> fsync does not work because it returns when everything (in the same thread? or whatever) has been written to disk, and doesn't let you wait for a particular block to have been written.

I believe fsync is per-file (or, really, per-file descriptor). I wasn't familiar with this specific problem, but I believe the problem is that for fsync to work, it has to issue commands to the hardware to flush hardware buffers to disk. Apparently that isn't well targeted in Linux, and has the effect that calling fsync on a file can slow down other threads and processes that happen to be writing to that same disk, even if they're writing to a different file.