6.0.1

Master and Slave Hosts
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 typically exists per
cluster and all other hosts are slave 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.
The master host in a cluster has a number of responsibilities:
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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.
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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.
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Managing the lists of cluster hosts and protected virtual machines.
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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 to orchestrate restarts of protected virtual machines. A
virtual machine is protected by a master host after vCenter Server observes that the virtual machine's power
state has changed from powered off to powered on in response to a user action. The master host persists the
list of protected virtual machines in the cluster's datastores. A newly elected master host uses this
information to determine which virtual machines to protect.
NOTE If you disconnect a host from a cluster, all of the virtual machines registered to that host are
unprotected by vSphere HA.
Host Failure Types and Detection
The master host of a vSphere HA cluster is responsible for detecting the failure of slave hosts. Depending on
the type of failure detected, the virtual machines running on the hosts might need to be failed over.
In a vSphere HA cluster, three types of host failure are detected:
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Failure- A host stops functioning.
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Isolation- A host becomes network isolated.
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Partition- A host loses network connectivity with the master host.
The master host monitors the liveness of the slave hosts in the cluster. This communication is done through
the exchange of network heartbeats every second. When the master host stops receiving these heartbeats
from a slave host, it checks for host liveness before declaring the host to have failed. The liveness check that
the master host performs is to determine whether the slave host is exchanging heartbeats with one of the
datastores. See “Datastore Heartbeating,” on page 21. Also, the master host checks whether the host
responds to ICMP pings sent to its management IP addresses.
vSphere Availability
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