual disks
|
|---|
| 54 | continue to operate before, but all write operations go into
|
|---|
| 55 | the differencing images. Each time you create another
|
|---|
| 56 | snapshot, for each hard disk attachment, another differencing
|
|---|
| 57 | image is created and attached, forming a chain or tree.
|
|---|
| 58 | </p>
|
|---|
| 59 | <p>In the above screenshot, you see that the original disk image is now attached to a snapshot, representing the
|
|---|
| 60 | state of the disk when the snapshot was taken. </p>
|
|---|
| 61 | <p>If you <i>restore</i> a snapshot, and want to go back to the exact machine state that was stored in the
|
|---|
| 62 | snapshot, the following happens: </p>
|
|---|
| 63 | <ul>
|
|---|
| 64 | <li>
|
|---|
| 65 | <p><ph conkeyref="vbox-conkeyref-phrases/product-name"/> copies the virtual machine settings that were
|
|---|
| 66 | copied into the snapshot back to the virtual machine. As a result, if you have made changes to the machine
|
|---|
| 67 | configuration since taking the snapshot, they are undone. </p>
|
|---|
| 68 | </li>
|
|---|
| 69 | <li>
|
|---|
| 70 | <p>If the snapshot was taken while the machine was running, it contains a saved machine state, and that
|
|---|
| 71 | state is restored as well. After restoring the snapshot, the machine will then be in Saved state and
|
|---|
| 72 | resume execution from there when it is next started. Otherwise the machine will be in Powered Off state
|
|---|
| 73 | and do a full boot. </p>
|
|---|
| 74 | </li>
|
|---|
| 75 | <li>
|
|---|
| 76 | <p>For each disk image attached to the machine, the differencing image holding all the write operations
|
|---|
| 77 | since the current snapshot was taken is thrown away, and the original parent image is made active again.
|
|---|
| 78 | If you restored the root snapshot, then this will be the root disk image for each attachment. Otherwise,
|
|---|
| 79 | some other differencing image descended from it. This effectively restores the old machine state. </p>
|
|---|
| 80 | </li>
|
|---|
| 81 | </ul>
|
|---|
| 82 | <p>If you later <i>delete</i> a snapshot in order to free disk space, for each disk attachment, one of the
|
|---|
| 83 | differencing images becomes obsolete. In this case, the differencing image of the disk attachment cannot
|
|---|
| 84 | simply be deleted. Instead, <ph conkeyref="vbox-conkeyref-phrases/product-name"/> needs to look at each sector
|
|---|
| 85 | of the differencing image and needs to copy it back into its parent. This is called "merging" images and can
|
|---|
| 86 | be a potentially lengthy process, depending on how large the differencing image is. It can also temporarily
|
|---|
| 87 | need a considerable amount of extra disk space, before the differencing image obsoleted by the merge operation
|
|---|
| 88 | is deleted. </p>
|
|---|
| 89 | </li>
|
|---|
| 90 | <li>
|
|---|
| 91 | <p><b outputclass="bold">Immutable images.</b> When an
|
|---|
| 92 | image is switched to immutable mode, a differencing image is
|
|---|
| 93 | created as well. As with snapshots, the parent image then
|
|---|
| 94 | becomes read-only, and the differencing image receives all the
|
|---|
| 95 | write operations. Every time the virtual machine is started,
|
|---|
| 96 | all the immutable images which are attached to it have their
|
|---|
| 97 | respective differencing image thrown away, effectively
|
|---|
| 98 | resetting the virtual machine's virtual disk with every
|
|---|
| 99 | restart.
|
|---|
| 100 | </p>
|
|---|
| 101 | </li>
|
|---|
| 102 | </ul>
|
|---|
| 103 | </body>
|
|---|
| 104 |
|
|---|
| 105 | </topic>
|
|---|
