6.5.1

Master and Subordinate Hosts
When you add a host to a vSphere HA cluster, an agent is uploaded to the host and congured 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 (that are not in standby, 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 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.
The master host in a cluster has several responsibilities:
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Monitoring the state of subordinate hosts. If a subordinate host fails or becomes unreachable, the master
host identies which virtual machines must 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 takes place.
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Managing the lists of cluster hosts and protected virtual machines.
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Acting as the vCenter Server management interface to the cluster and reporting the cluster health state.
The subordinate 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 subordinate 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 o 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.
N If you disconnect a host from a cluster, the virtual machines registered to that host are unprotected
by vSphere HA.
Host Failure Types
The master host of a VMware vSphere
®
High Availability cluster is responsible for detecting the failure of
subordinate 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 subordinate hosts in the cluster. This communication happens
through the exchange of network heartbeats every second. When the master host stops receiving these
heartbeats from a subordinate host, it checks for host liveness before declaring the host failed. The liveness
check that the master host performs is to determine whether the subordinate host is exchanging heartbeats
with one of the datastores. See “Datastore Heartbeating,” on page 17. Also, the master host checks whether
the host responds to ICMP pings sent to its management IP addresses.
vSphere Availability
12 VMware, Inc.