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

[Piotr Kliczewski]

On Tue, Apr 4, 2017 at 5:26 PM, 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 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.

How that would work when we would update how we collect stats in the future?
Would it require rerun host-deploy or Ansible?

Why not to expect different data from different versions like we did
for vm status changes.
We added the new way of obtaining it but we kept the old code as it
was for backward
compatibility purposes.

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