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Similarly to the domain config code it may be beneficial to control the cache size of images introduced as snapshots into the backing chain. Wire up handling of the 'metadata_cache' element. Signed-off-by: Peter Krempa <pkrempa@redhat.com> Reviewed-by: Ján Tomko <jtomko@redhat.com>
353 lines
16 KiB
XML
353 lines
16 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE html>
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<html xmlns="http://www.w3.org/1999/xhtml">
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<body>
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<h1>Snapshot XML format</h1>
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<ul id="toc"></ul>
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<h2><a id="SnapshotAttributes">Snapshot XML</a></h2>
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<p>
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Snapshots are one form
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of <a href="kbase/domainstatecapture.html">domain state
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capture</a>. There are several types of snapshots:
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</p>
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<dl>
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<dt>disk snapshot</dt>
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<dd>Contents of disks (whether a subset or all disks associated
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with the domain) are saved at a given point of time, and can
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be restored back to that state. On a running guest, a disk
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snapshot is likely to be only crash-consistent rather than
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clean (that is, it represents the state of the disk on a
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sudden power outage, and may need fsck or journal replays to
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be made consistent); on an inactive guest, a disk snapshot is
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clean if the disks were clean when the guest was last shut
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down. Disk snapshots exist in two forms: internal (file
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formats such as qcow2 track both the snapshot and changes
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since the snapshot in a single file) and external (the
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snapshot is one file, and the changes since the snapshot are
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in another file).</dd>
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<dt>memory state (or VM state)</dt>
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<dd>Tracks only the state of RAM and all other resources in use
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by the VM. If the disks are unmodified between the time a VM
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state snapshot is taken and restored, then the guest will
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resume in a consistent state; but if the disks are modified
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externally in the meantime, this is likely to lead to data
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corruption.</dd>
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<dt>full system</dt>
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<dd>A combination of disk snapshots for all disks as well as VM
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memory state, which can be used to resume the guest from where it
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left off with symptoms similar to hibernation (that is, TCP
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connections in the guest may have timed out, but no files or
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processes are lost).</dd>
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</dl>
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<p>
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Libvirt can manage all three types of snapshots. For now, VM
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state (memory) snapshots are created only by
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the <code>virDomainSave()</code>, <code>virDomainSaveFlags</code>,
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and <code>virDomainManagedSave()</code> functions, and restored
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via the <code>virDomainRestore()</code>,
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<code>virDomainRestoreFlags()</code>, <code>virDomainCreate()</code>,
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and <code>virDomainCreateWithFlags()</code> functions (as well
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as via domain autostart). With managed snapshots, libvirt
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tracks all information internally; with save images, the user
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tracks the snapshot file, but libvirt provides functions such
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as <code>virDomainSaveImageGetXMLDesc()</code> to work with
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those files.
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</p>
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<p>Full system snapshots are created
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by <code>virDomainSnapshotCreateXML()</code> with no flags, while
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disk snapshots are created by the same function with
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the <code>VIR_DOMAIN_SNAPSHOT_CREATE_DISK_ONLY</code>
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flag. Regardless of the flags provided, restoration of the
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snapshot is handled by
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the <code>virDomainRevertToSnapshot()</code> function. For
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these types of snapshots, libvirt tracks each snapshot as a
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separate <code>virDomainSnapshotPtr</code> object, and maintains
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a tree relationship of which snapshots descended from an earlier
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point in time.
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</p>
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<p>
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Attributes of libvirt snapshots are stored as child elements of
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the <code>domainsnapshot</code> element. At snapshot creation
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time, normally only the <code>name</code>, <code>description</code>,
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and <code>disks</code> elements are settable; the rest of the
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fields are ignored on creation, and will be filled in by
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libvirt in for informational purposes
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by <code>virDomainSnapshotGetXMLDesc()</code>. However, when
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redefining a snapshot (<span class="since">since 0.9.5</span>),
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with the <code>VIR_DOMAIN_SNAPSHOT_CREATE_REDEFINE</code> flag
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of <code>virDomainSnapshotCreateXML()</code>, all of the XML
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described here is relevant on input, even the fields that are
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normally described as readonly for output.
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</p>
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<p>
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Snapshots are maintained in a hierarchy. A domain can have a
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current snapshot, which is the most recent snapshot compared to
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the current state of the domain (although a domain might have
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snapshots without a current snapshot, if snapshots have been
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deleted in the meantime). Creating or reverting to a snapshot
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sets that snapshot as current, and the prior current snapshot is
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the parent of the new snapshot. Branches in the hierarchy can
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be formed by reverting to a snapshot with a child, then creating
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another snapshot. For now, the creation of external snapshots
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when checkpoints exist is forbidden, although future work will
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make it possible to integrate these two concepts.
