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

[Dan Kenigsberg]

On Tue, Apr 4, 2017 at 6:33 PM, Michal Skrivanek
<michal.skrivanek at redhat.com> wrote:
>
>> On 4 Apr 2017, at 17:26, Dan Kenigsberg <danken at redhat.com> wrote:
>>
>> 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 don’t mind either way
>
>>
>> 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.
>
> But why would you require restart for such a simple thing? We have the collection pretty well isolated already, it’s not even using periodic sampling, there are nice-to-drop things which can wait (repoStats, HE HA status?)

A specific verb for enablePeriodicStatsCollection(False) can work like that.

Roy suggested a more general configure verb, which I think can lead to
all sort of unsatisfiable assumptions.