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Table Of Contents
When you create a vSphere HA 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
heartbeating to determine the type of failure.
Sphere HA Clusters (http://link.brightcove.com/services/player/bcpid2296383276001?
bctid=ref:vSphereHAClusters)
Master and Subordinate 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
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:
n
Monitoring the state of subordinate hosts. If a subordinate host fails or becomes unreachable, the
master host identifies which virtual machines must be restarted.
n
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 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, the virtual machines registered to that host are unprotected
by vSphere HA.
vSphere Availability
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