[Users] two node ovirt cluster with HA

[Karli Sjöberg]

On Mon, 2014-01-27 at 11:11 +0000, Dafna Ron wrote:
> Powering off the host will never trigger vm migration.
> As far as engine is concerned it just lost connection to the host, but 
> has no way of telling if the host is down or if a router is down.

Can´t it at least check with power management if the Host status is down
first?

I mean, if the network is down there will be no response from either PM
or Host. But if PM is up and can tell you that the Host is down, sounds
rather clear cut to me...

Seems to me the VM's would be restarted sooner if the flow was altered
to first check with PM if it´s a network or Host issue, and if Host
issue, immediately restart VM's on another Host, instead of waiting for
a potentially problematic Host to boot up eventually. 

/K

> since vm's can continue running on the host even if engine has no access 
> to it, starting the vm's on the second host can cause split brain and 
> data corruption.
> 
> The way that the engine knows what's going on is by sending heath check 
> queries to the vdsm.
> Power management will try to reboot a host when the health checks to 
> vdsm will not be answered.
> So... if engine gets no reply and has no way of rebooting the host, the 
> host status will be changed to Non-Responsive and the vm's will be 
> unknown because engine has no way of knowing what's happening with the 
> vm's.
> Since reboot of the host will kill the vm's running on it - this will 
> never cause any vm migration but... along with the High-Availability vm 
> feature, you will be able to have some of the vm's re-started on the 
> second host after the host reboot (and that is only if Power Management 
> was confirmed as successful).
> 
> VM migration is only triggered when:
> 1. Cluster configuration states that the vm should be migrated in case 
> of failure
> 2. Engine has access to the host - so the failure is on the storage side 
> and not the host side.
> 3. the vms are not actively writing (although there might be a new RFE 
> for it).
> 
> hope this clears things up
> 
> Dafna
> 
> 
> 
> On 01/27/2014 10:11 AM, Andrew Lau wrote:
> >
> > Hi,
> >
> > Have you got power management enabled?
> >
> > That's the fencing feature required for the engine to ensure that the 
> > host is actually offline. It won't resume any other VMs to prevent 
> > potential VM corruption (eg. VM running on multiple hosts).
> >
> > Andrew.
> >
> > On Jan 27, 2014 5:12 PM, "Jaison peter" <urotrip2 at gmail.com 
> > <mailto:urotrip2 at gmail.com>> wrote:
> >
> >     Hi all ,
> >
> >     I was setting a two node ovirt cluster with ovirt engine on
> >     seperate node . I completed the configuration and tested VM  live
> >     migrations with out any issues . Then for checking cluster HA I
> >     powered down one host and expected vms running on that host to be
> >     migrated to the other one . But nothing happened , Engine detected
> >     host as un-rechable and marked it as non-operational and vm ran on
> >     that host went to 'unknown state' . Is that not possible to setup
> >     a fully HA ovirt cluster with two nodes ? or else is that my
> >     configuration problem ? please advice .
> >
> >     Thanks & Regards
> >
> >     Alex
> >
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> >
> >
> >
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> > http://lists.ovirt.org/mailman/listinfo/users
> 
> 
> -- 
> Dafna Ron
> _______________________________________________
> Users mailing list
> Users at ovirt.org
> http://lists.ovirt.org/mailman/listinfo/users



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Karli Sjöberg
Swedish University of Agricultural Sciences Box 7079 (Visiting Address
Kronåsvägen 8)
S-750 07 Uppsala, Sweden
Phone:  +46-(0)18-67 15 66
karli.sjoberg at slu.se