ASYNCH I/O

Ran across a statment that multiple database writters are useless if you are using
Asynch I/O. Does this statement apply to OS Asynch I/O, or the Oracle simulated
Asynch I/O created with USE_ASYNCH_IO = true, or is it true at all?

Incidentally, can someone shed some light on what this does specifically. In other words,
if you are not using Asynch I/O, then database blocks have to be written out in order?
As opposed to whatever the OS finds convenient by disk drive? Or what?

• Dc.