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Prepare for new backup APIs by describing the XML that will represent a backup. The XML resembles snapshots and checkpoints in being able to select actions for a set of disks, but has other differences. It can support both push model (the hypervisor does the backup directly into the destination file) and pull model (the hypervisor exposes an access port for a third party to grab what is necessary). Add testsuite coverage for some minimal uses of the XML. The <disk> element within <domainbackup> tries to model the same elements as a <disk> under <domain>, but sharing the RNG grammar proved to be hairy. That is in part because while <domain> use <source> to describe a host resource in use by the guest, a backup job is using a host resource that is not visible to the guest: a push backup action is instead describing a <target> (which ultimately could be a remote network resource, but for simplicity the RNG just validates a local file for now), and a pull backup action is instead describing a temporary local file <scratch> (which probably should not be a remote resource). A future refactoring may thus introduce some way to parameterize RNG to accept <disk type='FOO'>...</disk> so that the name of the subelement can be <source> for domain, or <target> or <scratch> as needed for backups. Future patches may improve this area of code. Signed-off-by: Eric Blake <eblake@redhat.com> Signed-off-by: Peter Krempa <pkrempa@redhat.com> Reviewed-by: Daniel P. Berrangé <berrange@redhat.com> Reviewed-by: Ján Tomko <jtomko@redhat.com>
199 lines
8.8 KiB
XML
199 lines
8.8 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>Checkpoint XML format</h1>
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<ul id="toc"></ul>
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<h2><a id="CheckpointAttributes">Checkpoint XML</a></h2>
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<p>
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One method of capturing domain disk backups is via the use of
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incremental backups. Right now, incremental backups are only
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supported for the QEMU hypervisor when using qcow2 disks at the
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active layer; if other disk formats are in use, capturing disk
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backups requires different libvirt APIs
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(see <a href="kbase/domainstatecapture.html">domain state
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capture</a> for a comparison between APIs).
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</p>
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<p>
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Libvirt is able to facilitate incremental backups by tracking
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disk checkpoints, which are points in time against which it is
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easy to compute which portion of the disk has changed. Given a
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full backup (a backup created from the creation of the disk to a
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given point in time), coupled with the creation of a disk
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checkpoint at that time, and an incremental backup (a backup
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created from just the dirty portion of the disk between the
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first checkpoint and the second backup operation), it is
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possible to do an offline reconstruction of the state of the
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disk at the time of the second backup without having to copy as
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much data as a second full backup would require. Most disk
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checkpoints are created in conjunction with a backup
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via <code>virDomainBackupBegin()</code>, although a future API
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addition of <code>virDomainSnapshotCreateXML2()</code> will also
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make this possible when creating external snapshots; however,
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libvirt also exposes enough support to create disk checkpoints
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independently from a backup operation
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via <code>virDomainCheckpointCreateXML()</code> <span class="since">since
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5.6.0</span>. Likewise, the creation of checkpoints when
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external snapshots exist is currently forbidden, although future
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work will make it possible to integrate these two concepts.
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</p>
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<p>
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Attributes of libvirt checkpoints are stored as child elements
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of the <code>domaincheckpoint</code> element. At checkpoint
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creation time, normally only
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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 libvirt
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in for informational purposes
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by <code>virDomainCheckpointGetXMLDesc()</code>. However, when
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redefining a checkpoint, with
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the <code>VIR_DOMAIN_CHECKPOINT_CREATE_REDEFINE</code> flag
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of <code>virDomainCheckpointCreateXML()</code>, all of the XML
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fields described here are relevant on input, even the fields
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that are normally described as readonly for output.
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</p>
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<p>
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The top-level <code>domaincheckpoint</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 checkpoint. 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 checkpoint.
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If the description is omitted when initially creating the
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checkpoint, then this field will be empty.
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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 checkpoints; it is needed when making a
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checkpoint on only a subset of the disks associated with a
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domain. In particular, since QEMU checkpoints require qcow2
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disks, this element may be needed on input for excluding guest
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disks that are not in qcow2 format. If the entire element was
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omitted on input, then all disks participate in the
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checkpoint, otherwise, only the disks explicitly listed which
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do not also use <code>checkpoint='no'</code> will
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participate. On output, this is the checkpoint state of each
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of the domain's disks.
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<dl>
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<dt><code>disk</code></dt>
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<dd>This sub-element describes the checkpoint properties of
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a specific disk with the following attributes:
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<dl>
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<dt><code>name</code></dt>
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<dd>A mandatory attribute which must match either
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the <code><target dev='name'/></code> or an
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unambiguous <code><source file='name'/></code>
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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
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the checkpoint.</dd>
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<dt><code>checkpoint</code></dt>
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<dd>An optional attribute; possible values
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are <code>no</code> when the disk does not participate
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in this checkpoint; or <code>bitmap</code> if the disk
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will track all changes since the creation of this
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checkpoint via a bitmap.</dd>
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<dt><code>bitmap</code></dt>
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<dd>The attribute <code>bitmap</code> is only valid
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if <code>checkpoint='bitmap'</code>; it describes the
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name of the tracking bitmap (defaulting to the
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checkpoint name).</dd>
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<dt><code>size</code></dt>
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<dd>The attribute <code>size</code> is ignored on input;
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on output, it is only present if
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the <code>VIR_DOMAIN_CHECKPOINT_XML_SIZE</code> flag
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was used to perform a dynamic query of the estimated
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size in bytes of the changes made since the checkpoint
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was created.</dd>
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</dl>
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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 checkpoint 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>parent</code></dt>
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<dd>Readonly, present if this checkpoint 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 list of related checkpoints.
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</dd>
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<dt><code>domain</code></dt>
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<dd>A readonly representation of the
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inactive <a href="formatdomain.html">domain configuration</a>
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at the time the checkpoint was created. This element may be
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omitted for output brevity by supplying
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the <code>VIR_DOMAIN_CHECKPOINT_XML_NO_DOMAIN</code> flag, but
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the resulting XML is no longer viable for use with
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the <code>VIR_DOMAIN_CHECKPOINT_CREATE_REDEFINE</code> flag
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of <code>virDomainCheckpointCreateXML()</code>. The domain
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will have security-sensitive information omitted unless the
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flag <code>VIR_DOMAIN_CHECKPOINT_XML_SECURE</code> is provided
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on a read-write connection.
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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 checkpoint of just vda on a qemu
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domain with two disks and a prior checkpoint:</p>
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<pre>
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<domaincheckpoint>
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<description>Completion of updates after OS install</description>
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<disks>
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<disk name='vda' checkpoint='bitmap'/>
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<disk name='vdb' checkpoint='no'/>
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</disks>
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</domaincheckpoint></pre>
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<p>will result in XML similar to this from
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<code>virDomainCheckpointGetXMLDesc()</code>:</p>
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<pre>
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<domaincheckpoint>
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<name>1525889631</name>
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<description>Completion of updates after OS install</description>
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<parent>
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<name>1525111885</name>
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</parent>
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<creationTime>1525889631</creationTime>
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<disks>
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<disk name='vda' checkpoint='bitmap' bitmap='1525889631'/>
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<disk name='vdb' checkpoint='no'/>
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</disks>
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<domain type='qemu'>
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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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<source file='/path/to/file1'/>
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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/file2'/>
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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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</domaincheckpoint></pre>
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<p>With that checkpoint created, the qcow2 image is now tracking
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all changes that occur in the image since the checkpoint via
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the persistent bitmap named <code>1525889631</code>.
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</p>
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</body>
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</html>
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