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

Tue Apr 4 11:20:53 UTC 2017

I wonder why we not go in the same approach as we went for vm status
changes. Why not to pull for older versions and expect a push from new
implementation.
This would allow as to keep current code as it is for backward
compatibility purposes and create new one (reuse parts of the old one) for
new approach.
We could change stats collection (collectd based) and still get important
data using push.

If we want to pass cluster version we could use connection negotiation like
we do for heartbeat interval. It can be passed every time engine connects
so there is no
need for persistence. We could define custom headers and as a result it
won't break vdsm-client code.

Please note that we are not good at deprecating api and I am not going to
say anything about cleanup or removal.
Whatever we introduce it is going to stay so we need to be careful.

On Tue, Apr 4, 2017 at 12:28 PM, Michal Skrivanek <
michal.skrivanek at redhat.com> wrote:

>
> On 4 Apr 2017, at 12:21, Roy Golan <rgolan 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:)
>>
>
> Intended for this as well.
>
>
> great!
>
>
>>
>> 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?
>>
>> What is the benefit?
>
>
> just so you do not need any configuration verb nor persistence, not much
> else.
>
>
>>   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
>>
>
> But we should prefer a deliberate action. Doing that as a side effect is
> surprising for an API verb. What would happen in case you invoke
> 'vdsm-client’ ?
>
>
> I do not mean to switch for any client, more like a single “upgrade” of
> communication once the new verb is called. So once the engine calls the new
> verb the legacy stats thread is stopped and vdsClient <oldVerb> stops
> returning meaningful data.
> Actually, moving host back to older cluster doesn’t really need switching
> back either - you still have the new engine capable of handling the new
> verb despite older cluster.
>
>
>
>
>>
>> Thanks,
>> michal
>>
>>
>>
>>
>>
>>
>> Nevertheless it can be potentially beneficial to more functions in vdsm.
>>
>> Thanks,
>> Roy
>>
>>
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>
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