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Re: [libvirt] RFCv2: virDomainSnapshotCreateXML enhancements

[ CCed qemu-devel, just in case someone's interested ]

Am 11.08.2011 15:23, schrieb Eric Blake:
>> [ Okay, some of it is handled later in this document, but I think it's
>> still useful to leave this summary in my mail. External VM state is
>> something that you don't seem to have covered yet - can't we do this
>> already with live migration to a file? ]
> Yes, external VM state is already covered with live migration to file, 
> and I will not be touching it while implementing this RFC, but future 
> extensions may be able to further unify the two concepts.

Thanks for your explanation regarding the multiple dimensions of
snapshot options, it all makes sense.

I also agree with your incremental approach. I just wanted to make sure
that we keep the possible extension in mind so that we won't end up in a
design that makes assumptions that don't hold true in the long run.

And in the long run I think that looking into unifying these features is
something that should be done, because they are really similar.

>>> libvirt can be taught to honor persistent=no for qemu by creating a
>>> qcow2 wrapper file prior to starting qemu, then tearing down that
>>> wrapper after the fact, although I'll probably leave that for later in
>>> my patch series.
>> qemu can already do this with -drive snapshot=on. It must be allowed to
>> create a temporary file for this to work, of course. Is that a problem?
>> If not, I would just forward the option to qemu.
> Where would that file be created?  If the main image is in a directory, 
> is the temporary would also live in that directory (shared storage 
> visible to another qemu for migration purposes) or in local storage 
> (preventing migration)?  

It uses whatever mkstemp() returns, i.e. usually something in /tmp.

> If migration is possible, would libvirt need to 
> be able to learn the name of the temporary file so as to tell the new 
> qemu on the destination the same temporary file name it should open?

That's a good point that I haven't thought of. Temporary disks isn't
something that immediately reminds me of VMs using live migration, but
there's really no reason against it. So maybe duplicating this in
libvirt could make some sense indeed.

> What about if the main image is a block device - there, the temporary 
> file obviously has to live somewhere else, but how does qemu decide 
> where, and should that decision be configurable by the user?  How will 
> things interact with SELinux labeling?  What about down the road when we 
> add enhancements to enforce that qemu cannot open() files on NFS, but 
> must instead receive fds by inheritance?

Yeah, that was basically my question, if letting qemu create a file in
/tmp would be a problem from a libvirt/SELinux perspective. Of course,
you're much more flexible if libvirt does it manually and allows to
specify where you want to create the temporary image etc.

> This certainly sounds like some fertile ground for design decisions on 
> how libvirt and qemu should interact; I don't know if -drive snapshot=on 
> is reliable enough for use by libvirt, or whether libvirt will end up 
> having to manage things itself.
> Obviously, my implementation of this RFC will start simple, by rejecting 
> persistent=no for qemu, until we've answered some of those other design 
> questions; I can get snapshot_blkdev support working before we have to 
> tackle this enhancement.
>>> The other thing to be aware of is that with internal snapshots, qcow2
>>> maintains a distinction between current state and a snapshot - that is,
>>> qcow2 is _always_ tracking a delta, and never modifies a named snapshot,
>>> even when you use 'qemu-img snapshot -a' to revert to different snapshot
>>> names.  But with named files, the original file now becomes a read-only
>>> backing file to a new active file; if we revert to the original file,
>>> and make any modifications to it, the active file that was using it as
>>> backing will be corrupted.  Therefore, the safest thing is to reject any
>>> attempt to revert to any snapshot (whether checkpoint or disk snapshot)
>>> that has an existing child snapshot consisting of an external disk
>>> snapshot.  The metadata for each of these children can be deleted
>>> manually, but that requires quite a few API calls (learn how many
>>> children exist, get the list of children, and for each child, get its
>>> xml to see if that child has the target snapshot as a parent, and if so
>>> delete the snapshot).  So as shorthand, virDomainRevertToSnapshot will
>>> be taught a new flag, VIR_DOMAIN_SNAPSHOT_REVERT_DELETE_CHILDREN, which
>>> first deletes any children of the snapshot about to be deleted prior to
>>> reverting to that particular child.
>> I think the API should make it possible to revert to a given external
>> snapshot without deleting all children, but by creating another qcow2
>> file that uses the same backing file. Basically this new qcow2 file
>> would be the equivalent to the "current state" concept qcow2 uses for
>> internal snapshots.
> Interesting idea.  But I'm not quite sure how to fit it into existing API.
> Remember, existing API is that you have an existing file name, and when 
> you call snapshot_blkdev, you are specifying a new file name that 
> becomes the live qcow2 file, rendering the previous file name as the 
> snapshot.  So:
> <disk name='vda' snapshot='external'>
>    <source file='/path/to/new'/>
> </disk>
> in the <domainsnapshot> is naming the new active file name, not the 
> snapshot.  If we go with that representation, then reverting to the 
> snapshot means that you want to re-create a new qcow2 file for new 
> active state, but what do we call it?  We can't call it 
> /path/to/original (since we want to reuse that as the backing file to 
> both branches in the snapshot hierarchy), and we can't call it 
> /path/to/new from the xml naming unless we get rid of the existing copy 
> of /path/to/new.  I see two options for future enhancements, but neither 
> has to be implemented right away (that is, this RFC is fine limiting the 
> reversion to a disk-snapshot to only occur when there are no 
> descendants, as long as we can later relax that restriction in the 
> future once we figure out how to do branched descendants).

Meh. I understand the problem you're describing, it just sounds so
banal. :-)

If we're taking the analogy with internal snapshots, then this "current
state" doesn't really have a name. It only gets one when you create a
new snapshot and then a new "current snapshot" is created on top.

Just that renaming external files while they are in use is probably only
a great idea if you intend to confuse everyone...

>> It should be possible to make both look the same to users if we think
>> this is a good idea.
> 1. As a user, I'd much rather have an interface where _I_ decide the 
> name of the snapshot, but keep the active file name unchanged.  That is, 
> the current semantics of snapshot_blkdev feel a bit backward (it 
> requires me to tell you the name of the new active file, and the 
> existing name becomes the snapshot), where I would naively expect to 
> have a mode where I tell you the name to rename() the existing file 
> into, at which point you then recreate the original name as a active 
> qcow2 file that has the new snapshot name as its backing file.  But I'm 
> not entirely sure how that would play with SELinux permissions.  Also, 
> while rename() works for files, it is lousy for the case of the original 
> name being a block device which can't be rename()'d, so I think the 
> current snapshot_blkdev semantics are correct even if they feel a bit 
> backwards.  But it would be nice if we could design future qemu 
> enhancements that would allow the creation of a snapshot of arbitrary 
> name while keeping the live file name unchanged.

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

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

> 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?

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.

> 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:
> <domainsnapshot>
>    <name>branched</name>
>    <parent>
>      <name>disk-snapstho</name>
>    </parent>
>    <disks>...</disks>
> </domainsnapshot>
> 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.


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