On Sat, Jan 30, 2021 at 3:32 PM Larry McVoy <lm@mcvoy.com> wrote:

On Sun, Jan 31, 2021 at 09:27:10AM +1100, Dave Horsfall wrote:
> 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.

I was taught the exact same thing and for the same reasons.

Yes. There's no actual code in the System V or earlier kernels to block any of the sync calls. At least none I could find in examining all the 'leaked sources' I could find on the internet.

sync just schedules I/O. There's nothing in the system call that waits for anything: it just goes through the list of all non-busy buffers calling the driver's strategy routine and marking the buffer as busy.

Later systems, like SunOS did have a wait, but that was a later addition, done a half a dozen different ways. BSD had one that waited for a while, but gave up after some tens of seconds...

Warner