| 1 | <?xml version='1.0' encoding='UTF-8'?>
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| 2 | <!DOCTYPE topic PUBLIC "-//OASIS//DTD DITA Topic//EN" "topic.dtd">
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| 3 | <topic xml:lang="en-us" id="diffimages">
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| 4 | <title>Differencing Images</title>
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| 5 |
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| 6 | <body>
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| 7 | <p>
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| 8 | The previous section mentioned differencing images and how they
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| 9 | are used with snapshots, immutable images, and multiple disk
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| 10 | attachments. This section describes in more detail how
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| 11 | differencing images work.
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| 12 | </p>
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| 13 | <p>
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| 14 | A differencing image is a special disk image that only holds the
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| 15 | differences to another image. A differencing image by itself is
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| 16 | useless, it must always refer to another image. The differencing
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| 17 | image is then typically referred to as a
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| 18 | <i>child</i>, which holds the differences to its
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| 19 | <i>parent</i>.
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| 20 | </p>
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| 21 | <p>
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| 22 | When a differencing image is active, it receives all write
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| 23 | operations from the virtual machine instead of its parent. The
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| 24 | differencing image only contains the sectors of the virtual hard
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| 25 | disk that have changed since the differencing image was created.
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| 26 | When the machine reads a sector from such a virtual hard disk, it
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| 27 | looks into the differencing image first. If the sector is present,
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| 28 | it is returned from there. If not, Oracle VM VirtualBox looks into the
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| 29 | parent. In other words, the parent becomes
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| 30 | <i>read-only</i>. It is never written to again, but
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| 31 | it is read from if a sector has not changed.
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| 32 | </p>
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| 33 | <p>
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| 34 | Differencing images can be chained. If another differencing image
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| 35 | is created for a virtual disk that already has a differencing
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| 36 | image, then it becomes a <i>grandchild</i> of the
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| 37 | original parent. The first differencing image then becomes
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| 38 | read-only as well, and write operations only go to the
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| 39 | second-level differencing image. When reading from the virtual
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| 40 | disk, Oracle VM VirtualBox needs to look into the second differencing
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| 41 | image first, then into the first if the sector was not found, and
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| 42 | then into the original image.
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| 43 | </p>
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| 44 | <p>
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| 45 | There can be an unlimited number of differencing images, and each
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| 46 | image can have more than one child. As a result, the differencing
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| 47 | images can form a complex tree with parents, siblings, and
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| 48 | children, depending on how complex your machine configuration is.
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| 49 | Write operations always go to the one <i>active</i>
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| 50 | differencing image that is attached to the machine, and for read
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| 51 | operations, Oracle VM VirtualBox may need to look up all the parents in
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| 52 | the chain until the sector in question is found. You can view such
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| 53 | a tree in the Virtual Media Manager.
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| 54 | </p>
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| 55 | <fig id="fig-diff-images">
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| 56 | <title>Differencing Images, Shown in Virtual Media Manager</title>
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| 57 | <image href="images/virtual-disk-manager-2.png" placement="break"/>
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| 58 | </fig>
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| 59 | <p>
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| 60 | In all of these situations, from the point of view of the virtual
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| 61 | machine, the virtual hard disk behaves like any other disk. While
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| 62 | the virtual machine is running, there is a slight run-time I/O
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| 63 | overhead because Oracle VM VirtualBox might need to look up sectors
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| 64 | several times. This is not noticeable however since the tables
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| 65 | with sector information are always kept in memory and can be
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| 66 | looked up quickly.
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| 67 | </p>
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| 68 | <p>
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| 69 | Differencing images are used in the following situations:
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| 70 | </p>
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| 71 | <ul>
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| 72 | <li>
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| 73 | <p><b outputclass="bold">Snapshots.</b> When you create a
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| 74 | snapshot, as explained in the previous section, Oracle VM VirtualBox
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| 75 | <i>freezes</i> the images attached to the
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| 76 | virtual machine and creates differencing images for each image
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| 77 | that is not in <i>write-through</i> mode. From
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| 78 | the point of view of the virtual machine, the virtual disks
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| 79 | continue to operate before, but all write operations go into
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| 80 | the differencing images. Each time you create another
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| 81 | snapshot, for each hard disk attachment, another differencing
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| 82 | image is created and attached, forming a chain or tree.
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| 83 | </p>
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| 84 | <p>
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| 85 | In the above screenshot, you see that the original disk image
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| 86 | is now attached to a snapshot, representing the state of the
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| 87 | disk when the snapshot was taken.
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| 88 | </p>
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| 89 | <p>
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| 90 | If you <i>restore</i> a snapshot, and want to go
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| 91 | back to the exact machine state that was stored in the
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| 92 | snapshot, the following happens:
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| 93 | </p>
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| 94 | <ul>
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| 95 | <li>
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| 96 | <p>
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| 97 | Oracle VM VirtualBox copies the virtual machine settings that
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| 98 | were copied into the snapshot back to the virtual machine.
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| 99 | As a result, if you have made changes to the machine
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| 100 | configuration since taking the snapshot, they are undone.
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| 101 | </p>
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| 102 | </li>
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| 103 | <li>
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| 104 | <p>
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| 105 | If the snapshot was taken while the machine was running,
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| 106 | it contains a saved machine state, and that state is
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| 107 | restored as well. After restoring the snapshot, the
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| 108 | machine will then be in Saved state and resume execution
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| 109 | from there when it is next started. Otherwise the machine
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| 110 | will be in Powered Off state and do a full boot.
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| 111 | </p>
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| 112 | </li>
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| 113 | <li>
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| 114 | <p>
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| 115 | For each disk image attached to the machine, the
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| 116 | differencing image holding all the write operations since
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| 117 | the current snapshot was taken is thrown away, and the
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| 118 | original parent image is made active again. If you
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| 119 | restored the root snapshot, then this will be the root
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| 120 | disk image for each attachment. Otherwise, some other
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| 121 | differencing image descended from it. This effectively
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| 122 | restores the old machine state.
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| 123 | </p>
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| 124 | </li>
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| 125 | </ul>
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| 126 | <p>
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| 127 | If you later <i>delete</i> a snapshot in order
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| 128 | to free disk space, for each disk attachment, one of the
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| 129 | differencing images becomes obsolete. In this case, the
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| 130 | differencing image of the disk attachment cannot simply be
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| 131 | deleted. Instead, Oracle VM VirtualBox needs to look at each sector
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| 132 | of the differencing image and needs to copy it back into its
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| 133 | parent. This is called "merging" images and can be a
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| 134 | potentially lengthy process, depending on how large the
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| 135 | differencing image is. It can also temporarily need a
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| 136 | considerable amount of extra disk space, before the
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| 137 | differencing image obsoleted by the merge operation is
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| 138 | deleted.
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| 139 | </p>
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| 140 | </li>
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| 141 | <li>
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| 142 | <p><b outputclass="bold">Immutable images.</b> When an
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| 143 | image is switched to immutable mode, a differencing image is
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| 144 | created as well. As with snapshots, the parent image then
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| 145 | becomes read-only, and the differencing image receives all the
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| 146 | write operations. Every time the virtual machine is started,
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| 147 | all the immutable images which are attached to it have their
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| 148 | respective differencing image thrown away, effectively
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| 149 | resetting the virtual machine's virtual disk with every
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| 150 | restart.
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| 151 | </p>
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| 152 | </li>
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| 153 | </ul>
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| 154 | </body>
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| 155 |
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| 156 | </topic>
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