Clustering in VMWare is based upon customer requirements.

Cluster-in-a-Box: Both the Nodes in same Physical Hosts, this type of configuration is suitable in case there is possibility of data crashes or administrative errors, but there is no cover if ESX host fails on hardware front.

Cluster-across-Boxes: Both the nodes are placed on seperate ESX host, and this takes of ESX host's hardware failure.

Physical-to-Virtual Cluster: Here Node A is actually physical box and Node B is Virtual Machine in ESX host, acting as standby host.

VMWARE HA solutions has some advantages which not very obvious. But we should any case apply VM HA for one simple reason, if the ESX host fails, all VM’s at least get started at other host. You don’t have to manually do that. Downtime will be Non-Zero

VMHA and VC 2.0 deals only with Host failures, for VM's (Node failure) you monitor Heart Beat using Alarm

• Each host must be able to poweron VM's i.e. Each host must have access to VM's files, in other words all VMotion requirements are met.
• ESX server is reachable when you type it's fully qualified domain name

For VMHA heartbeats it is recommended to set

• Two service console port on different virtual switch
• One service console with NIC teaming enable at virtual switch level

VMHA is fully integrated with DRS, which means when your host fails and all VM’s are moved to different hosts, DRS takes care of resource management. VMHA is reactive solution, which means it will act only when one or more host fails but VMDRS is proactive solution, it is always best to implement both VMHA & VMDRS

Failover capacity: When you enable cluster, two important configurations you need to do and they are again dependant upon client's requirement.

1. Number of host allowed failures allowed

   Maximum is 04 and Minimum is 01. This configuration help HA to determine if there are enough resources to power on VM in the cluster. But it is we who decided how much redudant capacity to be made available.

2. Admission Control
   1. Do not power ON VM if they violate availability constraints (Selected as default option)
   2. Allow virtual machines to be powered on if they violate availability constraints

Depending upon adminission control option you select, VM will be either powered ON or NOT. These values help VMHA to balance and calculated enough resource across hosts in case there is any host failures. Current failover capacity under Cluster's summary tab informs how many hosts are available at that time to hold the VM's

We only need to provide number of host, rest like resources required to power on VM’s across these host or only 1 host is alive, decision like this is taken by VMHA. If resources are not enough VMHA wouldn’t all VM’s to be powered ON(default option). You can force VMHA to start VM’s(when you like the constraints to be voilated), in this case Cluster will show RED sign, which means failover might not be guaranteed. It is not recommended that you work with red clusters. Also if you have 3 hosts and 2 fails cluster will turn RED.

So when you enable VMHA, you should design in such a way that hosts in ESX will be able to handle additional VM's without any over utilization of resource.

For example: Two ESX Host having equal capacity handling 50 VM's each. We should design in way that each Host should be able to handle 100 VM's.