io uring is similarly “fake async” because it’s also just a thread pool (just owned-operated by the kernel, not a runtime or library). I don’t think anyone has ever bothered to make FS code async, it’s already quite horrible as it is. Not in a mainstream kernel anyway. So you need to be on the FS-less happy path - properly aligned I/O to allocated blocks on some specific FSes - to get async disk IO, this automatically excludes all extending writes and metadata ops. Same for Windows disk IO with IOCPs.

>I don’t think anyone has ever bothered to make FS code async, it’s already quite horrible as it is. Not in a mainstream kernel anyway. [...] Same for Windows disk IO with IOCPs.

IOCP merely allows IO completion to happen on worker threads in a threadpool rather than the particular thread that initiated the IO request. Many concepts in the NT kernel come from a long line of production kernels in mainstream DEC operating systems going back to RSX-11M and VMS. That entire lineage of operating systems (culminating in NT) all have true asynchronous IO using IO Request Packets (IRPs) within the kernel to initiate/queue IO requests and immediately return. There are individual cases within NTFS where blocking will occur but those are special cases rather than the rule as the kernel IO system in general is entirely async.