Introduction to VMware vSphere Availability

Today we are going to talk about the VMware vSphere availability. Before we start with the VMware vSphere availability let's have a quick look what is VMware vSphere ?

VMware vSphere
VMware vSphere is the world's leading virtualization platform. As virtualization & the  vSphere platform have continued to grow, organizations have faced new challenges. vCenter Server unifies resources from individual hosts so that those resources can be  shared among virtual machines in the entire datacenter. 

It accomplishes this by  managing the assignment of virtual machines to the hosts and the assignment of  resources to the virtual machines within a given host based on the policies that the  system administrator sets.

What is vSphere Availability?
vSphere Availability provides high availability for virtual machines by pooling the virtual  machines and the hosts they reside on into a cluster. Hosts in the cluster are monitored  and in the event of a failure, the virtual machines on a failed host are restarted on  alternate hosts.

When you create a vSphere Availability cluster, a single host is automatically elected as  the master host. The master host communicates with vCenter Server and monitors the  state of all protected virtual machines and of the slave hosts.

Different types of host  failures are possible, and the master host must detect and appropriately deal with the  failure. The master host must distinguish between a failed host and one that is in a  network partition or that has become network isolated. 

The master host uses network and datastore heart beating to determine the type of failure. Also note that vSphere availability is a host function which means there is not a dependency on vCenter in order to effectively fail-over VMs to other hosts in the cluster.

Fig 1.1- VMware vSphere Master and Slave Role

VMware vSphere Master Role
When we add a host to a vSphere HA cluster, an agent is uploaded to the host and configured to communicate with other agents in the cluster. Each host in the cluster functions as a master host or a subordinate host.

When vSphere HA is enabled for a cluster, all active hosts participate in an election to choose the cluster's master host. The host that mounts the

greatest number of datastores has an advantage in the election. Only one master host typically exists per cluster and all other hosts are subordinate hosts. If the master host fails, is shut down or put in standby mode, or is removed from the cluster a new election is held.

VMware vSphere Slave Role
A slave monitors the runtime state of its locally running VMs and forwards any significant state changes to the master. 

It implements vSphere HA feature that do not require central co-ordination. It also monitor the health of the master. If master fails, it participates in the election process for a new master. It also maintains the list of powered on VMs.