Why overlap I/O and processing? clutches...

[utzoo!decvax!ittvax!swatt]

For about 95+% of the cases, synchronous I/O is the method of choice;
UNIX makes this commendably easy.  For the remaining uses, it is a
bloody pain and you spend infinite energy in various kludges to get
around it.  I disagree there is anything terribly ugly or clumsy about
anynchronous I/O; I've used it under RSX-11.  If you approach it with
coroutines, it is not much more complex than synchronous I/O.

We have a DEC TU78 and using George Goble's "dbuf" program (a double-
buffered version of "dd), I can copy /dev/rhp1g (141545 1024-byte
blocks) to tape in 7 minutes flat (VAX780).  However, to "dump" two
filesystems of this size, plus one root partition takes over 4 hours
elapsed time in single-user mode.  Than could be cut at least in half
with overlaped disk and tape I/O.

Programs like "dump" and "tar" would be significantly faster with
asynchronous I/O.  Programs like "cu", "tip", and communications tasks
in general would be infinitely cleaner as well.

	- Alan S. Watt