I don't think it's off-topic, and I think this answer is wrong.
On most Linux installations, update is the same as bdflush. bdflush used
to invoke the kernel flush mechanism, essentially providing a thread.
Now that there is the kflushd, the question is valid whether a user process
still needs to initiate flushing. And how does kflushd relate to what
update does on other systems?
Thanks,
Martin
P.S. I'm not sure about the answer, so I don't give one.