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<h1>Snapshot XML format</h1>
<ul>
<li>
<a href="#SnapshotAttributes">Snapshot XML</a>
</li>
<li>
<a href="#example">Examples</a>
</li>
</ul>
<h2>
<a id="SnapshotAttributes">Snapshot XML</a>
<a class="headerlink" href="#SnapshotAttributes" title="Permalink to this headline">¶</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>full system</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>Full system snapshots are created
by <code>virDomainSnapshotCreateXML()</code> with no flags, while
disk snapshots are created by the same function with
the <code>VIR_DOMAIN_SNAPSHOT_CREATE_DISK_ONLY</code>
flag. Regardless of the flags provided, restoration of the
snapshot is handled 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 on input, even the fields that are
normally described as readonly for output.
</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 optional name for this snapshot. 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 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 full system snapshots, this field is
ignored on input and omitted on output (a full system snapshot
implies that all disks participate in the snapshot process).
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><target
dev='name'/></code> (recommended) 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
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.
<p/>
The <code>source</code> element also may contain the
<code>seclabel</code> element (described in the
<a href="formatdomain.html#seclabel">domain XML documentation</a>)
which can be used to override the domain security labeling policy
for <code>source</code>.
</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><dt><code>seclabel</code></dt></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>A readonly representation of the time this snapshot was
created. The time is specified in seconds since the Epoch,
UTC (i.e. Unix time).
</dd>
<dt>
<code>state</code>
</dt>
<dd>A readonly representation of the state of the domain at the
time this snapshot was taken. If a full system snapshot was
created, 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 this state, unless overridden
by <code>virDomainRevertToSnapshot()</code> flags to revert to
a running or paused state. 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.
</dd>
<dt>
<code>parent</code>
</dt>
<dd>An optional readonly representation of the parent of this
snapshot. If present, this element contains exactly one child
element, <code>name</code>. This specifies the name of the
parent snapshot of this snapshot, and is used to represent
trees of snapshots.
</dd>
<dt>
<code>domain</code>
</dt>
<dd>A readonly representation of 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>). The domain will have
security-sensitive information omitted
unless the flag <code>VIR_DOMAIN_SNAPSHOT_XML_SECURE</code> is
provided on a read-write connection.
</dd>
<dt>
<code>cookie</code>
</dt>
<dd>An optional readonly representation of a 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.
</dd>
</dl>
<h2>
<a id="example">Examples</a>
<a class="headerlink" href="#example" title="Permalink to this headline">¶</a>
</h2>
<p>Using this XML to create a disk snapshot of just vda on a qemu
domain with two disks:</p>
<pre>
<domainsnapshot>
<description>Snapshot of OS install and updates</description>
<disks>
<disk name='vda'>
<source file='/path/to/new'/>
</disk>
<disk name='vdb' snapshot='no'/>
<disk name='vdc'>
<source file='/path/to/newc'>
<seclabel model='dac' relabel='no'/>
</source>
</disk>
</disks>
</domainsnapshot></pre>
<p>will result in XML similar to this from
<code>virDomainSnapshotGetXMLDesc()</code>:</p>
<pre>
<domainsnapshot>
<name>1270477159</name>
<description>Snapshot of OS install and updates</description>
<state>running</state>
<creationTime>1270477159</creationTime>
<parent>
<name>bare-os-install</name>
</parent>
<memory snapshot='no'/>
<disks>
<disk name='vda' snapshot='external'>
<driver type='qcow2'/>
<b><source file='/path/to/new'/></b>
</disk>
<disk name='vdb' snapshot='no'/>
</disks>
<domain>
<name>fedora</name>
<uuid>93a5c045-6457-2c09-e56c-927cdf34e178</uuid>
<memory>1048576</memory>
...
<devices>
<disk type='file' device='disk'>
<driver name='qemu' type='raw'/>
<b><source file='/path/to/old'/></b>
<target dev='vda' bus='virtio'/>
</disk>
<disk type='file' device='disk' snapshot='external'>
<driver name='qemu' type='raw'/>
<source file='/path/to/old2'/>
<target dev='vdb' bus='virtio'/>
</disk>
...
</devices>
</domain>
</domainsnapshot></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><domain></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>
<domain>
<name>fedora</name>
<uuid>93a5c045-6457-2c09-e56c-927cdf34e178</uuid>
<memory>1048576</memory>
...
<devices>
<disk type='file' device='disk'>
<driver name='qemu' type='qcow2'/>
<b><source file='/path/to/new'/></b>
<target dev='vda' bus='virtio'/>
</disk>
<disk type='file' device='disk' snapshot='external'>
<driver name='qemu' type='raw'/>
<source file='/path/to/old2'/>
<target dev='vdb' bus='virtio'/>
</disk>
...
</devices>
</domain></pre>
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