The linux aio syscalls (io_submit, ...) work well on some filesystems (xfs, ext4, ...) but block on others (btrfs). It limits your file system choices but allows for a single threaded, concurrent web-server. This is what I ended up using for my hobby web-server wwwee [1] as low memory usage and good performance on a single core was an important constraint. It is sort of an opinionated design though.

1: https://github.com/bwindels/wwwee