For filesystems, obviously the sync(2) family need to force data
all the way to non-volatile memory.  That's the whole purpose of
the calls.

For non-filesystem things (e.g. databases using raw partitions)
things aren't quite as simple.  Maybe add an argument to write(2)?
An ioctl to force the device into non-caching mode?  Either way
things are ugly and assorted nasty issues come up.

As far as disks that cache writes but don't give you a way to
force data to the platters, these would appear to be useless
for any data one cares about.  Maybe useful for swap?

-Dave