In this video, Russ Long discusses the different roles of hosts within an HA cluster. Learn about how each role and the responsibilities associated with them.
- In order to ensure that VM's are protected by HA, HA determines a host that oversees the other hosts in a cluster. There are two roles assigned to the host within a cluster. The first is a "master role", there is usually only one master per cluster which is considered a best practice. The second role is the "slave role". All other hosts within that HA cluster are considered a slave. HA requires that an HA agent is installed on all hosts within a cluster. This agent handles all communication which takes place between other agents that been installed on other hosts in the cluster.
Once the agents have been installed and communication between the hosts are established in the cluster, the very next thing that takes place is an election to determine who is the master. If, at some point the master fails, the re-election is done among the remaining hosts again using the agents for communication. Let's take a look at some of the master role details, when a host is established in the master role it is given the responsibility of overseeing the HA cluster. The master is responsible for the state of the slaves and determines if any of 'em have failed.
The master also maintains a list of protected VM's and is responsible for overseeing the restarting of failed VM's on other hosts in the cluster in case of a failure. The master keeps the list of protected VM's and another list within the cluster data stores. The list on the data store is used in case, you know, the master host fails. A restart engine is also used in HA. The master determines on which host the VM is placed by using this placement engine.
Last, but not least, the master host is responsible for reporting to vCenter Server for management and the health of the cluster. In the host slave role, hosts are required to monitor the protected VM's on that host. The slave host also communicates with the master host and to a small extent, the other slave hosts in the cluster. This communication ensures that a slave host is still active and its VM's are protected. It also ensures that in case of a master host failure, that communication can take place between other hosts within the cluster and a new election can take place.
Both host roles have a responsibility to run protected VM's and monitor them in case of a failure. Now, all hosts within an HA cluster are responsible for communicating to and from the data stores and the default gateway to ensure that there has been no loss of connectivity. It's one of the ways HA detects a failure. This ensures the overall integrity of the cluster and is used to verify that a host within the cluster is actually down or isolated. So let's talk for a moment on how this actually takes place.
First, high availability is configured, once that's done, all the agents are installed within hosts, within the cluster. Once the agents are up and communicating with each other, then an election is held to determine who is the master. Once the master has been elected, we create a host list of all the protected virtual machines and monitoring begins.
Note: This course will also help you prepare for the Configure and Administer vSphere Availability Solutions domain of the VMware Certified Professional – Data Center Virtualization exam. View the exam blueprint at https://mylearn.vmware.com/mgrReg/plan.cfm?plan=64180&ui=www_cert.
- How vSphere High Availability works
- The basics of clusters
- Understanding failure types and failure response
- Monitoring HA virtual machines and appliances
- Using heartbeats
- Creating and configuring clusters
- Configuring admission control
- Best practices: Networking, interoperability, and cluster monitoring