Well, there's no POSIX async file I/O, but there's libaio for Linux's aio syscalls (io_submit and co.), which works. (MySQL/InnoDB uses it for example.)
Upon return from a non-blocking I/O call the request is either failed or succeeded. Upon return from an async I/O call the request might additionally be in progress thus the fiber is required to wait for its completion. This case has little practical difference from a thread pool. Except with a thread pool it is the worker thread that notifies the fiber scheduler on I/O completion. With async I/O it is the kernel that notifies the application and then the application (from a signal handler or from an auxiliary thread) should propagate the notification to the fiber scheduler.