A cluster is a collection of physical hosts that share the same storage domains and have the same type of CPU. Because virtual machines can be migrated across hosts in the same cluster, the cluster is the highest level at which power and load-sharing policies can be defined. The Red Hat Enterprise Virtualization platform contains a default cluster in the default data center at installation.
Every cluster in the system must belong to a data center, and every host in the system must belong to a cluster. This enables the system to dynamically allocate a virtual machine to any host in the applicable cluster, according to policies defined on the Cluster tab and in the Configuration tool during runtime, thus maximizing memory and disk space, as well as virtual machine uptime.
At any given time, after a virtual machine runs on a specific host in the cluster, the virtual machine can be migrated to another host in the cluster using Migrate. This can be very useful when a host is shut down for maintenance. The migration to another host in the cluster is transparent to the user, and the user continues working as usual. Note that a virtual machine cannot be migrated to a host outside the cluster.
9.2.1. Creating a Cluster
Before creating a new cluster, ensure that there is at least one host available to be assigned to it. The hosts in a cluster all run the same type of CPU. That is, a cluster must not contain a mix of both Intel and AMD CPUs.
The default rhevm network cannot be modified once a cluster has been attached to a data center. Any configuration required for the rhevm network, such as enabling VLAN tagging, must be performed before a cluster is attached, and the data center is still in the Uninitialized state.
Procedure 9.1. To create a new host cluster:
Click the Clusters tab. A list of clusters displays.
Click the New button on the Clusters tab.
The New Cluster dialog displays.
On the General tab, Select an existing Data Center from the list, the cluster Name and Description. The name should not include spaces.
Select the CPU Name for hosts in this cluster. All hosts must run the same type of CPU. The CPU Name list displays all the CPU types supported by Red Hat Enterprise Virtualization. Finally on the General tab, select the Compatibility Level of the data center, from either 2.2 or 3.0.
Use the Memory Optimization tab to define how much of the host's memory can be used in excess of the permitted memory for a virtual machine in the cluster. For example, all virtual machines will not be using the full amount of allocated memory all the time. Memory sharing allows virtual machines that require additional memory at a certain time to use memory that is not being used at that time by other virtual machines. This feature allows you to fine tune how you wish to optimize the memory page sharing and large pages implementation on the hosts in the cluster.
Select from None, that disables memory page sharing, Optimized for Server Load, that sets the memory page sharing threshold to 150% of the system memory on each host, or Optimized for Desktop Load, that sets the memory page sharing threshold to 200% of the system memory on each host.
Select the Resilience Policy tab to define if high availability is to be implemented for the virtual machines in the cluster. If a host shuts down unexpectedly or is put into maintenance, the virtual machines running on the host can be re-run on another host in the same cluster. This field allows you to configure the migration settings for virtual machines.
Select from Migrate Virtual Machines, (migrates all machines); Migrate only Highly Available Virtual Machines or Do Not Migrate Virtual Machines.
Click OK to create the cluster. The new host cluster is added to the data center and displays on the Cluster tab.
The New Cluster - Guide Me dialog displays.
The Guide Me tab prompts you to add hosts to the new cluster. Click the Configure Hosts button, the New Host dialog displays.
Enter the details of the host to assign to the cluster. Click OK to close the New Host dialog. Now click Configure Later to close the New Cluster Guide Me dialog and return to the Clusters tab.
The Hosts tab on the Details pane displays the newly added hosts.