I don't think that's the reason actually, there is a big spike and then all the other COWs are ok, also copying 2MB is no way near to the 300 ms I was observing. Not sure if for some reason most pages where copied, or it was some other issue at all. It could be the code that fragments the huge page into multiple pages maybe? MOV -> fault -> split of page -> COW of a single 4k page.