> mmap absolutely copies the file contents into the kernel file system cache which is a buffer
Isn't this a bit misleading? mmaping a file doesn't cause the kernel to start loading the whole thing into RAM, it just sets things up for the kernel to later transparently load pages of it on demand, possibly with some prefetching.
"Completely unbuffered" is almost unattainable in practice, so I'm not sure that's a reasonable inference. About the best you can do in general is not do any buffering yourself, and usually explicitly bypass whatever buffering is going on at the next level of abstraction down. Ensuring you've cut out all buffering in the entire IO stack takes real effort.
Isn't this a bit misleading? mmaping a file doesn't cause the kernel to start loading the whole thing into RAM, it just sets things up for the kernel to later transparently load pages of it on demand, possibly with some prefetching.