[libvirt] Querying block device job status and semantics of virDomainBlockJobInfo()

[Eric Blake]

On 09/01/2016 08:57 AM, Kashyap Chamarthy wrote:
> So, I'm trying to understand how libvirt reports the "cur" and "end"
> values.  I've read the virDomainBlockJobInfo() struct, it wasn't crystal
> clear.  It states:
> 
> /*
>  * The following fields provide an indication of block job progress.  @cur
>  * indicates the current position and will be between 0 and @end.  @end is
>  * the final cursor position for this operation and represents completion.
>  * To approximate progress, divide @cur by @end.
>  */

Libvirt is (more or less) reporting numbers from qemu.  What's more,
@end need not be the same between calls; qemu is free to change the end
value as it comes up with more work to do (or sees that less work
remains than initially estimated), all that REALLY matters is that the
ratio between the two numbers converges, and is < 1 while busy, and == 1
when complete.  Except that there are cases in qemu where a block job
really has 0 work to do.

Prior to qemu exposing the "ready":true/false flag, libvirt had to guess
whether equal numbers really meant done, or if it merely meant nearly
done.  But now that we have "ready", I think the sanest course of action
for libvirt is to fudge the numbers from qemu.  After all, we've already
documented (both in libvirt and in qemu) that @end is not fixed, so much
as a moving target (it just doesn't move very much on operations where
we have a good grasp on how much work remains from the start, like a
deep copy; but is more prone to move on operations like live commit that
are influenced by how active the guest is at writing data that we are
attempting to commit at the same time).

> 
> Now, if a job hasn't started, what would libvirt report?  Talking to
> Michal Privoznik on IRC:
> 
> [mprivozn]
> 
>     I wonder if libvirt should report something else than start=0 end=0
>     in order to tell the caller that job hasn't been started yet.  I
>     suspect that libvirt does not report correctly whether job has
>     started already, which in turns forces Nova to use some workarounds.
> 
> [kashyap]
> 
>     So, if the job hasn't started yet, what should libvirt report? 
> 
> [mprivozn]
> 
>     That's the question. We can't change the [virDomainBlockJobInfo]
>     struct (otherwise we won't be ABI compatible), so we can't really
>     add a bolean there 'bool job_started'.
> 
>     Or we can introduce new "job type" which wouldn't really be a job
>     type, but we will fill status.job with it to say explicitly job
>     hasn't started yet
> 
> 
> So, any other thoughts here, on how to proceed here?

My preference would be:

If qemu doesn't report anything (because the job is not started yet),
then libvirt should report cur=0, end=1 (the job still has 100% to go).

If qemu reports 0/0 and "done":false, then libvirt should report cur=0,
end=1 (that is, we fudge the end to be larger, because the job is not
done yet).

If qemu reports 0/0 and "done":true (because the job was really a
no-op), then libvirt should report cur=1, end=1 (the job is 100% complete).

If qemu reports 0/0 and lacks "done" (older qemu), then libvirt just has
to guess.  I'm not sure which guess is most appropriate; maybe libvirt
itself will have to set up a timer and report 0/1 the first time, and
only report 1/1 after a minimum time has elapsed, to make sure qemu has
had a chance to do something about the job.  Or maybe we don't worry
about it, and just have libvirt report 0/0 because we really don't know
any better.

If qemu reports a/b, where a < b and b > 0, use those numbers as is.  We
don't even have to check "done".

If qemu reports a/a, where a > 0, then also check "done". If
"done":false is present, report a-1/a (the job is not quite done); if
"done" is absent or "done":true is present, report a/a (the job is done).


> I realize that a copy job has two phases, as clearly stated in the API
> documenation of virDomainBlockRebase()[3]:
> 
>   "[...] The job transitions to the second phase when the job info
>   states cur == end, and remains alive to mirror all further changes to
>   both source and destination.  The user must call
>   virDomainBlockJobAbort() to end the mirroring while choosing whether
>   to revert to source or pivot to the destination. [...]"

We may also want to enhance the libvirt documentation to mention cur ==
non-zero end as the phase change point, mentioning that 0/0 is a special
case (hopefully reserved for older qemu).

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

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