Hello,
Thanks for the answer.
I have set the memory reservation in order to save the ~500GB swap file created... with all my big VM servers, I am saving TB of space with way.
I had the same idea about the Admission Control, but I disabled it, so this is not the problem.
With trial and errors, I found out 449GB was the maximum I could go to cold migrate a VM. I'll try this with vMotion next.
As you said, that must be some kind of internal reservations but I cannot see anything in VMware's documentations and Maximums...