Re: [libvirt] [PATCH] snapshots: More clarification about REDEFINE

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). But during REDEFINE, the API wants the complete XML produced by an earlier virDomainSnapshotGetXMLDesc, which includes a <domain> sub-element capturing the state a the time the snapshot was first created. As the domain state has likely changed in the meantime, omitting <domain> during REDEFINE is extremely risky (to the point that virDomainSnapshotRevert() requires the use of FORCE flag to use such a snapshot) - we only support it because of backwards-compatibility to snapshots created before 0.9.5 started capturing <domain>. When checkpoints are introduced, the <domain> element will be mandatory in REDEFINE. Note that because <domain> can be potentially huge, we also are unable to include a bulk listing or redefine of all <domainsnapshot>s for a single domain in one XML dump (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(a)redhat.com&gt; --- Question: 0.9.5 is so long ago, is it worth reworking the qemu snapshot code to declare that REDEFINE will no longer import a snapshot definition that lacks a <domain> subelement? (Note: the vbox driver also supports snapshots and the REDEFINE flag, but does not output <domain> and thus should still allow it to be omitted on redefine). The current API wording DOES have the disclaimer regarding REDEFINE that "the hypervisor may validate that reverting to the snapshot appears to be possible", which we can use as our escape clause whereby the modern qemu driver is merely adding a validation that <domain> must be present for the redefine to be viable, even though it is at odds with our usual efforts to always parse XML that used to work in earlier versions.

diff --git a/src/libvirt-domain-snapshot.c b/src/libvirt-domain-snapshot.c index 947547627a..be9bf71af9 100644 --- a/src/libvirt-domain-snapshot.c +++ b/src/libvirt-domain-snapshot.c @@ -117,15 +117,21 @@ virDomainSnapshotGetConnect(virDomainSnapshotPtr snapshot) * * If @flags includes VIR_DOMAIN_SNAPSHOT_CREATE_REDEFINE, then this * is a request to reinstate snapshot metadata that was previously - * discarded, rather than creating a new snapshot. This can be used - * to recreate a snapshot hierarchy on a destination, then remove it - * on the source, in order to allow migration (since migration - * normally fails if snapshot metadata still remains on the source - * machine). When redefining snapshot metadata, the current snapshot - * will not be altered unless the VIR_DOMAIN_SNAPSHOT_CREATE_CURRENT - * flag is also present. It is an error to request the - * VIR_DOMAIN_SNAPSHOT_CREATE_CURRENT flag without - * VIR_DOMAIN_SNAPSHOT_CREATE_REDEFINE. On some hypervisors, + * captured from virDomainSnapshotGetXMLDesc() before removing that + * metadata, rather than creating a new snapshot. This can be used to + * recreate a snapshot hierarchy on a destination, then remove it on + * the source, in order to allow migration (since migration normally + * fails if snapshot metadata still remains on the source machine). + * Note that while original creation can omit a number of elements + * from @xmlDesc (and libvirt will supply sane defaults based on the + * domain state at that point in time), a redefinition must supply + * more elements (as the domain may have changed in the meantime, so + * that libvirt no longer has a way to resupply correct + * defaults). When redefining snapshot metadata, the domain's current + * snapshot will not be altered unless the + * VIR_DOMAIN_SNAPSHOT_CREATE_CURRENT flag is also present. It is an + * error to request the VIR_DOMAIN_SNAPSHOT_CREATE_CURRENT flag + * without VIR_DOMAIN_SNAPSHOT_CREATE_REDEFINE. On some hypervisors, * redefining an existing snapshot can be used to alter host-specific * portions of the domain XML to be used during revert (such as * backing filenames associated with disk devices), but must not alter