formatsnapshot.html.in

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    <h1>Snapshot XML format</h1>

    <ul id="toc"></ul>

    <h2><a id="SnapshotAttributes">Snapshot XML</a></h2>

    <p>
      There are several types of snapshots:
    </p>
    <dl>
      <dt>disk snapshot</dt>
      <dd>Contents of disks (whether a subset or all disks associated
        with the domain) are saved at a given point of time, and can
        be restored back to that state.  On a running guest, a disk
        snapshot is likely to be only crash-consistent rather than
        clean (that is, it represents the state of the disk on a
        sudden power outage, and may need fsck or journal replays to
        be made consistent); on an inactive guest, a disk snapshot is
        clean if the disks were clean when the guest was last shut
        down.  Disk snapshots exist in two forms: internal (file
        formats such as qcow2 track both the snapshot and changes
        since the snapshot in a single file) and external (the
        snapshot is one file, and the changes since the snapshot are
        in another file).</dd>
      <dt>memory state (or VM state)</dt>
      <dd>Tracks only the state of RAM and all other resources in use
        by the VM.  If the disks are unmodified between the time a VM
        state snapshot is taken and restored, then the guest will
        resume in a consistent state; but if the disks are modified
        externally in the meantime, this is likely to lead to data
        corruption.</dd>
      <dt>system checkpoint</dt>
      <dd>A combination of disk snapshots for all disks as well as VM
        memory state, which can be used to resume the guest from where it
        left off with symptoms similar to hibernation (that is, TCP
        connections in the guest may have timed out, but no files or
        processes are lost).</dd>
    </dl>

    <p>
      Libvirt can manage all three types of snapshots.  For now, VM
      state (memory) snapshots are created only by
      the <code>virDomainSave()</code>, <code>virDomainSaveFlags</code>,
      and <code>virDomainManagedSave()</code> functions, and restored
      via the <code>virDomainRestore()</code>,
      <code>virDomainRestoreFlags()</code>, <code>virDomainCreate()</code>,
      and <code>virDomainCreateWithFlags()</code> functions (as well
      as via domain autostart).  With managed snapshots, libvirt
      tracks all information internally; with save images, the user
      tracks the snapshot file, but libvirt provides functions such
      as <code>virDomainSaveImageGetXMLDesc()</code> to work with
      those files.
    </p>
    <p>System checkpoints are created
      by <code>virDomainSnapshotCreateXML()</code> with no flags, and
      disk snapshots are created by the same function with
      the <code>VIR_DOMAIN_SNAPSHOT_CREATE_DISK_ONLY</code> flag; in
      both cases, they are restored by
      the <code>virDomainRevertToSnapshot()</code> function.  For
      these types of snapshots, libvirt tracks each snapshot as a
      separate <code>virDomainSnapshotPtr</code> object, and maintains
      a tree relationship of which snapshots descended from an earlier
      point in time.
    </p>

