Information Technology Reference
In-Depth Information
while still providing all the same functionality from earlier versions of vSphere. FDM also offers
a few signii cant improvements over AAM:
FDM uses a master/slave architecture that does not rely on primary/secondary host
designations.
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FDM uses both the management network and storage devices for communication.
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FDM introduces support for IPv6.
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FDM addresses the issues of both network partition and network isolation.
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FDM uses the concept of an agent that runs on each ESXi host. This agent is separate and
decoupled from the vCenter management agents that vCenter uses to communicate with ESXi
hosts (this management agent is known as vpxa). This agent gets installed into the ESXi hosts at
/opt/vmware/fdm and stores its coni guration i les at /etc/opt/vmware/fdm (note that you must
enable SSH and the ESXi shell in order to view these directories).
Although FDM is markedly different from AAM, as an end user you will notice very little
difference in how vSphere HA operates. Therefore, we generally won't refer to FDM directly, but
instead we'll refer to vSphere HA. We did want to bring it to your attention, though, so that you
are aware of the underlying differences.
When vSphere HA is enabled, the vSphere HA agents participate in an election to pick a
vSphere HA master. The vSphere HA master is responsible for the following key tasks within
a vSphere HA-enabled cluster:
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Monitors slave hosts and will restart VMs in the event of a slave host failure.
Monitors the power state of all protected VMs. If a protected VM fails, the vSphere HA
master will restart the VM.
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Manages the list of hosts that are members of the cluster and manages the process of add-
ing and removing hosts from the cluster.
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Manages the list of protected VMs. It updates this list after each user-initiated power-on or
power-off operation. These updates are at the request of vCenter Server, which requests the
master to protect or unprotect VMs.
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Caches the cluster coni guration. The master notii es and informs slave hosts of changes in
the cluster coni guration.
The vSphere HA master host sends heartbeat messages to the slave hosts so that the slave
hosts know the master is alive.
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Reports state information to vCenter Server. vCenter Server typically communicates only
with the master.
As you can see, the role of the vSphere HA master is quite important. For this reason, if the
existing master fails, a new vSphere HA master is automatically elected. The new master will
then take over the responsibilities listed here, including communication with vCenter Server.
