libvirt users - Feb 2017 - Re: Determining domain job kind from job stats?

Jiri Denemark <jdenemar@redhat.com> writes:
> On Fri, Feb 10, 2017 at 21:50:19 +0100, Milan Zamazal wrote:
>> Hi, is there a reliable way to find out to what kind of job does the
>> information returned from virDomainGetJobStats or provided in
>> VIR_DOMAIN_EVENT_ID_JOB_COMPLETED event callback belong to?
>
> No, libvirt expects that the caller knows what job it started. All jobs
> currently reported using virDomainGetJobStats API or
> VIR_DOMAIN_EVENT_ID_JOB_COMPLETED event are internally implemented as
> migration in QEMU driver (either to a file or to a network socket),
> which may confuse any heuristics for detecting the job type from the set
> of fields returned by libvirt.
I see, thank you for explanation.
> What is the problem you are trying to solve?
There are basically two problems:

- When the job completion callback is called, I need to distinguish what
  kind of job was it to perform the appropriate actions.  It would be
  easier if I knew the job type directly in the callback (no need to
  coordinate anything), but "external" job tracking is also possible.

- If I lost track of my jobs (e.g. because of a crash and restart), I'd
  like to find out whether a given VM is migrating.  Examining the job
  looked like a good candidate to get the information, but apparently
  it's not.  Again, I can probably arrange things to handle that, but to
  get the information directly from libvirt (not necessarily via job
  info) would be easier and more reliable.

Thanks,
Milan

On Fri, Feb 17, 2017 at 12:38:24 +0100, Milan Zamazal
wrote:
> Jiri Denemark <jdenemar@redhat.com> writes:
> 
> > On Fri, Feb 10, 2017 at 21:50:19 +0100, Milan Zamazal wrote:
> >> Hi, is there a reliable way to find out to what kind of job does
the
> >> information returned from virDomainGetJobStats or provided in
> >> VIR_DOMAIN_EVENT_ID_JOB_COMPLETED event callback belong to?
> >
> > No, libvirt expects that the caller knows what job it started. All
jobs
> > currently reported using virDomainGetJobStats API or
> > VIR_DOMAIN_EVENT_ID_JOB_COMPLETED event are internally implemented as
> > migration in QEMU driver (either to a file or to a network socket),
> > which may confuse any heuristics for detecting the job type from the
set
> > of fields returned by libvirt.
> 
> I see, thank you for explanation.
> 
> > What is the problem you are trying to solve?
> 
> There are basically two problems:
> 
> - When the job completion callback is called, I need to distinguish what
>   kind of job was it to perform the appropriate actions.  It would be
>   easier if I knew the job type directly in the callback (no need to
>   coordinate anything), but "external" job tracking is also
possible.
An immediate answer would be: "don't rely on the completion callback
and
just check the return value of the API which started the job", but I
guess you want it because checking the return value is not possible when
the process which started the job is not running anymore as described
below.
> - If I lost track of my jobs (e.g. because of a crash and restart), I'd
>   like to find out whether a given VM is migrating.  Examining the job
>   looked like a good candidate to get the information, but apparently
>   it's not.  Again, I can probably arrange things to handle that, but
to
>   get the information directly from libvirt (not necessarily via job
>   info) would be easier and more reliable.
Apparently you are talking about peer-to-peer migration, otherwise the
migration would be automatically canceled when the process which started
it disappears. I'm afraid this is not currently possible in general. You
might be able to get something by checking the domain's status, but it
won't work in all cases.

Jirka

Jiri Denemark <jdenemar@redhat.com> writes:
> On Fri, Feb 17, 2017 at 12:38:24 +0100, Milan Zamazal wrote:
>> Jiri Denemark <jdenemar@redhat.com> writes:
>> 
>> > On Fri, Feb 10, 2017 at 21:50:19 +0100, Milan Zamazal wrote:
>> >> Hi, is there a reliable way to find out to what kind of job
does the
>> >> information returned from virDomainGetJobStats or provided in
>> >> VIR_DOMAIN_EVENT_ID_JOB_COMPLETED event callback belong to?
>> >
>> > No, libvirt expects that the caller knows what job it started. All
jobs
>> > currently reported using virDomainGetJobStats API or
>> > VIR_DOMAIN_EVENT_ID_JOB_COMPLETED event are internally implemented
as
>> > migration in QEMU driver (either to a file or to a network
socket),
>> > which may confuse any heuristics for detecting the job type from
the set
>> > of fields returned by libvirt.
>> 
>> I see, thank you for explanation.
>> 
>> > What is the problem you are trying to solve?
>> 
>> There are basically two problems:
>> 
>> - When the job completion callback is called, I need to distinguish
what
>>   kind of job was it to perform the appropriate actions.  It would be
>>   easier if I knew the job type directly in the callback (no need to
>>   coordinate anything), but "external" job tracking is also
possible.
>
> An immediate answer would be: "don't rely on the completion
callback and
> just check the return value of the API which started the job", but I
> guess you want it because checking the return value is not possible when
> the process which started the job is not running anymore as described
> below.
Well, avoiding using the completion callback is probably OK for me.
(In case of the process restart, I don't expect having everything
perfectly working, just some basic sanity.)
>> - If I lost track of my jobs (e.g. because of a crash and restart),
I'd
>>   like to find out whether a given VM is migrating.  Examining the job
>>   looked like a good candidate to get the information, but apparently
>>   it's not.  Again, I can probably arrange things to handle that,
but to
>>   get the information directly from libvirt (not necessarily via job
>>   info) would be easier and more reliable.
>
> Apparently you are talking about peer-to-peer migration, 
Yes.
> otherwise the migration would be automatically canceled when the
> process which started it disappears. I'm afraid this is not currently
> possible in general. You might be able to get something by checking
> the domain's status, but it won't work in all cases.
Too bad.  Could some future libvirt version provide that information?

Thank you for clarification,
Milan