    <p>
      Attributes of libvirt snapshots are stored as child elements of
      the <code>domainsnapshot</code> element.  At snapshot creation
      time, normally only the <code>name</code>, <code>description</code>,
      and <code>disks</code> elements are settable; the rest of the
      fields are ignored on creation, and will be filled in by
      libvirt in for informational purposes
      by <code>virDomainSnapshotGetXMLDesc()</code>.  However, when
      redefining a snapshot (<span class="since">since 0.9.5</span>),
      with the <code>VIR_DOMAIN_SNAPSHOT_CREATE_REDEFINE</code> flag
      of <code>virDomainSnapshotCreateXML()</code>, all of the XML
      described here is relevant.
    </p>
    <p>
      Snapshots are maintained in a hierarchy.  A domain can have a
      current snapshot, which is the most recent snapshot compared to
      the current state of the domain (although a domain might have
      snapshots without a current snapshot, if snapshots have been
      deleted in the meantime).  Creating or reverting to a snapshot
      sets that snapshot as current, and the prior current snapshot is
      the parent of the new snapshot.  Branches in the hierarchy can
      be formed by reverting to a snapshot with a child, then creating
      another snapshot.
    </p>
    <p>
      The top-level <code>domainsnapshot</code> element may contain
      the following elements:
    </p>
    <dl>
      <dt><code>name</code></dt>
      <dd>The name for this snapshot.  If the name is specified when
        initially creating the snapshot, then the snapshot will have
        that particular name.  If the name is omitted when initially
        creating the snapshot, then libvirt will make up a name for
        the snapshot, based on the time when it was created.
      </dd>
      <dt><code>description</code></dt>
      <dd>A human-readable description of the snapshot.  If the
        description is omitted when initially creating the snapshot,
        then this field will be empty.
      </dd>
      <dt><code>memory</code></dt>
      <dd>On input, this is an optional request for how to handle VM
        memory state.  For an offline domain or a disk-only snapshot,
        attribute <code>snapshot</code> must be <code>no</code>, since
        there is no VM state saved; otherwise, the attribute can
        be <code>internal</code> if the memory state is piggy-backed with
        other internal disk state, or <code>external</code> along with
        a second attribute <code>file</code> giving the absolute path
        of the file holding the VM memory state.  <span class="since">Since
        1.0.1</span>
      </dd>
      <dt><code>disks</code></dt>
      <dd>On input, this is an optional listing of specific
        instructions for disk snapshots; it is needed when making a
        snapshot of only a subset of the disks associated with a
        domain, or when overriding the domain defaults for how to
        snapshot each disk, or for providing specific control over
        what file name is created in an external snapshot.  On output,
        this is fully populated to show the state of each disk in the
        snapshot, including any properties that were generated by the
        hypervisor defaults.  For system checkpoints, this field is
        ignored on input and omitted on output (a system checkpoint
        implies that all disks participate in the snapshot process,
        and since the current implementation only does internal system
        checkpoints, there are no extra details to add); a future
        release may allow the use of <code>disks</code> with a system
        checkpoint.  This element has a list of <code>disk</code>
        sub-elements, describing anywhere from zero to all of the
        disks associated with the domain.  <span class="since">Since
        0.9.5</span>
        <dl>
          <dt><code>disk</code></dt>
          <dd>This sub-element describes the snapshot properties of a
            specific disk.  The attribute <code>name</code> is
            mandatory, and must match either the <code>&lt;target
            dev='name'/&gt;</code> or an unambiguous <code>&lt;source
            file='name'/&gt;</code> of one of
            the <a href="formatdomain.html#elementsDisks">disk
            devices</a> specified for the domain at the time of the
            snapshot.  The attribute <code>snapshot</code> is
            optional, and the possible values are the same as the
            <code>snapshot</code> attribute for
             <a href="formatdomain.html#elementsDisks">disk devices</a>
            (<code>no</code>, <code>internal</code>,
            or <code>external</code>).  Some hypervisors like ESX
            require that if specified, the snapshot mode must not
            override any snapshot mode attached to the corresponding
            domain disk, while others like qemu allow this field to
            override the domain default.

            <dl>
              <dt><code>source</code></dt>
              <dd>If the snapshot mode is external (whether specified
              or inherited), then there is an optional sub-element
              <code>source</code>, with an attribute <code>file</code>
              giving the name of the new file.
              If <code>source</code> is not
              given and the disk is backed by a local image file (not
              a block device or remote storage), a file name is
              generated that consists of the existing file name
              with anything after the trailing dot replaced by the
              snapshot name.  Remember that with external
              snapshots, the original file name becomes the read-only
              snapshot, and the new file name contains the read-write
              delta of all disk changes since the snapshot.
              </dd>
              <dt><code>driver</code></dt>
              <dd>An optional sub-element <code>driver</code>,
              with an attribute <code>type</code> giving the driver type (such
              as qcow2), of the new file created by the external
              snapshot of the new file.
              </dd>
            </dl>

            <span class="since">Since 1.2.2</span> the <code>disk</code> element
            supports an optional attribute <code>type</code> if the
            <code>snapshot</code> attribute is set to <code>external</code>.
            This attribute specifies the snapshot target storage type and allows
            to overwrite the default <code>file</code> type. The <code>type</code>
            attribute along with the format of the <code>source</code>
            sub-element is identical to the <code>source</code> element used in
            domain disk definitions. See the
            <a href="formatdomain.html#elementsDisks">disk devices</a> section
            documentation for further information.

            Libvirt currently supports the <code>type</code> element in the qemu
            driver and supported values are <code>file</code>, <code>block</code>
            and <code>network</code> with a protocol of <code>gluster</code>
            <span class="since">(since 1.2.2)</span>.
          </dd>
        </dl>
      </dd>
      <dt><code>creationTime</code></dt>
      <dd>The time this snapshot was created.  The time is specified
        in seconds since the Epoch, UTC (i.e. Unix time).  Readonly.
      </dd>
      <dt><code>state</code></dt>
      <dd>The state of the domain at the time this snapshot was taken.
        If the snapshot was created as a system checkpoint, then this
        is the state of the domain at that time; when the domain is
        reverted to this snapshot, the domain's state will default to
        whatever is in this field unless additional flags are passed
        to <code>virDomainRevertToSnapshot()</code>.  Additionally,
        this field can be the value "disk-snapshot"
        (<span class="since">since 0.9.5</span>) when it represents
        only a disk snapshot (no VM memory state), and reverting to this
        snapshot will default to an inactive guest.  Readonly.
      </dd>
      <dt><code>parent</code></dt>
      <dd>The parent of this snapshot.  If present, this element
        contains exactly one child element, name.  This specifies the
        name of the parent snapshot of this snapshot, and is used to
        represent trees of snapshots.  Readonly.
      </dd>
      <dt><code>domain</code></dt>
      <dd>The domain that this snapshot was taken against.  Older
        versions of libvirt stored only a single child element, uuid;
        reverting to a snapshot like this is risky if the current
        state of the domain differs from the state that the domain was
        created in, and requires the use of the
        <code>VIR_DOMAIN_SNAPSHOT_REVERT_FORCE</code> flag
        in <code>virDomainRevertToSnapshot()</code>.  Newer versions
        of libvirt (<span class="since">since 0.9.5</span>) store the entire
        inactive <a href="formatdomain.html">domain configuration</a>
        at the time of the snapshot (<span class="since">since
        0.9.5</span>).  Readonly.
      </dd>
      <dt><code>cookie</code></dt>
      <dd>Save image cookie containing additional data libvirt may need to
        properly restore a domain from an active snapshot when such data
        cannot be stored directly in the <code>domain</code> to maintain
        compatibility with older libvirt or hypervisor. Readonly.
      </dd>
    </dl>

