While porting some new IO code to lots of OSes I noticed in passing that there is now a way to do synchronous fdatasync() on Windows. This mechanism doesn't have an async variant, which is what I was actually looking for (which turns out to doable with bleeding edge IoRings, more on that later), but I figured this might be useful anyway. I see that at least one other open source database has discovered it and seen speedups. Like some other file API improvements discussed recently, it's Windows 10+ and NTFS only. I tried out a quick POC patch and it runs a bit faster than fsync(), as expected. I'm not sure if it's worth bothering with or not given the other options, but figured it was worth sharing.
While testing that I also couldn't resist adding an extra output line to pg_test_fsync to run open_datasync in buffered I/O mode, like PostgreSQL actually does in real life. I guess I should really change it to duplicate less code, though...