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Re: [libvirt] RFCv2: virDomainSnapshotCreateXML enhancements

On 08/11/2011 08:11 AM, Kevin Wolf wrote:
I agree with you. It feels a bit backwards for snapshots, but it's
really the only reasonable thing to do if you're using external
snapshots. That you can't rename block devices is actually a very point
point, too.

There's one more point to consider: If creating a snapshot of foo.img
just creates a new bar.img, but I keep working on foo.img, I might
expect that by deleting bar.img I remove the snapshot, but foo.img keeps

More ideas on this front:

One of the ideas of 'live snapshot' is to grab state that I can copy to an independent backup, taking as much time as needed, with minimal interruption to qemu. Given an original 'file' of any format, then we can consider the sequence:

rename file to file.tmp (assuming we figure out how to teach qemu about renames)
use snapshot_blkdev to recreate file with file.tmp as backup
in parallel:
  copy file.tmp to file.snapshot
  block pull the contents of file.tmp back into file
when both tasks have completed, remove file.tmp

Now, I have created a snapshot file.snap, which can safely be deleted without breaking 'file', and with minimal downtime to the qemu process. It's just that there is a window of time where the the snapshot is still in progress (that is, until both file.snap and the block pull have completed); dealing with the wrinkle that this forces 'file' to now be qcow2, even if it started out raw; and dealing with rename() issues not being usable on block devices. And a non-zero window of time between starting the sequence and reaching a stable completion implies ramifications to whether other commands would be locked out in the meantime, or whether it can be broken into multiple steps with progress checks along the way, whether events need to be exposed to track when pieces complete, and so on.

Another idea is that if qemu would ever gain a way to export the contents of an internal snapshot or backing file (aka external snapshot), independently of how that state differs from the current state, then another operation would be:

with qcow2 file, create an internal snapshot
use new API to copy out the snapshot state into file.snap, while qemu is still actively modifying current state
remove the internal snapshot

with the net result that appears the same as creating file.snap as an external snapshot of a given state in time, but where the original qcow2 file is not impacted if file.snap is deleted.

So working with renames might turn out to be tricky in many ways, and
not only technical ones.

Hopefully we're leaving enough flexibility to support these additional snapshot modes, even if we don't implement everything in the first round.

2. It is possible to add a new libvirt API, virDomainSnapshotCreateFrom,
which takes an existing snapshot as a child of the given snapshot passed
in as its parent.  This would combine the action of reverting to a
disk-snapshot along with the xml argument necessary for naming a new
live file, so that you could indeed support branching off the
disk-snapshot with a user-specified or libvirt-generated new active file
name without having to delete the existing children that were branched
off the old active file name, and making the original base file the
backing file to both branches.  Unfortunately, adding a new API is out
of the question for backporting purposes.

This API would be completely pointless with internal snapshots, right?

On the contrary, it might be useful as a way to convert an internal snapshot into an external one. But yes, we can already do branching children off internal snapshots without needing this new feature, so the new feature's main point is for use in creating a branching child off an external disk snapshot.

The ideal result would be an API where the user doesn't really have to
deal with internal vs. external snapshots other than setting the right
flag/XML option/whatever and libvirt would do the mapping to the
low-level functions.

Of course, if we want to avoid renames (for which there are good
reasons), then maybe we can't really get a unified API for internal and
external snapshots. In this case, maybe using completely different
functions to signal that we have different semantics might be appropriate.

This looks like it still needs a lot of thought.

Different functions at the qemu level, at the libvirt level, or both? I agree that the ideal libvirt semantics is a single interface with enough expressivity to properly map to all the underlying qemu options, where libvirt correctly decides between migrate to disk and qemu-img, savevm, snapshot_blkdev, block pull, or any other underlying operations, while still properly rejecting any combinations that are possible in the XML matrix but unsupported by current qemu capabilities.

2a. But thinking about it a bit more, maybe we don't need a new API, but
just an XML enhancement to the existing virDomainSnapshotCreateXML!
That is, if I specify:

then we can accomplish your goal, without any qemu changes, and without
any new libvirt API.  That is, right now,<parent>  is an output-only
aspect of snapshot xml, but by allowing it to be an input element
(probably requiring the use of a new flag,
VIR_DOMAIN_SNAPSHOT_CREATE_BRANCH), then it is possible to both revert
to the state of the old snapshot and specify the new file name to use to
collect the branched delta data from that point in time.  It also means
that creation of a branched snapshot would have to learn some of the
same flags as reverting to a snapshot (can you create the branch as well
as run a new qemu process?)  I'll play with the ideas, once I get the
groundwork of this RFC done first.

Thanks for forcing me to think about it!

Yes, this sounds like a nice solution for this case, and it looks
consistent with your existing proposal.

It still doesn't change anything for the fundamental problem that you
pointed me at, that internal snapshots give you different semantics than
external snapshots. So I think this is where we need some more discussion.

I guess at this point, my biggest concern is whether my RFC locks out any useful extensions, or if it still looks like we have enough flexibility by adding new XML constructs to cover new cases later on, while we wait for resolution of additional discussion on these sorts of internal vs. external issues.

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

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