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</p>
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<p>
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The top-level <code>domainsnapshot</code> element may contain
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the following elements:
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</p>
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<dl>
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<dt><code>name</code></dt>
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<dd>The optional name for this snapshot. If the name is
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omitted, libvirt will create a name based on the time of the
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creation.
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</dd>
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<dt><code>description</code></dt>
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<dd>An optional human-readable description of the snapshot. If
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the description is omitted when initially creating the
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snapshot, then this field will be empty.
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</dd>
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<dt><code>memory</code></dt>
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<dd>On input, this is an optional request for how to handle VM
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memory state. For an offline domain or a disk-only snapshot,
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attribute <code>snapshot</code> must be <code>no</code>, since
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there is no VM state saved; otherwise, the attribute can
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be <code>internal</code> if the memory state is piggy-backed with
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other internal disk state, or <code>external</code> along with
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a second attribute <code>file</code> giving the absolute path
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of the file holding the VM memory state. <span class="since">Since
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1.0.1</span>
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</dd>
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<dt><code>disks</code></dt>
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<dd>On input, this is an optional listing of specific
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instructions for disk snapshots; it is needed when making a
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snapshot of only a subset of the disks associated with a
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domain, or when overriding the domain defaults for how to
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snapshot each disk, or for providing specific control over
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what file name is created in an external snapshot. On output,
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this is fully populated to show the state of each disk in the
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snapshot, including any properties that were generated by the
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hypervisor defaults. For full system snapshots, this field is
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ignored on input and omitted on output (a full system snapshot
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implies that all disks participate in the snapshot process).
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This element has a list of <code>disk</code>
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sub-elements, describing anywhere from zero to all of the
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disks associated with the domain. <span class="since">Since
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0.9.5</span>
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<dl>
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<dt><code>disk</code></dt>
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<dd>This sub-element describes the snapshot properties of a
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specific disk. The attribute <code>name</code> is
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mandatory, and must match either the <code><target
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dev='name'/></code> (recommended) or an unambiguous
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<code><source file='name'/></code> of one of
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the <a href="formatdomain.html#elementsDisks">disk
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devices</a> specified for the domain at the time of the
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snapshot. The attribute <code>snapshot</code> is
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optional, and the possible values are the same as the
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<code>snapshot</code> attribute for
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<a href="formatdomain.html#elementsDisks">disk devices</a>
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(<code>no</code>, <code>internal</code>,
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or <code>external</code>). Some hypervisors like ESX
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require that if specified, the snapshot mode must not
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override any snapshot mode attached to the corresponding
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domain disk, while others like qemu allow this field to
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override the domain default.
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<dl>
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<dt><code>source</code></dt>
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<dd>If the snapshot mode is external (whether specified
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or inherited), then there is an optional sub-element
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<code>source</code>, with an attribute <code>file</code>
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giving the name of the new file.
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If <code>source</code> is not
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given and the disk is backed by a local image file (not
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a block device or remote storage), a file name is
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generated that consists of the existing file name
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with anything after the trailing dot replaced by the
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snapshot name. Remember that with external
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snapshots, the original file name becomes the read-only
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snapshot, and the new file name contains the read-write
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delta of all disk changes since the snapshot.
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<p/>
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The <code>source</code> element also may contain the
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<code>seclabel</code> element (described in the
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<a href="formatdomain.html#seclabel">domain XML documentation</a>)
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which can be used to override the domain security labeling policy
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for <code>source</code>.
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</dd>
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<dt><code>driver</code></dt>
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<dd>An optional sub-element <code>driver</code>,
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with an attribute <code>type</code> giving the driver type (such
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as qcow2), of the new file created by the external
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snapshot of the new file.
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Optionally <code>metadata_cache</code> sub-element can be used
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with same semantics as the identically named subelement of the
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domain definition disk's driver.
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</dd>
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<dt><code>seclabel</code></dt>
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</dl>
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<span class="since">Since 1.2.2</span> the <code>disk</code> element
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supports an optional attribute <code>type</code> if the
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<code>snapshot</code> attribute is set to <code>external</code>.
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This attribute specifies the snapshot target storage type and allows
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to overwrite the default <code>file</code> type. The <code>type</code>
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attribute along with the format of the <code>source</code>
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sub-element is identical to the <code>source</code> element used in
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domain disk definitions. See the
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<a href="formatdomain.html#elementsDisks">disk devices</a> section
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documentation for further information.
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Libvirt currently supports the <code>type</code> element in the qemu
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driver and supported values are <code>file</code>, <code>block</code>
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and <code>network</code> with a protocol of <code>gluster</code>
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<span class="since">(since 1.2.2)</span>.
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</dd>
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</dl>
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</dd>
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<dt><code>creationTime</code></dt>
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<dd>A readonly representation of the time this snapshot was
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created. The time is specified in seconds since the Epoch,
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UTC (i.e. Unix time).
