gistfile1.txt

buffer = new Buffer(1024 * 1024);

buffer.readFrom(socketA);
buffer.writeTo(socketB);

... is this sys.pump, with a one megabyte buffer?

Make write throw an error when it would block (making sure people handle writes in the wouldblock case, not blocking nor throwing data away)

Make an enlargeable buffer for cases where you want the old growing-memory behavior of write()

And make a non-buffering case. 

Instead of either not having a buffer, or disabling it in some cases ... why not let one have control of it?