VMware even has a vSphere Fault Tolerance product that creates a "live shadow instance" of a VM that mirrors the primary virtual machine (with up to 4 vCPUs). So you can do a quick failover in case of an "immediate planned" failover case, but apparently even when the primary DB goes down. I guess this might work when some external system (like a storage array) goes down in the primary, you can just switch to the other VM (with latest memory/CPU state) and replay that I/O there and keep going... But if there's a hard crash of the primary, if it actually does work, then they must be doing lots of reasoning about internal state change ordering & external device side-effect (somewhat like Antithesis, but for a different purpose). Back in the day, they supported only uniprocessor VMs (with something called vLockstep) and later up to 4 vCPUs with something called Fast Checkpointing.
I've always wanted to test this out for fun, by now 15 years have gone by and I've never got to it...
Live migration had some very cool demos. They would have an intensive workload such as a game playing and cause a crash and the VM would resume with 0 buffering.
