Several typical situations can benefit from the use of vSphere Fault Tolerance.
Fault Tolerance provides a higher level of business continuity than vSphere HA. When a Secondary VM is called upon to replace its Primary VM counterpart, the Secondary VM immediately takes over the Primary VM’s role with the entire state of the virtual machine preserved. Applications are already running, and data stored in memory does not need to be re-entered or reloaded. This differs from a failover provided by vSphere HA, which restarts the virtual machines affected by a failure.
This higher level of continuity and the added protection of state information and data informs the scenarios when you might want to deploy Fault Tolerance.
Applications that need to be available at all times, especially those that have long-lasting client connections that users want to maintain during hardware failure.
Custom applications that have no other way of doing clustering.
Cases where high availability might be provided through custom clustering solutions, which are too complicated to configure and maintain.
On-Demand Fault Tolerance
Another key use case for protecting a virtual machine with Fault Tolerance can be described as On-Demand Fault Tolerance. In this case, a virtual machine is adequately protected with vSphere HA during normal operation. During certain critical periods, you might want to enhance the protection of the virtual machine. For example, you might be executing a quarter-end report which, if interrupted, might delay the availability of mission critical information. With vSphere Fault Tolerance, you can protect this virtual machine prior to running this report and then turn off or disable Fault Tolerance after the report has been produced. You can use On-Demand Fault Tolerance to protect the virtual machine during a critical time period and return the resources to normal during non-critical operation.