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<h1>Checkpoint XML format</h1>
<ul id="toc"></ul>
<h2><a id="CheckpointAttributes">Checkpoint XML</a></h2>
<p>
One method of capturing domain disk backups is via the use of
incremental backups. Right now, incremental backups are only
supported for the QEMU hypervisor when using qcow2 disks at the
active layer; if other disk formats are in use, capturing disk
backups requires different libvirt APIs
(see <a href="kbase/domainstatecapture.html">domain state
capture</a> for a comparison between APIs).
</p>
<p>
Libvirt is able to facilitate incremental backups by tracking
disk checkpoints, which are points in time against which it is
easy to compute which portion of the disk has changed. Given a
full backup (a backup created from the creation of the disk to a
given point in time), coupled with the creation of a disk
checkpoint at that time, and an incremental backup (a backup
created from just the dirty portion of the disk between the
first checkpoint and the second backup operation), it is
possible to do an offline reconstruction of the state of the
disk at the time of the second backup without having to copy as
much data as a second full backup would require. Most disk
checkpoints are created in conjunction with a backup
via <code>virDomainBackupBegin()</code>, although a future API
addition of <code>virDomainSnapshotCreateXML2()</code> will also
make this possible when creating external snapshots; however,
libvirt also exposes enough support to create disk checkpoints
independently from a backup operation
via <code>virDomainCheckpointCreateXML()</code> <span class="since">since
5.6.0</span>. Likewise, the creation of checkpoints when
external snapshots exist is currently forbidden, although future
work will make it possible to integrate these two concepts.
</p>
<p>
Attributes of libvirt checkpoints are stored as child elements
of the <code>domaincheckpoint</code> element. At checkpoint
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>virDomainCheckpointGetXMLDesc()</code>. However, when
redefining a checkpoint, with
the <code>VIR_DOMAIN_CHECKPOINT_CREATE_REDEFINE</code> flag
of <code>virDomainCheckpointCreateXML()</code>, all of the XML
fields described here are relevant on input, even the fields
that are normally described as readonly for output.
</p>
<p>
The top-level <code>domaincheckpoint</code> element may contain
the following elements:
</p>
<dl>
<dt><code>name</code></dt>
<dd>The optional name for this checkpoint. If the name is
omitted, libvirt will create a name based on the time of the
creation.
</dd>
<dt><code>description</code></dt>
<dd>An optional human-readable description of the checkpoint.
If the description is omitted when initially creating the
checkpoint, then this field will be empty.
</dd>
<dt><code>disks</code></dt>
<dd>On input, this is an optional listing of specific
instructions for disk checkpoints; it is needed when making a
checkpoint on only a subset of the disks associated with a
domain. In particular, since QEMU checkpoints require qcow2
disks, this element may be needed on input for excluding guest
disks that are not in qcow2 format. If the entire element was
omitted on input, then all disks participate in the
checkpoint, otherwise, only the disks explicitly listed which
do not also use <code>checkpoint='no'</code> will
participate. On output, this is the checkpoint state of each
of the domain's disks.
<dl>
<dt><code>disk</code></dt>
<dd>This sub-element describes the checkpoint properties of
a specific disk with the following attributes:
<dl>
<dt><code>name</code></dt>
<dd>A mandatory attribute which must match either
the <code><target dev='name'/></code> or an
unambiguous <code><source file='name'/></code>
of one of
the <a href="formatdomain.html#elementsDisks">disk
devices</a> specified for the domain at the time of
the checkpoint.</dd>
<dt><code>checkpoint</code></dt>
<dd>An optional attribute; possible values
are <code>no</code> when the disk does not participate
in this checkpoint; or <code>bitmap</code> if the disk
will track all changes since the creation of this
checkpoint via a bitmap.</dd>
<dt><code>bitmap</code></dt>
<dd>The attribute <code>bitmap</code> is only valid
if <code>checkpoint='bitmap'</code>; it describes the
name of the tracking bitmap (defaulting to the
checkpoint name).</dd>
<dt><code>size</code></dt>
<dd>The attribute <code>size</code> is ignored on input;
on output, it is only present if
the <code>VIR_DOMAIN_CHECKPOINT_XML_SIZE</code> flag
was used to perform a dynamic query of the estimated
size in bytes of the changes made since the checkpoint
was created.</dd>
</dl>
</dd>
</dl>
</dd>
<dt><code>creationTime</code></dt>
<dd>A readonly representation of the time this checkpoint was
created. The time is specified in seconds since the Epoch,
UTC (i.e. Unix time).
</dd>
<dt><code>parent</code></dt>
<dd>Readonly, present if this checkpoint has a parent. The
parent name is given by the sub-element <code>name</code>. The
parent relationship allows tracking a list of related checkpoints.
</dd>
<dt><code>domain</code></dt>
<dd>A readonly representation of the
inactive <a href="formatdomain.html">domain configuration</a>
at the time the checkpoint was created. This element may be
omitted for output brevity by supplying
the <code>VIR_DOMAIN_CHECKPOINT_XML_NO_DOMAIN</code> flag, but
the resulting XML is no longer viable for use with
the <code>VIR_DOMAIN_CHECKPOINT_CREATE_REDEFINE</code> flag
of <code>virDomainCheckpointCreateXML()</code>. The domain
will have security-sensitive information omitted unless the
flag <code>VIR_DOMAIN_CHECKPOINT_XML_SECURE</code> is provided
on a read-write connection.
</dd>
</dl>
<h2><a id="example">Examples</a></h2>
<p>Using this XML to create a checkpoint of just vda on a qemu
domain with two disks and a prior checkpoint:</p>
<pre>
<domaincheckpoint>
<description>Completion of updates after OS install</description>
<disks>
<disk name='vda' checkpoint='bitmap'/>
<disk name='vdb' checkpoint='no'/>
</disks>
</domaincheckpoint></pre>
<p>will result in XML similar to this from
<code>virDomainCheckpointGetXMLDesc()</code>:</p>
<pre>
<domaincheckpoint>
<name>1525889631</name>
<description>Completion of updates after OS install</description>
<parent>
<name>1525111885</name>
</parent>
<creationTime>1525889631</creationTime>
<disks>
<disk name='vda' checkpoint='bitmap' bitmap='1525889631'/>
<disk name='vdb' checkpoint='no'/>
</disks>
<domain type='qemu'>
<name>fedora</name>
<uuid>93a5c045-6457-2c09-e56c-927cdf34e178</uuid>
<memory>1048576</memory>
...
<devices>
<disk type='file' device='disk'>
<driver name='qemu' type='qcow2'/>
<source file='/path/to/file1'/>
<target dev='vda' bus='virtio'/>
</disk>
<disk type='file' device='disk' snapshot='external'>
<driver name='qemu' type='raw'/>
<source file='/path/to/file2'/>
<target dev='vdb' bus='virtio'/>
</disk>
...
</devices>
</domain>
</domaincheckpoint></pre>
<p>With that checkpoint created, the qcow2 image is now tracking
all changes that occur in the image since the checkpoint via
the persistent bitmap named <code>1525889631</code>.
</p>
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