HA clusters/how do they know when to kick in

can I ask for vmware HA, how do other hosts joined to a cluster know when one of the other hosts has died? And what formula comes into play to determine which of the other hosts takes over the guest machines, if say you have 5 hosts joined to a cluster, how do you know which of the other 4 would kick in were 1 host to die/fail. Is there anything you need to "do"  or "configure" on the other hosts to ensure they are aware when a host fails/would be made aware if another host fais?
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pma111Asked:
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Andrew Hancock (VMware vExpert / EE MVE^2)Connect With a Mentor VMware and Virtualization ConsultantCommented:
At least two ESXi Host Servers, added to a Cluster with VMware HA (High Availability) enabled. HA Agents are installed on both ESXi servers, and vCenter Server is used to configure VMware HA, but does not take part or control the HA Agents - vCenters role is only to configure VMware.

One server will be the Master, and the other will be the slave, this can be seen in the Host Summary.

VMs hosted on both ESXi Host Servers, become vSphere HA Protecti-ed - and there should be a green tick, which states Protected. This can be confirmed for the VM, under VM Summary.

The HA Agents on the hosts...

vSphere HA State - Master - A server which is elected as the master. This agent monitors the VMs on this server, and other operational Hosts, and it WILL attempt to restart VMs on failure.

vSphere HA State - Slave - This server is connected to the Master Agent, via the Management Network. The vSphere HA Protected VMs on this server are monitored by one or more vSphere HA Master Agents, and the agent will attempt to restart VMs after a failure.

vSphere HA Protected VM - vSphere will attempt to restart the VM after a supported failure of the VM.

VM is HA Protected on the following conditions:-

VM is in a vSphere HA enabled cluster.
VM is powered on successfully after a successful user power on.
vSphere HA has recorded that the power state is ON.

When an ESXi Host Server Fails (which is part of a VMware HA Cluster), all the Virtual Machines, which are hosted on that Host, also go down, e.g. fail.

A Host Failure could be:-

1. Pink/Purple Screen of Death - caused by memory fault.
2. Pink/Purple Screen of Death  - cause by cpu fault.
3. Power supply failure (if only a single power supply)

A Host Manual Shutdown, reboot, restart is not considered a host failure. Because it's a controlled shutdown.

So we have a Host which has failed, and ALL the VMs it was hosting are now DOWN!

Is there anything you need to "do"  or "configure" on the other hosts to ensure they are aware when a host fails/would be made aware if another host fais?

Nothing than ensure, it has been configured and tested correctly.       

The Master Host Decides where to restart virtual machines.
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