Varnish caches the rendered html and stores it in disk for os to cache like mongodb with mmap.

Wordpress super cache plugin generates .html files from the pages and saves it in the disk for the os to cache.

This is what I meant.

Varnish leaves it to the OS to decide what pages to keep in RAM and which to leave on disk; essentially to avoid the double-buffering problem and because the OS has a larger view of the total machine's requirements.

But with wp-supercache I already have that approximate architecture. Files are on-disk, Nginx selects them, the OS notices that some files are frequently accessed and silently caches them in RAM. Everyone wins.

And I don't need to add ESI directives to my themes to get Varnish to handle frequently-updated material correctly.