[Qemu-devel] QMP event for suspend to disk

[Qemu-devel] QMP event for suspend to disk

[Luiz Capitulino]

Hi,

QEMU can now distinguish between S4 and power off for the guest OSes that
suspends to disk through S4. This means that we can have a QMP event for S4
(so that QMP clients can distinguish between S3 and S4).

However, as we already emit the SHUTDOWN event for this, it turns out that
there are three different ways of supporting the new event. We could:

 1. not emit the SHUTDOWN event and only emit the SUSPEND_DISK event

    cons: breaks compatibility, as we'll stop sending an event we send today.

 2. emit both, that is, first SUSPEND_DISK and then SHUTDOWN

    cons: emitting two very similar events.

 3. extend the SHUTDOWN event to say whether it's a suspend to disk or not,
    like this:

       { "event": "SHUTDOWN", "data": { "suspend-to-disk": true }, ... }

    cons: we already have the SUSPEND event, so from an API perspective
          it would make sense to have SUSPEND_DISK.

I prefer item 3, because it seems to be cleanest one.

Re: [Qemu-devel] QMP event for suspend to disk

[Eric Blake]

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On 07/18/2012 08:02 AM, Luiz Capitulino wrote:
> Hi,
> 
> QEMU can now distinguish between S4 and power off for the guest OSes that
> suspends to disk through S4. This means that we can have a QMP event for S4
> (so that QMP clients can distinguish between S3 and S4).
> 
> However, as we already emit the SHUTDOWN event for this, it turns out that
> there are three different ways of supporting the new event. We could:
> 
>  1. not emit the SHUTDOWN event and only emit the SUSPEND_DISK event
> 
>     cons: breaks compatibility, as we'll stop sending an event we send today.

At least the new 'query-events' monitor command will allow libvirt to
know which events to listen for; so this is workable.

> 
>  2. emit both, that is, first SUSPEND_DISK and then SHUTDOWN
> 
>     cons: emitting two very similar events.

This has the most back-compat, but is noisier in that it generates more
events.

> 
>  3. extend the SHUTDOWN event to say whether it's a suspend to disk or not,
>     like this:
> 
>        { "event": "SHUTDOWN", "data": { "suspend-to-disk": true }, ... }
> 
>     cons: we already have the SUSPEND event, so from an API perspective
>           it would make sense to have SUSPEND_DISK.
> 
> I prefer item 3, because it seems to be cleanest one.

I'm also liking 3, then 1, then 2; but think any of them will work from
libvirt's POV.

-- 
Eric Blake   eblake@redhat.com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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