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>
a6a25d5cb6
