Master and Slave Hosts

Master and Slave Hosts in ESXi 

When you 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 slave host.

When vSphere HA is enabled for a cluster, all active hosts (those not in standby or maintenance mode, or not disconnected) 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 exists per cluster and all other hosts are slave hosts. If the master host fails, is shut down, or is removed from the cluster a new election is held.

The master host in a cluster has a number of responsibilities:

■	Monitoring the state of slave hosts. If a slave host fails or becomes unreachable, the master host identifies which virtual machines need to be restarted.
■	Monitoring the power state of all protected virtual machines. If one virtual machine fails, the master host ensures that it is restarted. Using a local placement engine, the master host also determines where the restart should be done.
■	Managing the lists of cluster hosts and protected virtual machines.
■	Acting as vCenter Server management interface to the cluster and reporting the cluster health state.

The slave hosts primarily contribute to the cluster by running virtual machines locally, monitoring their runtime states, and reporting state updates to the master host. A master host can also run and monitor virtual machines. Both slave hosts and master hosts implement the VM and Application Monitoring features.

One of the functions performed by the master host is virtual machine protection. When a virtual machine is protected, vSphere HA guarantees that it attempts to power it back on after a failure. A master host commits to protecting a virtual machine when it observes that the power state of the virtual machine changes from powered off to powered on in response to a user action. If a failover occurs, the master host must restart the virtual machines that are protected and for which it is responsible. This responsibility is assigned to the master host that has exclusively locked a system-defined file on the datastore that contains a virtual machine's configuration file.

vCenter Server reports whether a host is a master host or a slave host using a vSphere HA host state. This state is reported on the host's Summary tab in the vSphere Client and in the Host List view for a cluster or datacenter, if the HA State column has been enabled. An HA state of "Running (Master)" indicates the host is serving as a vSphere HA master host. A state of "Connected (Slave)" indicates the host is serving as a vSphere HA slave host. Several other states are provided to indicate when an election is underway or an error condition has occurred. The host'sSummary tab provides a link next to the vSphere HA state of the host that explains the current state.