[libvirt] RFCv2: virDomainSnapshotCreateXML enhancements

[Eric Blake]

On 08/10/2011 04:08 PM, Eric Blake wrote:
> Undefining
> ++++++++++
>
> In one regards, undefining a domain that has snapshots is just as bad as
> undefining a domain with managed save state - since libvirt is
> maintaining metadata about snapshot hierarchies, leaving this metadata
> behind _will_ interfere with creation of a new domain by the same name.
> However, since both checkpoints and snapshots are stored in
> user-accessible disk images, and only the metadata is stored by libvirt,
> it should eventually be possible for the user to decide whether to
> discard the metadata but keep the snapshot contents intact in the disk
> images, or to discard both the metadata and the disk image snapshots.
>
> Meanwhile, I propose changing the default behavior of
> virDomainUndefine[Flags] to reject attempts to undefine a domain with
> any defined snapshots, and to add a new flag for virDomainUndefineFlags,
> virDomainUndefineFlags(,VIR_DOMAIN_UNDEFINE_SNAPSHOTS), to act as
> shorthand for calling virDomainSnapshotDelete for all snapshots tied to
> the domain. Note that this deletes the metadata, but not the underlying
> storage volumes.

Hmm.  VIR_DOMAIN_UNDEFINE_MANAGED_SAVE is only needed for 
virDomainUndefineFlags, since managed save is a persistent-only 
possibility (a running domain does not have a managed save).  But 
VIR_DOMAIN_UNDEFINE_SNAPSHOTS is needed for both virDomainUndefineFlags 
and virDomainDestroyFlags (since transient domains can have both 
checkpoints and disk snapshots).

And while we added virDomainDestroyFlags in 0.9.4, we missed adding 
virDomainShutdownFlags.  Oh well - no convenience flag for the shutdown 
case.  But it does mean that both virDomainDestroy and virDomainShutdown 
will have to fail by default if they would strand some snapshot metadata.

Also, I need to clarify the change to failure case.  It is not fatal to 
have snapshot metadata when converting a running guest from persistent 
to transient, nor is it fatal to shutdown or destroy a persistent guest 
with snapshots - in those cases, the domain still exists.  The fatal 
case is only whens stranding snapshot data (undefine on an inactive 
domain, or destroy/shutdown on a transient domain).

-- 
Eric Blake   eblake at redhat.com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org