[libvirt] RFCv2: virDomainSnapshotCreateXML enhancements

[Eric Blake]

On 08/10/2011 04:08 PM, Eric Blake wrote:
> The <disks> element has an optional number of <disk> subelements; at
> most one per <disk> in the <devices> section of <domain>. Each <disk>
> element has a mandatory attribute name='name', which must match the
> <target dev='name'/> of the <domain> xml, as a way of getting 1:1
> correspondence between domainsnapshot/disks/disk and domain/devices/disk
> while using names that should already be unique. Each <disk> also has an
> optional snapshot='no|internal|external' attribute, similar to the
> proposal for <domain>/<devices>/<disk>; if not provided, the attribute
> defaults to the one from the <domain>. If snapshot=external, then there
> may be an optional subelement <source file='path'/>, which gives the
> desired new file name. If external is requested, but the <source>
> subelement is not present, then libvirt will generate a suitable
> filename, probably by concatenating the existing name with the snapshot
> name, and remembering that the snapshot name is generated as a timestamp
> if not specified. Also, for external snapshots, the <disk> element may
> have an optional sub-element specifying the driver (useful for selecting
> qcow2 vs. qed in the qemu 'snapshot_blkdev' monitor command); again,
> this can normally be generated by default.

I realized I never had an example of this last sentence, and almost 
omitted it from my xml parser.  It will look like:

<domainsnapshot>
   <disks>
     <disk name='vda'>
       <driver type='qcow2'/>
       <source file='path'/>
     </disk>
   </disks>
</domainsnapshot>

> Quick Summary
> =============

Status update: more than a week later, and patches have been flying - 
I'm now at patch 34/26 in my v2 series (don't you love it when a patch 
series grows after the first send?), not counting some of the other 
prereq patches I have submitted and pushed in the meantime.

https://www.redhat.com/archives/libvir-list/2011-August/msg00620.html

Anyone up for code reviews?  :)

For convenience, I've pushed things to my git repo, so you can:

git fetch git://repo.or.cz/libvirt/ericb.git snapshot

or browse online at:

http://repo.or.cz/w/libvirt/ericb.git/shortlog/refs/heads/snapshot

>
> These are the changes I plan on making soon; I mentioned other possible
> future changes above that would depend on these being complete first, or
> which involve creation of new API.
>
> The following API patterns currently "succeed", but risk data loss or
> other bugs that can get libvirt into an inconsistent state; they will
> now fail by default:
>
> virDomainRevertToSnapshot to go from a running VM to a stopped
> checkpoint will now fail by default. Justification: stopping a running
> domain is a form of data loss. Mitigation: use
> VIR_DOMAIN_SNAPSHOT_REVERT_FORCE for old behavior.

Still needs to be implemented.

>
> virDomainRevertToSnapshot to go from a running VM to a live checkpoint
> with an ABI-incompatible <domain> will now fail by default.
> Justification: qemu does not handle ABI incompatibilities, and even if
> the 'loadvm' may have succeeded, this generally resulted in fullscale
> guest corruption. Mitigation: use VIR_DOMAIN_SNAPSHOT_REVERT_FORCE to
> start a new qemu process that properly conforms to the snapshot's ABI.

ABI incompatibilities are detected, but VIR_DOMAIN_SNAPSHOT_REVERT_FORCE 
still needs to be implemented.

>
> virDomainUndefine will now fail to undefine a domain with any snapshots.
> Justification: leaving behind libvirt metadata can corrupt future
> defines, comparable to recent managed save changes, plus it is a form of
> data loss. Mitigation: use virDomainUndefineFlags.

Done.

>
> virDomainUndefineFlags will now default to failing an undefine of a
> domain with any snapshots. Justification: leaving behind libvirt
> metadata can corrupt future defines, comparable to recent managed save
> changes, plus it is a form of data loss. Mitigation: separately delete
> all snapshots (or at least all snapshot metadata) first, or use
> VIR_DOMAIN_UNDEFINE_SNAPSHOTS.

Done.

>
> virDomainMigrate/virDomainMigrate2 will now default to fail if the
> source has any snapshots. Justification: metadata must be transferred
> along with the domain for the migration to be complete. Mitigation:
> until an improved migration protocol can automatically do the
> handshaking necessary to migrate all the snapshot metadata, a user can
> manually loop over each snapshot prior to migration, using
> virDomainSnapshotCreateXML with VIR_DOMAIN_SNAPSHOT_CREATE_REDEFINE on
> the destination, then virDomainSnapshotDelete with
> VIR_DOMAIN_SNAPSHOT_DELETE_METADATA_ONLY on the source.

Migration is forbidden with snapshots, but 
VIR_DOMAIN_SNAPSHOT_CREATE_REDEFINE still needs to be implemented.

>
> Add the following XML:
> in <domain>/<devices>/<disk>:
> add optional attribute snapshot='no|internal|external'
> add optional attribute persistent='yes|no'

XML parsing done (persistent='yes|no' got renamed to an optional 
sub-element <transient/>).  Qemu support for <transient/> is missing.

> in <domainsnapshot>:
> expand <domainsnapshot>/<domain> to be full domain, not just uuid

Done.

> add <state>disk-snapshot</state>

Done.

> add optional <disks>/<disk>, where each <disk> maps back to
> <domain>/<devices>/<disk> and controls how to do external disk snapshots

XML parsing done, but no hypervisor is yet taking advantage of the new 
domain_conf fields.

>
> Add the following flags to existing API:
>
> virDomainSnapshotCreateXML:
> VIR_DOMAIN_SNAPSHOT_CREATE_HALT

Done.

> VIR_DOMAIN_SNAPSHOT_CREATE_DISK_ONLY

Not done - next on my list (and nearly there!)

> VIR_DOMAIN_SNAPSHOT_CREATE_REDEFINE

Not done, further out.

>
> virDomainSnapshotGetXMLDesc
> VIR_DOMAIN_XML_SECURE

Done.

>
> virDomainRevertToSnapshot
> VIR_DOMAIN_SNAPSHOT_REVERT_START
> VIR_DOMAIN_SNAPSHOT_REVERT_PAUSE
> VIR_DOMAIN_SNAPSHOT_REVERT_FORCE

Not done.

> VIR_DOMAIN_SNAPSHOT_REVERT_DELETE_CHILDREN
> VIR_DOMAIN_SNAPSHOT_REVERT_DISCARD

Done.

>
> virDomainSnapshotDelete
> VIR_DOMAIN_SNAPSHOT_DELETE_CHILDREN_ONLY
> VIR_DOMAIN_SNAPSHOT_DELETE_METADATA_ONLY

Done.

>
> virDomainUndefineFlags
> VIR_DOMAIN_UNDEFINE_SNAPSHOTS

Done.

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