You can use vSphere Fault Tolerance (FT) for most mission critical virtual machines. FT provides continuous availability for such a virtual machine by creating and maintaining another VM that is identical and continuously available to replace it in the event of a failover situation.
The protected virtual machine is called the Primary VM. The duplicate virtual machine, the Secondary VM, is created and runs on another host. The Secondary VM's execution is identical to that of the Primary VM and it can take over at any point without interruption, thereby providing fault tolerant protection.
The Primary and Secondary VMs continuously monitor the status of one another to ensure that Fault Tolerance is maintained. A transparent failover occurs if the host running the Primary VM fails, in which case the Secondary VM is immediately activated to replace the Primary VM. A new Secondary VM is started and Fault Tolerance redundancy is reestablished automatically. If the host running the Secondary VM fails, it is also immediately replaced. In either case, users experience no interruption in service and no loss of data.
A fault tolerant virtual machine and its secondary copy are not allowed to run on the same host. This restriction ensures that a host failure cannot result in the loss of both VMs.
You can also use VM-Host affinity rules to dictate which hosts designated virtual machines can run on. If you use these rules, be aware that for any Primary VM that is affected by such a rule, its associated Secondary VM is also affected by that rule. For more information about affinity rules, see the vSphere Resource Management documentation.
Fault Tolerance avoids "split-brain" situations, which can lead to two active copies of a virtual machine after recovery from a failure. Atomic file locking on shared storage is used to coordinate failover so that only one side continues running as the Primary VM and a new Secondary VM is respawned automatically.
vSphere Fault Tolerance can accommodate symmetric multiprocessor (SMP) virtual machines with up to four vCPUs. Earlier versions of vSphere used a different technology for Fault Tolerance (now known as legacy FT), with different requirements and characteristics (including a limitation of single vCPUs for legacy FT VMs). If compatibility with these earlier requirements is necessary, you can instead use legacy FT. However, this involves the setting of an advanced option for each VM. See Legacy Fault Tolerance for more information.