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Re: [libvirt] Determining domain job kind from job stats?



[Starting to move to the development list.]

Milan Zamazal <mzamazal redhat com> writes:

> Jiri Denemark <jdenemar redhat com> writes:
>
>> On Fri, Feb 17, 2017 at 12:38:24 +0100, Milan Zamazal wrote:
>>> 
>>> 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.

Thinking about it more, it's not very nice: I have to use the callback
to get the completed job stats (I'm not guaranteed the domain still
exists on the source host when I ask it for the stats explicitly) *and*
to track the jobs outside the callback to know whether the callback is
related to the type of domain jobs I'm going to handle.

Although not absolutely necessary, it would be much nicer if the job
type was identified in the callback.

> (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?

If libvirt provided information about the job type, it would help with
several things, for instance: With the callback problem above, with
using libvirt as the ultimate single source of information about the
VMs, or with handling VMs not running under complete control of a
particular piece of software.

If some piece of information about a VM is missing from libvirt I need
to store it somewhere.  Domain metadata is a natural place for that,
since it's closely bound to the corresponding VM and keeps libvirt as
the ultimate single source of information.  But putting there
information about migration is weird at best.

Based on those thoughts I think libvirt should really provide a simple
way to find out whether the VM is migrating to another host and to
identify the domain job type in the job completed callback.  Is there
anything preventing to add that information?

Thanks,
Milan


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