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Re: [Qemu-block] [Qemu-devel] [PATCH v2 02/11] blockjob: centralize QMP

From: Kashyap Chamarthy
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH v2 02/11] blockjob: centralize QMP event emissions
Date: Mon, 10 Oct 2016 18:45:42 +0200
User-agent: Mutt/ (2016-04-01)

On Wed, Oct 05, 2016 at 05:00:29PM -0400, John Snow wrote:

[Arbitrarily chiming here, and still catching up with the details of the
> On 10/05/2016 03:24 PM, Eric Blake wrote:
> > On 10/05/2016 01:49 PM, John Snow wrote:


> > > Hmm, do we want to make it so some jobs are invisible and others are
> > > not? Because as it stands right now, neither case is strictly true. We
> > > only emit cancelled/completed events if it was started via QMP, however
> > > we do emit events for error and ready regardless of who started the job.
> > 
> > Libvirt tries to mirror any block job event it receives to upper layers.
> >  But if it cannot figure out which upper-layer disk the event is
> > associated with, it just drops the event.  So I think that from the
> > libvirt perspective, you are okay if if you always report job events,
> > even for internal jobs.  (Do we have a quick and easy way to set up an
> > internal job event, to double-check if I can produce some sort of
> > libvirt scenario to trigger the event and see that it gets safely ignored?)
> > 
> Not in a QEMU release yet, I think.

If not from an official QEMU release, it'd still be useful to work out a
a way to reproduce what Eric asked even from upstream Git master.

> > > That didn't seem particularly consistent to me; either all events should
> > > be controlled by the job layer itself or none of them should be.
> > > 
> > > I opted for "all."
> > > 
> > > For "internal" jobs that did not previously emit any events, is it not
> > > true that these jobs still appear in the block job list and are
> > > effectively public regardless? I'd argue that these messages may be of
> > > value for management utilities who are still blocked by these jobs
> > > whether or not they are 'internal' or not.

It'd certainly be useful durign debugging (for the said management
utilities), if it's possible to distinguish an event that was triggerred
by an internal block job vs. an event emitted by a job explicitly
triggerred by a user action.

For example, OpenStack Nova calls libvirt API virDomainBlockRebase(),
which internally calls QMP `drive-mirror` that emits events.  An "event
origin classification" system, if were to exist, allows one to pay
attention to only those events that are emitted due to an explicit
action and ignore all the rest ('internal').

But I'm not quite sure if it's desirable to have this event
classification for cleanliness reasons as Eric points out below.

> > > I'll push for keeping it mandatory and explicit. If it becomes a
> > > problem, we can always add a 'silent' job property that silences ALL qmp
> > > events, including all completion, error, and ready notices.
> > 
> > Completely silencing an internal job seems a little cleaner than having
> > events for the job but not being able to query it.  But if nothing
> > breaks by exposing the internal jobs, that seems even easier than trying
> > to decide which jobs are internal and hidden vs. user-requested and public.
> >
> Well, at the moment anything requested directly via blockdev.c is "public."
> Before 2.8, all jobs were public ones, with the exception of those in
> qemu-img which is a bit of a different/special case.
> We have this block/replication.c beast now, though, and it uses backup_start
> and commit_active_start as it sees fit without direct user intervention.
> As it stands, I believe the jobs that replication creates are user-visible
> via query, will not issue cancellation or completion events, but WILL emit
> error events. It may emit ready events for the mirror job it uses, but I
> haven't traced that. (It could, at least.)

Thanks, the above is useful to know. 


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