[Sesse__]

mmap is great for rapid prototyping. For anything I/O-heavy, it's a mess. You have zero control over how large your I/Os are (you're very much at the mercy of heuristics that are optimized for loading executables), readahead is spotty at best (practical madvise implementation is a mess), async I/O doesn't exist, you can't interleave compression in the page cache, there's no way of handling errors (I/O error = SIGBUS/SIGSEGV), and write ordering is largely inaccessible. Also, you get issues such as page table overhead for very large files, and address space limitations for 32-bit systems.

In short, it's a solution that looks so enticing at first, but rapidly costs much more than it's worth. As systems grow more complex, they almost inevitably have to throw out mmap.