In order to power on FT VM, we have to enable vSPhere HA on cluster as well. If say primary OS came across BSOD, even secondary VM also will be BSOD'ed. At this time, vSphere HA can detect the OS failure(we need to enable VM monitoring feature of vSphere HA) and it will restart the primary VM on the same host & it will recreate the secondary VM on the other host. In this case, there will be minimal downtime as VM is getting restarted on failure.
I hope above explanation answered your question
What happens during a failure?
When a host running the Primary virtual machine fails, a transparent failover occurs to the corresponding Secondary virtual machine. During this failover, there is no data loss or noticeable service interruption. In addition, VMware HA automatically restores redundancy by restarting a new Secondary virtual machine on another host. Similarly, if the host running the Secondary virtual machine fails, VMware HA starts a new Secondary virtual machine on a different host. In either case there is no noticeable outage.
Let me know if you have any other question
Thanks for the reply.
But I want to know how FT will work if my primary FT VM is corrupted or not available but the ESXi host which has the primary FT VM is running fine?
Does even in that case my secondary FT VM will resume job?
In fact, same explanation applies here as well.
Say, your FT VM is corrupted, your secondary VM can not resume the job. Even secondary VM also will be corrupted as well. FT will not handle this situation. In this case, vSphere HA will come in picture, as vSphere HA is not getting VM heartbeat from primary FT VM (as VM is corrupted), it will restart the VM on the same host (as same host is running fine) and as FT is enabled on the restarted VM, secondary VM also will be recreated on another host. As primary FT VM is restarted, VM will undergo minimal downtime.
Note that FT will handle the case where host itself is failed. Hence when host fails, secondary VM will resume the job without any downtime but if primary VM fails, vSphere HA will handle this case with minimal downtime.
Learn how vSPhere HA VM monitoring works :: vSphere HA VM Monitoring - Back to Basics | VMware vSphere Blog - VMware Blogs
Let me know if you have any doubts.