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6.14. Improving Uptime with Virtual Machine High Availability
6.14.1. What is High Availability?
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 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. With storage domains V4 or later, virtual machines have the additional capability to acquire a lease on a special volume on the storage, enabling a virtual machine to start on another host even if the original host loses power. The functionality also prevents the virtual machine from being started on two different hosts, which may lead to corruption of the virtual machine disks.
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.
6.14.2. Why Use High Availability?
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.
A high availability virtual machine is automatically restarted, either on its original host or another host in the cluster.
6.14.3. High Availability Considerations
A highly available host requires a power management device and its fencing parameters configured. In addition, for a virtual machine to be highly available when its host becomes non-operational, it needs to be started on another available host in the cluster. To enable the migration of highly available virtual machines:
- Power management must be configured for the hosts running the highly available virtual machines.
- The host running the highly available virtual machine must be part of a cluster which has other available hosts.
- The destination host must be running.
- The source and destination host must have access to the data domain on which the virtual machine resides.
- The source and destination host must have access to the same virtual networks and VLANs.
- There must be enough CPUs on the destination host that are not in use to support the virtual machine's requirements.
- There must be enough RAM on the destination host that is not in use to support the virtual machine's requirements.
6.14.4. Configuring a Highly Available Virtual Machine
High availability must be configured individually for each virtual machine.
Procedure 6.36. Configuring a Highly Available Virtual Machine
- Click the Virtual Machines tab and select a virtual machine.
- Click Edit.
- Click the High Availability tab.
Figure 6.12. The High Availability Tab
- Select the Highly Available check box to enable high availability for the virtual machine.
- Select the storage domain to hold the virtual machine lease, or select No VM Lease to disable the functionality, from the Target Storage Domain for VM Lease drop-down list. See Section 6.14.1, “What is High Availability?” for more information about virtual machine leases.
ImportantThis functionality is only available on storage domains that are V4 or later.
- Select Low, Medium, or High from the Priority drop-down list. When migration is triggered, a queue is created in which the high priority virtual machines are migrated first. If a cluster is running low on resources, only the high priority virtual machines are migrated.
- Click OK.