    <h2><a id="example">Examples</a></h2>

    <p>Using this XML to create a disk snapshot of just vda on a qemu
      domain with two disks:</p>
    <pre>
&lt;domainsnapshot&gt;
  &lt;description&gt;Snapshot of OS install and updates&lt;/description&gt;
  &lt;disks&gt;
    &lt;disk name='/path/to/old'&gt;
      &lt;source file='/path/to/new'/&gt;
    &lt;/disk&gt;
    &lt;disk name='vdb' snapshot='no'/&gt;
  &lt;/disks&gt;
&lt;/domainsnapshot&gt;</pre>

    <p>will result in XML similar to this from
      <code>virDomainSnapshotGetXMLDesc()</code>:</p>
    <pre>
&lt;domainsnapshot&gt;
  &lt;name&gt;1270477159&lt;/name&gt;
  &lt;description&gt;Snapshot of OS install and updates&lt;/description&gt;
  &lt;state&gt;running&lt;/state&gt;
  &lt;creationTime&gt;1270477159&lt;/creationTime&gt;
  &lt;parent&gt;
    &lt;name&gt;bare-os-install&lt;/name&gt;
  &lt;/parent&gt;
  &lt;memory snapshot='no'/&gt;
  &lt;disks&gt;
    &lt;disk name='vda' snapshot='external'&gt;
      &lt;driver type='qcow2'/&gt;
      <b>&lt;source file='/path/to/new'/&gt;</b>
    &lt;/disk&gt;
    &lt;disk name='vdb' snapshot='no'/&gt;
  &lt;/disks&gt;
  &lt;domain&gt;
    &lt;name&gt;fedora&lt;/name&gt;
    &lt;uuid&gt;93a5c045-6457-2c09-e56c-927cdf34e178&lt;/uuid&gt;
    &lt;memory&gt;1048576&lt;/memory&gt;
    ...
    &lt;devices&gt;
      &lt;disk type='file' device='disk'&gt;
        &lt;driver name='qemu' type='raw'/&gt;
        <b>&lt;source file='/path/to/old'/&gt;</b>
        &lt;target dev='vda' bus='virtio'/&gt;
      &lt;/disk&gt;
      &lt;disk type='file' device='disk' snapshot='external'&gt;
        &lt;driver name='qemu' type='raw'/&gt;
        &lt;source file='/path/to/old2'/&gt;
        &lt;target dev='vdb' bus='virtio'/&gt;
      &lt;/disk&gt;
      ...
    &lt;/devices&gt;
  &lt;/domain&gt;
&lt;/domainsnapshot&gt;</pre>

    <p>With that snapshot created, <code>/path/to/old</code> is the
      read-only backing file to the new active
      file <code>/path/to/new</code>.  The <code>&lt;domain&gt;</code>
      element within the snapshot xml records the state of the domain
      just before the snapshot; a call
      to <code>virDomainGetXMLDesc()</code> will show that the domain
      has been changed to reflect the snapshot:
    </p>
    <pre>
&lt;domain&gt;
  &lt;name&gt;fedora&lt;/name&gt;
  &lt;uuid&gt;93a5c045-6457-2c09-e56c-927cdf34e178&lt;/uuid&gt;
  &lt;memory&gt;1048576&lt;/memory&gt;
  ...
  &lt;devices&gt;
    &lt;disk type='file' device='disk'&gt;
      &lt;driver name='qemu' type='qcow2'/&gt;
      <b>&lt;source file='/path/to/new'/&gt;</b>
      &lt;target dev='vda' bus='virtio'/&gt;
    &lt;/disk&gt;
    &lt;disk type='file' device='disk' snapshot='external'&gt;
      &lt;driver name='qemu' type='raw'/&gt;
      &lt;source file='/path/to/old2'/&gt;
      &lt;target dev='vdb' bus='virtio'/&gt;
    &lt;/disk&gt;
    ...
  &lt;/devices&gt;
&lt;/domain&gt;</pre>
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