[Users] two node ovirt cluster with HA

[Dafna Ron]

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.
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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-- 
Dafna Ron