Re: [PATCH v2 (BROKEN) 0/6] migration: bring improved savevm/loadvm/delvm to QMP

[Markus Armbruster <armbru@redhat.com>]

Daniel P. Berrangé <berrange@redhat.com> writes:

> On Thu, Aug 27, 2020 at 01:04:43PM +0200, Markus Armbruster wrote:
>> Daniel P. Berrangé <berrange@redhat.com> writes:
>> 
>> > On Wed, Aug 26, 2020 at 05:52:06PM +0200, Markus Armbruster wrote:
>> > From the POV of practicality, making a design that unifies internal
>> > and external snapshots is something I'm considering out of scope.
>> > It increases the design time burden, as well as implementation burden.
>> > On my side, improving internal snapshots is a "spare time" project,
>> > not something I can justify spending weeks or months on.
>> 
>> I'm not demanding a solution that unifies internal and external
>> snapshots.  I'm asking for a bit of serious thought on an interface that
>> could compatibly evolve there.  Hours, not weeks or months.
>> 
>> > My goal is to implement something that is achievable in a short
>> > amount of time that gets us out of the hole we've been in for 10
>> > years. Minimal refactoring of the internal snapshot code aside
>> > from fixing the critical limitations we have today around choice
>> > of disks to snapshot.
>> >
>> > If someone later wants to come up with a grand unified design
>> > for everything that's fine, we can deprecate the new QMP commands
>> > I'm proposing now.
>> 
>> Failing at coming up with an interface that has a reasonable chance to
>> be future-proof is okay.
>> 
>> Not even trying is not okay.
>
> This was raised in my v1 posting:
>
>   https://lists.gnu.org/archive/html/qemu-devel/2020-07/msg01346.html
>
> but the conclusion was that it was a non-trivial amount of extra
> implementation work

Thanks for the pointer.  I've now read that review thread.

>> Specifically, I'd like you to think about monolothic snapshot command
>> (internal snapshots only by design) vs. transaction of individual
>> snapshot commands (design is not restricted to internal snapshots, but
>> we may want to accept implementation restrictions).
>> 
>> We already have transactionable individual storage snapshot commands.
>> What's missing is a transactionable machine state snapshot command.
>
> At a high level I consider what I've proposed as being higher level
> syntax sugar vs a more generic future impl based on multiple commands
> for snapshotting disk & state. I don't think I'd claim that it will
> evolve to become the design you're suggesting here, as they are designs
> from different levels in the stack. IOW, I think the would ultimately
> just exist in parallel. I don't think that's a real problem from a
> maint POV, as the large burden from the monolithic snapshot code is
> not the HMP/QMP interface, but rather the guts of the internal
> impl in the migration/savevm.c and block/snapshot.c files. That code
> will exist for as long as the HMP commands exist, and adding a QMP
> interface doesn't make that situation worse unless we were intending
> to drop the existing HMP commands. If we did change our minds though,
> we can just deprecate the QMP command at any time we like.
>
>
>> One restriction I'd readily accept at this time is "the machine state
>> snapshot must write to a QCOW2 that is also internally snapshot in the
>> same transaction".
>> 
>> Now explain to me why this is impractical.
>
> The issues were described by Kevin here:
>
>   https://lists.gnu.org/archive/html/qemu-devel/2020-07/msg02057.html
>
> Assuming the migration impl is actually possible to solve, there is
> still the question of actually writing it. That's a non-trivial
> amount of work someone has to find time for.

Kevin explained how the transactionable machine state snapshot command
should be made non-blocking: post-copy.

I don't dispute that creating such a post-copy snapshot is a non-trivial
task.  It is out of reach for you and me.  I didn't actually ask for it,
though.

You argue that providing a blocking snapshot in QMP is better than
nothing, and good enough for quite a few applications.  I agree!  I
blocked prior attempts at porting HMP's savevm/loadvm to QMP not because
they were blocking, but because they stuck to the HMP interface, and the
HMP interface is bonkers.  I would accept the restriction "snapshotting
machine state is blocking, i.e. it stops the machine."  As I wrote in
2016, "Limitations: No live internal machine snapshot, yet."

Aside: unless I'm mistaken, taking an internal block device snapshot is
also blocking, but unlike taking a machine state snapshot, it's fast
enough for the blocking not to matter.  That's the "sync" in
blockdev-snapshot-internal-sync.

I asked you to consider the interface design I proposed back in 2016.
You point out above that your interface is more high-level, and could be
turned into sugar for a low level interface.

If true, this means your proposal doesn't box us into a corner, which is
good.

Let me elaborate a bit on the desugaring, just to make sure we're on the
same page.  Please correct me where I'm talking nonsense.

snapshot-save creates job that snapshots a set of block devices and the
machine state.  The snapshots are consistent, i.e. they are all taken at
the same point in time.  The block device snapshots are all internal.
The machine state snapshot is saved together with one of the (internal)
block device snapshots.

This is basically a transaction of blockdev-snapshot-internal-sync
(which exists) plus machine-snapshot-internal-sync (which doesn't exist)
wrapped in a job.

Likweise for snapshot-load, except there not even the command for block
snapshots exists.

I doubt creating the (transactionable, but blocking) low-level commands
is actually out of reach.  It's a matter of adding interfaces to parts
of the code you already got working.

I'm not demanding you do that, though.  As I said, my chief concerns are
keeping "bonkers" out of QMP, and not boxing us in needlessly.