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

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