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Based on recent list questions about the proposed addition of virDomainCheckpointCreateXML(REDEFINE), it is worth adding some clarification to the existing snapshot redefine documentation that is serving as the basis for checkpoints. Normal snapshot creation requires very few elements from the user XML (libvirt can pick sane defaults for items that are omitted, and many fields, including <domain>, are documented as readonly output fields ignored on input, produced by drivers that track it). But during REDEFINE, the API wants the complete XML produced by an earlier virDomainSnapshotGetXMLDesc; as the domain definition has likely changed since the snapshot was first created, libvirt is unable to recreate a <domain> sub-element that matches the original output representing the domain state at the time the snapshot was first created. In fact, reverting without a <domain> sub-element is risky enough that we had to add a FORCE flag for virDomainSnapshotRevert(). In short, we only support omitting domain for qemu because of backwards-compatibility to snapshots created before 0.9.5 started capturing <domain>; even though there are other drivers like vbox that do not output <domain> because they have other reliable ways to revert. And based on the confusion caused when omitting <domain> from snapshot XML, the initial design for checkpoints in later patches will make <domain> a mandatory element during its REDEFINE. [Side note: the fact that <domain> can appear in <domainsnapshot> is a reason we cannot add a new API for a bulk listing or redefine of all snapshots of a single domain in one XML call (for example, a 1M <domain> XML * 16 snapshots explodes into 16M in a bulk form, which gets difficult to send over RPC). Perhaps we could add a flag to request that the <domain> sub-element be omitted on output, but such output is no longer suitable for sane REDEFINE input.] Signed-off-by: Eric Blake <eblake@redhat.com> Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
331 lines
15 KiB
XML
331 lines
15 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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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.
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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 name for this snapshot. If the name is specified when
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initially creating the snapshot, then the snapshot will have
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that particular name. If the name is omitted when initially
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creating the snapshot, then libvirt will make up a name for
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the snapshot, based on the time when it was created.
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</dd>
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<dt><code>description</code></dt>
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<dd>A human-readable description of the snapshot. If the
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description is omitted when initially creating the snapshot,
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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> or an unambiguous <code><source
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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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</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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</dd>
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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>The time this snapshot was created. The time is specified
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in seconds since the Epoch, UTC (i.e. Unix time). Readonly.
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</dd>
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<dt><code>state</code></dt>
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<dd>The state of the domain at the time this snapshot was taken.
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If a full system snapshot was created, then this
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is the state of the domain at that time. When the domain is
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reverted to this snapshot, the domain's state will default to
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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,
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this field can be the value "disk-snapshot"
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(<span class="since">since 0.9.5</span>) when it represents
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only a disk snapshot (no VM memory state), and reverting to this
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snapshot will default to an inactive guest. Readonly.
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</dd>
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<dt><code>parent</code></dt>
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<dd>The parent of this snapshot. If present, this element
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contains exactly one child element, name. This specifies the
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name of the parent snapshot of this snapshot, and is used to
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represent trees of snapshots. Readonly.
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</dd>
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<dt><code>domain</code></dt>
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<dd>The domain that this snapshot was taken against. Older
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versions of libvirt stored only a single child element, uuid;
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reverting to a snapshot like this is risky if the current
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state of the domain differs from the state that the domain was
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created in, and requires the use of the
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<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 entire
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inactive <a href="formatdomain.html">domain configuration</a>
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at the time of the snapshot (<span class="since">since
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0.9.5</span>). Readonly.
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</dd>
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<dt><code>cookie</code></dt>
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<dd>Save image cookie containing additional data libvirt may need to
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properly restore a domain from an active snapshot when such data
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cannot be stored directly in the <code>domain</code> to maintain
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compatibility with older libvirt or hypervisor. Readonly.
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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='/path/to/old'>
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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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</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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