[WGH_]

> Even today if you are running async IO in userspace (e.g. using epoll), it's not very obvious where something went wrong, because no task is seemingly blocked.

It doesn't apply to file IO, which is never non-blocking, and can't be made async with epoll. Epoll always considers files ready for any IO. And if the device is slow, the thread is blocked with dreaded "D" state.

[CodesInChaos]

The fundamental problem is that readiness based async IO and random access to not mix well. You'd need a way to poll readiness for different positions in the same file at the same time.

Completion based async (including io_uring on Linux or IO completion ports on Windows) doesn't suffer from this problem.