[Users] When does oVirt auto-migrate, and what does HA do?

[Eli Mesika]

----- Original Message -----
> From: "Rob Zwissler" <rob at zwissler.org>
> To: users at ovirt.org
> Sent: Tuesday, March 5, 2013 10:09:43 PM
> Subject: [Users] When does oVirt auto-migrate, and what does HA do?
> 
> In what scenarios does oVirt auto-migrate VMs?  I'm aware that it
> currently migates VMs when putting a host into maintenance, or when
> manually selecting migration via the web interface, but when else
> will
> hosts be migrated?  Is there any automatic compensation for resource
> imbalances between hosts?  I could find no documentation on this
> subject, if I missed it I apologize!
The following is taken from the upcoming 3.2 docs:

Automatic Virtual Machine Migration
Red Hat Enterprise Virtualization Manager automatically initiates live migration of all virtual machines running on a host when the host is moved into maintenance mode. The destination host for each virtual machine is assessed as the virtual machine is migrated, in order to spread the load across the cluster.
The Manager automatically initiates live migration of virtual machines in order to maintain load balancing or power saving levels in line with cluster policy. While no cluster policy is defined by default, it is recommended that you specify the cluster policy which best suits the needs of your environment. You can also disable automatic, or even manual, live migration of specific virtual machines where required. 


> 
> In a related question, exactly what does enabling HA (Highly
> Available) mode do?  The only documentation I could find on this is
> at
> http://www.ovirt.org/OVirt_3.0_Feature_Guide#High_availability but it
> is a bit vague, and being from 3.0, possibly out of date.  Can
> someone
> briefly describe the HA migration algorithm?

 High availability is recommended for virtual machines running critical workloads.
High availability can ensure that virtual machines are restarted in the following scenarios:

    When a host becomes non-operational due to hardware failure.
    When a host is put into maintenance mode for scheduled downtime.
    When a host becomes unavailable because it has lost communication with an external storage resource.
    When a virtual machine fails due to an operating system crash. 

 High availability means that a virtual machine will be automatically restarted if its process is interrupted. This happens if the virtual machine is terminated by methods other than powering off from within the guest or sending the shutdown command from the Manager. When these events occur, the highly available virtual machine is automatically restarted, either on its original host or another host in the cluster.
High availability is possible because the Red Hat Enterprise Virtualization Manager constantly monitors the hosts and storage, and automatically detects hardware failure. If host failure is detected, any virtual machine configured to be highly available is automatically restarted on another host in the cluster. In addition, all virtual machines are monitored, so if the virtual machine's operating system crashes, a signal is sent to automatically restart the virtual machine.
With high availability, interruption to service is minimal because virtual machines are restarted within seconds with no user intervention required. High availability keeps your resources balanced by restarting guests on a host with low current resource utilization, or based on any workload balancing or power saving policies that you configure. This ensures that there is sufficient capacity to restart virtual machines at all times. 


> 
> Thanks,
> 
> Rob
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