Re: [Qemu-block] Addressability of Block-jobs after bdrv_swap removal

[Kevin Wolf]

Am 09.12.2015 um 21:59 hat John Snow geschrieben:
> I have a question about how the device name of block jobs gets reported
> now after the bdrv_swap removal patch.
> 
> Previously, it appears that the job stayed attached to the root-node (if
> somewhat hackishly, apparently) so that we could at all stages report
> our name without having to cache the device we started under.

Yes, I think that was effectively what happened.

I don't remember the exact details, but what I remember from the
bdrv_swap() removal work is that it was confusing. After removing
bdrv_swap() doing it either way turned out wrong because the job had to
stay at the same BDS and at the same time at the root, too, so that the
device name would continue to exist. Doing both is impossible without
swapping the C objects.

> However, since QMP commands refer to block-jobs solely through their
> device name, do we have any cases post-removal where a job becomes
> "unreachable" through its advertised name?
> 
> e.g.
> 
> the block-job-ready event uses the device name to advertise which job
> has just converged. The user/client would then be responsible for
> sending qmp-block-job-complete device=XXX to finish the job when desired.
> 
> I don't see one at a quick glance, but we don't have any cases where we
> perform any graph manipulation before we expect the user to interface
> with the job again, right?
> 
> (It's always done right at completion time, at least for drive-mirror.
> Do any other jobs adjust the graph? If it's ever anything except right
> before completion time, we may lose the ability to pause/resume,
> set-speed, etc.)
> 
> Does this sound about right, or have I fatally misunderstood the situation?

Other jobs change the graph as well, but none do so before completion,
so I don't think theere will be further user interaction. The only
interesting part so far is the device name that is sent in the
completion event.

> (post-script: I was thinking of adding a unique per-job ID that could be
> reported alongside any events or errors where the job's device name was
> reported, so that users could provide this ID to find the job. Each BB
> would have a per-tree list of jobs with globally unique IDs, and
> regardless of what node the job was currently attached to, we could
> retrieve that job unambiguously. This would be useful if the above
> question reveals an API problem, or more generally for multiple block
> jobs where we'll need IDs to reference jobs anyway.)

I actually introduced a job->id field, but we're not using it
consistently yet. I needed it so I could still access the right name
for the completion message, as I said above.

What could turn out a bit nasty is that we have to maintain API
compatibility. The best option that I could think of so far is that we
change the current device name in all QMP commands into a block job ID
while still calling it 'device' for compatibility. If an ID isn't
specified in the command that starts a block job, the device_name is
used like today. If that default ID is already taken, we fail the
command; this doesn't impact compatibility because today you can't set
any ID other than the device_name and you can only have one job per
device.

If we don't like the misnomer 'device' for the block job ID, we would
have to store whether the id == device_name default was applied and then
add a 'device' key in all return values and events where this was the
case.

> Each BB would have a per-tree list of jobs with globally unique IDs

Why that? Block jobs don't belong to a single BB. They are artificially
attached to a single BDS today, but design-wise that doesn't make a lot
of sense for any job other than streaming, because all other currently
existing jobs have a source and a target rather than a single node.

Or maybe we should now introduce a completely new API for background jobs
(not just block jobs) and implement the old API on top of that. In the
long run we could move e.g. migration to the same API.

Kevin