Being able to open files with io_uring is important because there is no other way to do it without an unpredictable delay. Some systems like Erlang end up using separate OS threads just to be able to open files without blocking the main interpreter thread.
That's not true. There is iosubmit on Linux which is used by some database systems. It's hard to use and requires IODIRECT, which means you can't use the kernel file buffer, writes and reads have to be aligned and a multiple of the alignment size (sector? page?)
AIO has been around forever and it gives a crappy way to do async i/o on disk files that are already open, but it doesn't give a way to asynchronously open a file. If you issue an open(2) call, your thread blocks until the open finishes, which can involve disk and network waits or whatever. To avoid the block, you have to put the open in another thread or process, or use io_uring, as far as I can tell.
It is terrible, but it's worth pointing out that there has been an API for real async file IO for a while. I believe it works with ext4 as well, although maybe there are edge cases. Iouring is a big improvement.
Threads are the simplest way but I think you can also use ancillary messages on unix domain sockets, to open the files in a separate process and then send the file descriptors across.
