Comment by johncolanduoni

Kernel-mediated async disk IO is still a mess on all major platforms except for newer Linux kernels with io_uring. There's no way to call the APIs in a way that won't block sometimes, and to even have a snowball's chance in hell of not blocking requires giving up on the kernel's disk cache.

Also you're probably going to want to do multithreaded decompression anyway, and it'll be more efficient if you have the threads completing the reads do the decompression themselves. So in any case you probably want multiple threads handling the completion events.