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

From: John Snow
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH v2 02/11] blockjob: centralize QMP event emissions
Date: Wed, 5 Oct 2016 17:00:29 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.3.0

On 10/05/2016 03:24 PM, Eric Blake wrote:
On 10/05/2016 01:49 PM, John Snow wrote:

Here we have an additional caller in block/replication.c and qemu-img,
so the parameters must stay. For qemu-img, nothing changes. For
replication, the block job events are added as a side effect.

Not sure if we want to emit such events for an internal block job, but
if we do want the change, it should be explicit.

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.

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.

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.)

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