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</dd>
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<dt><code>state</code></dt>
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<dd>A readonly representation of the state of the domain at the
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time this snapshot was taken. If a full system snapshot was
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created, then this is the state of the domain at that
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time. When the domain is reverted to this snapshot, the
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domain's state will default to this state, unless overridden
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by <code>virDomainRevertToSnapshot()</code> flags to revert to
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a running or paused state. Additionally, this field can be the
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value "disk-snapshot" (<span class="since">since 0.9.5</span>)
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when it represents only a disk snapshot (no VM memory state),
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and reverting to this snapshot will default to an inactive
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guest.
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</dd>
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<dt><code>parent</code></dt>
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<dd>Readonly, present only if this snapshot has a parent. The
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parent name is given by the sub-element <code>name</code>. The
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parent relationship allows tracking a tree of related snapshots.
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</dd>
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<dt><code>domain</code></dt>
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<dd>A readonly representation of the domain that this snapshot
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was taken against. Older versions of libvirt stored only a
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single child element, uuid; reverting to a snapshot like this
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is risky if the current state of the domain differs from the
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state that the domain was created in, and requires the use of
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the <code>VIR_DOMAIN_SNAPSHOT_REVERT_FORCE</code> flag
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in <code>virDomainRevertToSnapshot()</code>. Newer versions
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of libvirt (<span class="since">since 0.9.5</span>) store the
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entire inactive <a href="formatdomain.html">domain
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configuration</a> at the time of the snapshot
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(<span class="since">since 0.9.5</span>). The domain will have
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security-sensitive information omitted
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unless the flag <code>VIR_DOMAIN_SNAPSHOT_XML_SECURE</code> is
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provided on a read-write connection.
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</dd>
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<dt><code>cookie</code></dt>
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<dd>An optional readonly representation of a save image cookie
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containing additional data libvirt may need to properly
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restore a domain from an active snapshot when such data cannot
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be stored directly in the <code>domain</code> to maintain
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compatibility with older libvirt or hypervisor.
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</dd>
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</dl>
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<h2><a id="example">Examples</a></h2>
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<p>Using this XML to create a disk snapshot of just vda on a qemu
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domain with two disks:</p>
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<pre>
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<domainsnapshot>
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<description>Snapshot of OS install and updates</description>
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<disks>
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<disk name='vda'>
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<source file='/path/to/new'/>
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</disk>
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<disk name='vdb' snapshot='no'/>
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<disk name='vdc'>
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<source file='/path/to/newc'>
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<seclabel model='dac' relabel='no'/>
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</source>
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</disk>
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</disks>
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</domainsnapshot></pre>
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<p>will result in XML similar to this from
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<code>virDomainSnapshotGetXMLDesc()</code>:</p>
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<pre>
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<domainsnapshot>
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<name>1270477159</name>
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<description>Snapshot of OS install and updates</description>
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<state>running</state>
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<creationTime>1270477159</creationTime>
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<parent>
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<name>bare-os-install</name>
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</parent>
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<memory snapshot='no'/>
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<disks>
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<disk name='vda' snapshot='external'>
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<driver type='qcow2'/>
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<b><source file='/path/to/new'/></b>
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</disk>
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<disk name='vdb' snapshot='no'/>
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</disks>
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<domain>
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<name>fedora</name>
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<uuid>93a5c045-6457-2c09-e56c-927cdf34e178</uuid>
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<memory>1048576</memory>
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...
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<devices>
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<disk type='file' device='disk'>
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<driver name='qemu' type='raw'/>
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<b><source file='/path/to/old'/></b>
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<target dev='vda' bus='virtio'/>
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</disk>
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<disk type='file' device='disk' snapshot='external'>
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<driver name='qemu' type='raw'/>
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<source file='/path/to/old2'/>
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<target dev='vdb' bus='virtio'/>
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</disk>
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...
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</devices>
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</domain>
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</domainsnapshot></pre>
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<p>With that snapshot created, <code>/path/to/old</code> is the
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read-only backing file to the new active
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file <code>/path/to/new</code>. The <code><domain></code>
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element within the snapshot xml records the state of the domain
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just before the snapshot; a call
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to <code>virDomainGetXMLDesc()</code> will show that the domain
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has been changed to reflect the snapshot:
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</p>
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<pre>
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<domain>
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<name>fedora</name>
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<uuid>93a5c045-6457-2c09-e56c-927cdf34e178</uuid>
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<memory>1048576</memory>
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...
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<devices>
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<disk type='file' device='disk'>
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<driver name='qemu' type='qcow2'/>
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<b><source file='/path/to/new'/></b>
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<target dev='vda' bus='virtio'/>
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</disk>
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<disk type='file' device='disk' snapshot='external'>
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<driver name='qemu' type='raw'/>
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<source file='/path/to/old2'/>
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<target dev='vdb' bus='virtio'/>
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</disk>
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...
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</devices>
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</domain></pre>
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</body>
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</html>
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