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Re: [Qemu-devel] [PATCH 2/8] qmp: create qmp_savevm command

From: Peter Krempa
Subject: Re: [Qemu-devel] [PATCH 2/8] qmp: create qmp_savevm command
Date: Tue, 26 Jan 2016 13:39:28 +0100
User-agent: Mutt/1.5.24 (2015-08-30)

On Tue, Jan 19, 2016 at 19:11:05 +0100, Markus Armbruster wrote:
> "Denis V. Lunev" <address@hidden> writes:
> > On 01/18/2016 06:58 PM, Markus Armbruster wrote:
> >> "Denis V. Lunev" <address@hidden> writes:


> The overloaded interface you propose is more complex than it seems.  You
> hide the complexity by not documenting its workings.  Not even to the
> (insufficient!) degree the HMP interface documents how its overloaded
> name parameters work.
> Merely copying over the HMP documentation won't cut it.  The bar for new
> QMP interfaces is a fair bit higher than "no worse than HMP".  The new
> interface must reasonably sane for *QMP*, and sufficiently documented.
> If we can't make a sane QMP interface, I'd rather have no QMP interface.
> However, I believe we *can* make a sane QMP interface if we put in the
> design work.

I have to agree on this one. A broken interface will introduce just more
pain in the long run. Management tools still can fall back to the old
command if they need to (granted that error handling sucks there though
.. [1]) and use it as previously.


> The design work must start with a review of what we're trying to
> accomplish, and how to fit it into the rest of the system.  Here's my
> attempt.  Since my knowledge on snapshots is rather superficial, I'm
> cc'ing Kevin for additional snapshot expertise.  Kevin, please correct
> me when I talk nonsense.  I'm further cc'ing Eric and Peter for the
> management layer perspective.
> A point-in-time snapshot of a system consists of a snapshot of its
> machine state and snapshots of its storage.  All the snapshots need to
> be made at the same point in time for the result to be consistent.
> Snapshots of read-only storage carry no information and are commonly
> omitted.
> Isolated storage snapshots can make sense, but snapshotting the machine
> state without also snapshotting the machine's storage doesn't sound
> useful to me.

Well, technically if all your storage is read-only then a full snapshot
is reduced to a machine state snapshot. Libvirt actually allows this for
external snapshots. For internal ones, it obviously doesn't work.

> Both storage and machine state snapshots come in two flavours: internal
> and external.
> External ones can be made with any block backend, but internal storage
> snapshots work only with certain formats, notably qcow2.  QMP supports
> both kinds of storage snapshots.
> Both kinds of storage snapshots need exclusive access while they work.
> They're relatively quick, but the delay could be noticable for large
> internal snapshots, and perhaps for external snapshots on really slow
> storage.
> Internal machine state snapshots are currently only available via HMP's
> savevm, which integrates internal machine state and storage snapshots.
> This is non-live, i.e. the guest is stopped while the snapshot gets
> saved.  I figure we could make it live if we really wanted to.  Another
> instance of the emerging background job concept.
> On the implementation level, QCOW2 can't currently store a machine state
> snapshot without also storing a storage snapshot.  I guess we could
> change this if we really wanted to.
> External machine state snapshots are basically migrate to file.
> Supported by QMP.
> Live migration to file is possible, but currently wastes space, because
> memory dirtied during migration gets saved multiple times.  Could be
> fixed either by making migration update previously saved memory instead
> of appending (beware, random I/O), or by compacting the file afterwards.
> Non-live migration to file doesn't waste space that way.
> To take multiple *consistent* snapshots, you have to bundle them up in a
> transaction.  Transactions currently support only *storage* snapshots,
> though.
> Work-around for external machine state snapshot: migrate to file
> (possibly live), leaving the guest sopped on completion, take storage
> snapshots, resume guest.

This is exactly what I've implemented in libvirt.

> You can combine internal and external storage snapshots with an external
> machine state snapshot to get a mixed system snapshot.

Libvirt allows this for two separate snapshots of a single vm, but it's
really buggy. You can take such snapshot, but reverting or deleting it
is where the fun begins. In a mixed snapshot you need to track the
backing files along with the internal snapshots in qcow properly and
libvirt currently doesn't really allow that.

> You currently can't do that with an internal machine state snapshot: the
> only way to take one is HMP savevm, which insists on internally
> snapshotting all writable storage, and doesn't transact together with
> external storage snapshots.
> Except for the case "purely internal snapshot with just one writable
> storage device", a system snapshot consists of multiple parts stored in
> separate files.  Tying the parts together is a management problem.  QEMU
> provides rudimentary management for purely internal snapshots, but it's
> flawed: missing storage isn't detected, and additional storage can creep
> in if snapshot tags or IDs happen to match.  I guess managing the parts
> is better left to the management layer.
> I figure a fully general QMP interface would let you perform a system
> snapshot by combining storage snapshots of either kind with either kind
> of machine state snapshot.

That'd be the ideal state. It's questionable though whether combining
internal and external snapshot targets for a single snapshot is useful
or desirable, but having a general interface is probably the best

> We already have most of the building blocks: we can take external and
> internal storage snapshots, and combine them in transactions.
> What's missing is transactionable machine state snapshots.

That will be totally great. Especially in combination with the ability
to do live snapshot and discard updated storage pages.

> We know how to work around it for external machine state snapshots (see
> above), but a transaction-based solution might be nicer.
> Any solution for internal machine state snapshots in QMP should at least
> try to fit into this.  Some restrictions are okay.  For instance, we
> probably need to restrict internal machine state snapshots to piggyback
> on an internal storage snapshot for now, so we don't have to dig up
> QCOW2 just to get QMP support.

That's fine. Libvirt already mandates at least one QCOW2 storage file
for a VM for an internal snapshot. Allowing us to specify which one will
be used would be great though.

> We can talk about more convenient interfaces for common special cases,
> but I feel we need to design for the general case.  We don't have to
> implement the completely general case right away, though.  As long as we
> know where we want to go, incremental steps towards that goal are fine.
> Can we create a transactionable QMP command to take an internal machine
> state snapshot?
> This would be like HMP savevm with the following differences:
> * Separate parameters for tag and ID.  I'll have none of this
>   overloading nonsense in QMP.
> * Specify the destination block device.  I'll have none of this "pick a
>   device in some magic, undocumented way" in QMP.
> * Leave alone the other block devices.  Adding transaction actions to
>   snapshot all the writable block devices to get a full system snapshot
>   is the user's responsibility.
> Limitations:
> * No live internal machine snapshot, yet.
> * The storage device taking the internal snapshot must also be
>   internally snapshot for now.  In fact, the command does both
>   (tolerable wart).

Having a boolean to switch to live although it will not be implemented
yet would be great. That

> Open questions:
> * Do we want the QMP command to delete existing snapshots with
>   conflicting tag / ID, like HMP savevm does?  Or do we want it to fail
>   the transaction?

For libvirt this doesn't matter that much. When you create all snapshots
using the APIs we make sure that the IDs don't collide. If you tamper
with it externally though, you are on your own.

> * Do we need transactions for switching to a system snapshot, too?
> Opinions?
> Denis, I understand doing this right will be more work than simply
> rebadging HMP's savevm etc for QMP, and probably a good deal more than
> you anticipated.  Sorry about that.  I'm trying to accomodate you as far
> as possible without screwing up QMP.  Not screwing up QMP too much is my
> job as maintainer.



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