libvirt/docs/formatsnapshot.html.in
Eric Blake af65695af0 snapshot: allow recreation of metadata
The first two flags are essential for being able to replicate
snapshot hierarchies across multiple hosts, which will come in
handy for supervised migrations.  It also allows a management app
to take a snapshot of a transient domain, save the metadata, stop
the domain, recreate a new transient domain by the same name,
redefine the snapshot, then revert to it.

This is not quite as convenient as leaving the metadata behind
after a domain is no longer around, but doing that has a few
problems: 1. the libvirt API can only delete snapshot metadata
if there is a valid domain handle to use to get to that snapshot
object - if stale data is left behind without a domain, there is
no way to request that the data be cleaned up. 2. creating a new
domain with the same name but different uuid than the older
domain where a snapshot existed cannot use the older snapshot
data; this risks confusing libvirt, and forbidding the stale
data is similar to the recent patch to forbid stale managed save.

The first two flags might be useful on hypervisors with no metadata,
but only for modifying the notion of the current snapshot;
however, I don't know how to do that for ESX or VBox.

The third flag is a convenience option, to combine a creation with
a delete metadata into one step.  It is trivial for hypervisors
with no metadata.

The qemu changes will be involved enough to warrant a separate patch.

* include/libvirt/libvirt.h.in
(VIR_DOMAIN_SNAPSHOT_CREATE_REDEFINE)
(VIR_DOMAIN_SNAPSHOT_CREATE_CURRENT)
(VIR_DOMAIN_SNAPSHOT_CREATE_NO_METADATA): New flags.
* src/libvirt.c (virDomainSnapshotCreateXML): Document them, and
enforce mutual exclusion.
* src/esx/esx_driver.c (esxDomainSnapshotCreateXML): Trivial
implementation.
* src/vbox/vbox_tmpl.c (vboxDomainSnapshotCreateXML): Likewise.
* docs/formatsnapshot.html.in: Document re-creation.
2011-09-02 17:44:42 -06:00

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<body>
<h1>Snapshot XML format</h1>
<ul id="toc"></ul>
<h2><a name="SnapshotAttributes">Snapshot XML</a></h2>
<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>
and <code>description</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>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.
When the domain is reverted to this snapshot, the domain's
state will default to whatever is in this field. Readonly.
</dd>
<dt><code>parent</code></dt>
<dd>The parent of this snapshot. 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, as described above. Readonly.
</dd>
<dt><code>domain</code></dt>
<dd>The domain that this snapshot was taken against. This
element contains exactly one child element, uuid. This
specifies the uuid of the domain that this snapshot was taken
against. Readonly.
</dd>
</dl>
<h2><a name="example">Example</a></h2>
<pre>
&lt;domainsnapshot&gt;
&lt;name&gt;os-updates&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;domain&gt;
&lt;uuid&gt;93a5c045-6457-2c09-e56c-927cdf34e178&lt;/uuid&gt;
&lt;/domain&gt;
&lt;/domainsnapshot&gt;</pre>
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