You can only enable Fault Tolerance on a virtual machine with a maximum of 64GB of memory.
Enabling Fault Tolerance on a virtual machine with more than 64GB memory can fail. Migrating a running fault tolerant virtual machine using vMotion also can fail if its memory is greater than 15GB or if memory is changing at a rate faster than vMotion can copy over the network.
This occurs if, due to the virtual machine’s memory size, there is not enough bandwidth to complete the vMotion switchover operation within the default timeout window (8 seconds).
To resolve this problem, before you enable Fault Tolerance, power off the virtual machine and increase its timeout window by adding the following line to the vmx file of the virtual machine:
ft.maxSwitchoverSeconds = "30"
where 30 is the timeout window in number in seconds. Enable Fault Tolerance and power the virtual machine back on. This solution should work except under conditions of very high network activity.
If you increase the timeout to 30 seconds, the fault tolerant virtual machine might become unresponsive for a longer period of time (up to 30 seconds) when enabling FT or when a new Secondary VM is created after a failover.