> I don't believe POSIX has any restriction such as you describe - or if
> it does, and I don't know about it, then most UNIX file systems (if
> not most file systems on any platform) are not POSIX compliant.
>
I suspect that indeed there are two different issues here in that the
file mutex relates to updates
to the file, not passing the buffers through into the drive, which
indeed might be delayed.
Been using direct io too much recently. :-(