If the option to turn on Fault Tolerance is available, this task still must be validated and can fail if certain requirements are not met.
Several validation checks are performed on a virtual machine before Fault Tolerance can be turned on.
SSL certificate checking must be enabled in the vCenter Server settings.
The host must be in a vSphere HA cluster or a mixed vSphere HA and DRS cluster.
The host must have ESXi 6.x or greater installed (ESX/ESXi 4.x or greater for legacy FT).
The virtual machine must not have snapshots.
The virtual machine must not be a template.
The virtual machine must not have vSphere HA disabled.
The virtual machine must not have a video device with 3D enabled.
Checks for Powered-On Virtual Machines
Several additional validation checks are performed for powered-on virtual machines (or those that are in the process of being powered on).
The BIOS of the hosts where the fault tolerant virtual machines reside must have Hardware Virtualization (HV) enabled.
The host that supports the Primary VM must have a processor that supports Fault Tolerance.
Your hardware should be certified as compatible with Fault Tolerance. To confirm that it is, use the VMware Compatibility Guide at http://www.vmware.com/resources/compatibility/search.php and select Search by Fault Tolerant Compatible Sets.
The configuration of the virtual machine must be valid for use with Fault Tolerance (for example, it must not contain any unsupported devices).
Secondary VM Placement
When your effort to turn on Fault Tolerance for a virtual machine passes the validation checks, the Secondary VM is created. The placement and immediate status of the Secondary VM depends upon whether the Primary VM was powered-on or powered-off when you turned on Fault Tolerance.
If the Primary VM is powered on:
The entire state of the Primary VM is copied and the Secondary VM is created, placed on a separate compatible host, and powered on if it passes admission control.
The Fault Tolerance Status displayed for the virtual machine is Protected.
If the Primary VM is powered off:
The Secondary VM is immediately created and registered to a host in the cluster (it might be re-registered to a more appropriate host when it is powered on.)
The Secondary VM is not powered on until after the Primary VM is powered on.
The Fault Tolerance Status displayed for the virtual machine is Not Protected, VM not Running.
When you attempt to power on the Primary VM after Fault Tolerance has been turned on, the additional validation checks listed above are performed.
After these checks are passed, the Primary and Secondary VMs are powered on and placed on separate, compatible hosts. The virtual machine's Fault Tolerance Status is tagged as Protected.