[ovirt-devel] [VDSM] [ENGINE] [RFC] A configuration verb for contexual vdsm operation mode

[Dan Kenigsberg]

On Tue, Apr 4, 2017 at 1:16 PM, Michal Skrivanek
<michal.skrivanek at redhat.com> wrote:
>
> On 4 Apr 2017, at 12:10, Roy Golan <rgolan at redhat.com> wrote:
>
>
>
> On Tue, Apr 4, 2017 at 12:49 PM Yaniv Kaul <ykaul at redhat.com> wrote:
>>
>> On Tue, Apr 4, 2017 at 12:29 PM, Roy Golan <rgolan at redhat.com> wrote:
>>>
>>> I'm working on a POC lately on a change to stats collection and retrieval
>>> by VDSM. The moto is to cut all we can from host/vm stats (possibly caps)
>>> and report only core-business stuff to the engine. Engine will retrieve the
>>> rest through a 3rd party provider
>>>
>>> (nevermind what is it atm)
>
>
> I hope it’s the same one as for VM stats, collectd:)
>
>>>
>>> Being backward compatible by design, I have to support 2 API versions for
>>> Host.getStats , '4.1' and '4.2'.
>>> Except from supplying less parameters, I want VDSM to do less stuff. It
>>> doesn't need to sample what it doesn't report. In other words I want
>>> '4.1-sampling' and '4.2-sampling'
>>>
>>> # Introducing 'configuration' Verb:
>>>
>>> As engine knows always(Hosted Engine as well) what cluster version this
>>> host belongs to, it can configure VDSM to operate in cluster version mode.
>
>
> why not running it in parallel for one version?
>
>>>
>>>   Host.configure(config={version: 4.2}
>>>
>>> Consider this verb, pre-activating using 'Host.getCaps' to set the
>>> context.
>>> It will set the righjt sampling method, and other stuff if needed then
>>> API endpoints will have the right permutation of the api to answer it.
>>>
>>> 4.2 host can operate in 4.1 mode:
>>>   Host.configure(config={version: 4.1}
>>>
>>> Issue: moving a 4.2 host from 4.2 cluster to 4.1 is a problem since
>>> engine needs to know this is a new vdsm that has the verb available. One way
>>> to overcome that is to fire the verb for every host regardless of the
>>> version and disregard an error that implies the verb doesn't exist.
>>
>>
>> Isn't it solved by host re-installation?
>
>
> We allow maintenance + change host cluster so not always. Was this changed?
>>
>>
>>>
>>> # Engine:
>>> Engine will have a handling of the verb per version.
>>> Host/Vms monitoring should be changed - I suggest to move out of the
>>> monitoring code the whole stats collection as it is a different task which
>>> is orthogonal to 'monitoring' and in 4.2 more than before.
>>>
>>>
>>> I know configuration for VDSM has been discussed before and there are
>>> probably tons of ways to do it. When you share your thoughts please remember
>>> that configuration is a by-product of the effort.
>>
>>
>> How do we persist this level on VDSM? Or we don't, and if VDSM is
>> restarted it is again back to 4.1 mode until Engine tells it otherwise?
>>
>> Y.
>
>
> Must persist it somehow otherwise there is a race when the engine will send
> send a stats request and will get the wrong answer.  I'm wondering if using
> differnt endpoints is the right solution here to prevent that from
> happening.
>   method: Host.getStats version: 4.1
>
>
> would it be a problem? assuming that the code is easily started/stopped
> within vdsm, we can just change the behavior based on receiving one or the
> other verb for the first time after vdsm starts

It does not feel right to have a such a state in Vdsm. and making this
state depend implicitly on a verb feels even worse than an explicit
"configure" verb. We already have something like that in the
debug-oriented setLogLevel verb; but that's not how client/server
applications usually operate.

I think that the proper way to do this would be to reconfigure
vdsm.conf, set there collect_statistics=false (via ovirt-host-deploy
or Anible), and restart vdsmd+supervdsmd. This way we are sure that
all threads and services see the new config and act accordingly. This
can be done by Engine whenever a host is added to a new cluster, based
on the statistic-gathering policy in that cluster.