Basically he's just saying that, hey, if you want to do a caching proxy server - doing it the way squid did it was all fine and good, but it actually doesn't ta

Basically he's just saying that, hey, if you want to do a caching proxy server - doing it the way squid did it was all fine and good, but it actually doesn't take into account how VM paging will interact with things - so you end up with potentially duplicated data in pagefiles and on disk. By making use of the mmap() method, he just ensures that when data he wants stored on disk somehow needs to be paged out of ram, it won't be duplicated.