At Sun, 31 Jan 2021 09:27:10 +1100 (EST), Dave Horsfall <dave@horsfall.org> wrote:
Subject: Re: [TUHS] reboot(2) system call
>
> On Tue, 26 Jan 2021, Greg A. Woods wrote:
>
> > The lore I was told at the time was that you alwasy ran three and
> > that it didn't matter if they were all on the same line with
> > semicolons or not because of the very fact that the second one would
> > block.
>
> What I was taught was:
>
> % sync
> % sync
> % sync
>
> and never:
>
> % sync; sync; sync
>
> The theory was that by waiting for the shell prompt each time, it gave
> the buffer pool enough time to be flushed.
If waiting was the true reason, then any sane person would have put a
sleep in there instead so as to avoid any variance in typing (and
terminal) speed.
On at least a large number of old systems I've used either the first or
the second invocation did block and not return if there were still any
dirty blocks it made the sync() call. It was trivial to see that the
system was busy writing while one waited for the shell prompt to
re-appear if one could see the disk activity lights (or hear them) from
the console, as was usually easy to do on desktop systems.
Since many of those old systems I used were Xenix of one flavour or
another, perhaps it was only those that waited for sync I/O to complete.
Would be nice to know which one so I can go check. I've not seen leaked xenix code though, so it may be possible.
Warner