Objectives

After completing this section, you should be able to remove a host from an existing data center for maintenance or reassigning, and add a host to increase the capability of a cluster in a data center.

Scaling the Infrastructure

Enterprise virtualization workloads can be diverse and complex; Red Hat Virtualization is designed to be flexible enough to support even the most demanding requirements. RHV data centers and clusters can be reconfigured to match changing requirements, providing an ability to scale both smaller and larger, to reconfigure existing clusters for new workloads, and to reprovision hypervisor hosts to handle those workloads.

Hypervisor hosts may be added or removed from clusters for a number of reasons, including:

Using Maintenance Mode

Many common maintenance tasks, including network configuration and deployment of software updates, require that hosts be placed into maintenance mode. Hosts should be placed into maintenance mode before any event that might cause VDSM to stop working properly, such as a reboot, or issues with networking or storage. Because VDSM provides the communication between the RHV Manager and hosts, placing a host in maintenance mode signals VDSM, which temporarily disables engine health checking. VDSM does not stop while the host is in maintenance mode.

When a host is placed in maintenance mode, the RHV Manager attempts to migrate all running virtual machines to alternative hosts. The standard prerequisites for live migration apply, in particular there must be at least one active host in the cluster with capacity to run the migrated virtual machines.

Virtual machines that are pinned to the host and cannot be migrated are shut down. Check which virtual machines are pinned to the host by clicking Pinned to Host in the Virtual Machines tab of the host Details screen.

If the host is the Storage Pool Manager (SPM), the SPM role is migrated to another host. The Status field of the host changes to Preparing for Maintenance, and finally Maintenance when the operation completes successfully. Advanced host editing options become available in the Edit Host window.

Placing a Data Center into Maintenance Mode

If a RHV host is the last active host in a data center, then the data center must also be placed into maintenance mode because a data center with no hosts cannot function. Placing the data center in maintenance mode will place all of the data center storage domains into maintenance mode. The Storage Pool Manager, which was migrated to the last active host as other hosts were placed in maintenance mode or shut down, will change the data center storage configuration as storage domains become inactive.

When all storage domains in a data center are in maintenance mode, the data center automatically transitions to maintenance mode and the SPM no longer has storage tasks to manage. The data center stops producing log output. Log output restarts only when the master storage domain becomes active again. The last active host may now be placed in maintenance mode.

Removing a Host from Red Hat Virtualization

To remove a host from the RHV infrastructure, use the Administration Portal as a user with a sufficient role. Select the host in the Hosts table, then select Maintenance from the drop-down list on the Management button. Existing virtual machines on the host may need to be manually migrated or shut down before the host can enter maintenance mode. When the host displays a Maintenance status, the Remove button becomes active. Verify that the correct host is selected in the table, then click the Remove button, and then click the OK button to confirm the removal.

Removing the host only removes its cluster and data center associations in the RHV engine database, but does not physically remove or wipe the host system, nor alter the host operating system configuration. Unless maintenance tasks or additional storage or network configuration is required, the host can be added to any existing cluster where it meets the joining criteria, such as a sufficient CPU family architecture.

Adding a Host to Red Hat Virtualization

After a RHV host has been installed, it is added to a Red Hat Virtualization environment by joining a data center and, on the same screen, a cluster of the selected data center. Logical networks and storage domains created in a data center then become available to the host. You must configure a host to connect to the storage domains and logical networks required by the cluster it has joined.

To function properly, all hosts in a single cluster must share access to the same storage domains and logical networks, which requires that host be configured to access the shared storage back ends and network segments. Hosts in the same cluster should standardize on the same CPU vendor and model, storage adapter and network interface hardware, and firmware versions, for consistent behavior and performance.

To add a host to the RHV infrastructure, use the Administration Portal as a user with a sufficient role. In the Hosts screen, click the New button to open the New Host window. In addition to entering a unique name and a helpful comment, you must provide the correct information for functioning SSH access. For stronger security, SSH public key access is preferred.

The Hostname field can be entered as either a fully qualified domain name (FQDN) or as the host IP address. To function properly, the host FQDN must be DNS resolvable, both forward and reverse.

By default, the RHV Manager automatically updates the host firewall rules to allow the required network protocols. If you wish to manually manage the host firewall rules, open the Advanced Parameters accordion icon to disable this behavior by clearing the Automatically configure host firewall checkbox.

Moving a Host Between Clusters or Data Centers

To move a host from one cluster to another, use the Administration Portal as user with a sufficient role. Select the host in the Hosts table, and place it in maintenance mode as previously described. When the host transitions to the Maintenance status, click the Edit button to open the Edit Host window.

Using the Host Cluster drop-down list, choose a new cluster in the same or a different data center. Click the OK button twice, to exit the editing window and confirm the move. The host remains in maintenance mode until you activate it using the Activate option on the Management button drop-down list. When the host successfully activates, its status icon changes from red to green to indicate that the host is now available in the new cluster and data center.