VMware's HA feature just restarts the VM affected by a server crash on another ESX server. From the perspective of the VM it looks like a spontaneous reboot. This guarantees minimum downtime, but does not make the crash invisible to the application.
> I doubt that the vmware Fault Tolerance would work for any system that used random numbers [...]
Remember that all the HW excepting the CPU is emulated. Fault Tolerance works by replicating the external events (e.g. an incoming network packet) seen by the original VM at the same point in time on the backup VM. Usual sources of entropy will be equal to both machines.
Maybe someone could deliberately construct a program that runs differently in the original and the backup VM. The only thing I can think of right now is measuring time via a covert channel (the normal means like RDTSC are covered, of course) -- but this is too vague still to make into an "exploit".
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