what is VMware Fault Tolerance

VMware HA protects against unplanned physical server failure by providing a way to automatically
restart virtual machines upon physical host failure. This need to restart a virtual machine
in the event of a physical host failure means that some downtime—generally less than three
minutes—is incurred. VMware FT goes even further and eliminates any downtime in the event of
a physical host failure

Fault tolarance uses vLockstep technology

VMware FT maintains a mirrored secondary VM on a separate physical host that is kept in lockstep with the primary VM

Everything that occurs on the primary (protected) VM also occurs simultaneously on the secondary (mirrored) VM, so that if the physical host on which the primary VM is running fails, the secondary VM can
immediately step in and take over without any loss of connectivity. VMware FT will also automatically
re-create the secondary (mirrored) VM on another host if the physical host on which the
secondary VM is running fails

If, the hosts running both the primary and secondary VMs failed then
VMware HA will reboot the primary VM on another available server, and VMware
FT will automatically create a new secondary VM. Again, this ensures protection for the primary
VM at all times.
VMware FT can work in conjunction with VMotion, but it cannot work with DRS, so DRS must
be manually disabled on VMs that are protected with VMware FT.