Date:        Fri, 20 Jun 2003 13:26:26 -0400 (EDT)
    From:        "Greg A. Woods" <woods@weird.com>
    Message-ID:  <m19TPec-000B44C@proven.weird.com>


  | Remember that the original reason UNIX System admins were instructed to
  | type:
(3 sync's)
  | before halting the CPU was that it took long enough to type the second
  | two commands, and to get the prompt back each time, on a real Teletype
  | ASR33, or similar clunker of a console, that the first command would
  | have actually finished working in the background by the time you got
  | around to halting the CPU.

As I recall it, sync marked the super block busy, and wrote that out last.
A second sync would simply wait for the super block to become free, before
starting - so when the 2nd sync started (when its prompt came back) you knew
the first one was done.   And that could take markedly longer to happen than
the time to echo a few characters, even on an ASR33.   The third one was
mostly superstition (if 2 is good, 3 must be better) combined with a little
bit of "make sure the 2nd sync finished, in case anything else happened
while the first one was running".   The 2nd sync would usually finish very
quickly (you'd know that happened when the 3rd one started).

kre