The clever trick here is that they'll pre-copy most of the memory without bothering to do it consistently, but mark pages that the source had written to as "dirty". The network cutover is stop-the-world, but VMware doesn't copy the dirty pages during the stop. Instead, it simply treats them as "swapped to pagefile", where the pagefile is actually the source machine memory. When computation resumes at the target, the source is used to page memory back in on-demand. This allows very fast cutovers.

> VM is stunned for a millisecond

This conjures up hilarious mental imagery, thanks

You just bop it on the head, and move it to the new machine quickly. By the time the VM comes to it won't even realize that it is in a new home.

Up to 500ms per your source, depending on how much churn there is in the memory from the source system.

Very cool.

That is really interesting I didn't realize it was so fast. Thanks for the post I will give it a read!