tmpfs will use the swap area in low memory situations, it doesn't 'map' to it. I'm a little confused about how you are wasting RAM. tmpfs lives entirely in th

tmpfs will use the swap area in low memory situations, it doesn't 'map' to it. I'm a little confused about how you are wasting RAM. tmpfs lives entirely in the page cache and mmaped files are always page cache resident as well so they always map to the same page cache. It is possible that file system overhead will be less as tmpfs is simpler but maybe you should simply change the mongodb source code to mmap the database anonymously without a file backing in the first place.