sync sync, was Re: Problem with crash post-install

["Thomas Schmitt" <scdbackup@gmx.net>]

Hi,

tomas@tuxteam.de wrote:
> > While I've read about issuing sync *twice* with the explanation
> > that sysadmins are a supersticious bunch

Polyna-Maude Racicot-Summerside wrote:
> I'm really asking why do it 3 time.
> In case it came back to the prompt *before* finishing to do it's job ?

I dimly remember that it was part of the shutdown procedure of one of
the earliest Unix machines which i met. IBM RT, Apollo DN3000, microVAX ... ?
The shutdown spell was something like

  sync ; sync ; halt

Googling "sync halt" leads me to an interesting theory at
  https://bsdimp.blogspot.com/2020/07/when-unix-learned-to-reboot2.html

According to section "That sync; sync; sync Thing..." two bugs of early
systems' sync existed (one explored by the blog's author, one as rumor):

1: The sync system call returned without waiting.
   Workaround:
   Keep the human operator busy while the machine is still not done with
   flushing buffers. E.g. prescribe to type three lines of "sync" before
   typing "halt".

2: Only the first sync call returned early, whereas the second sync
   properly blocked until flushing was done.
   That seems to be the origin of my memory.


Have a nice day :)

Thomas