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

On 4 Apr 2017, at 17:26, Dan Kenigsberg <danken(a)redhat.com&gt; wrote: On Tue, Apr 4, 2017 at 1:16 PM, Michal Skrivanek <michal.skrivanek(a)redhat.com&gt; wrote: > > On 4 Apr 2017, at 12:10, Roy Golan <rgolan(a)redhat.com&gt; wrote: > > > > On Tue, Apr 4, 2017 at 12:49 PM Yaniv Kaul <ykaul(a)redhat.com&gt; wrote: >> >> On Tue, Apr 4, 2017 at 12:29 PM, Roy Golan <rgolan(a)redhat.com&gt; 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.