From mboxrd@z Thu Jan  1 00:00:00 1970
From: jnc@mercury.lcs.mit.edu (Noel Chiappa)
Date: Wed, 16 Jul 2014 23:55:59 -0400 (EDT)
Subject: [TUHS] shutdown for pre-v7 unix
Message-ID: <20140717035559.0375D18C0BE@mercury.lcs.mit.edu>

    > From: Norman Wilson <norman at oclsc.org>

    > I believe that when sync(2) returned, all unflushed I/O had been queued
    > to the device driver, but not necessarily finished

Yes. I have just looked at update() (the internal version of 'sync') again,
and it does three things: writes out super-blocks, any modified inodes, and
(finally) any cached disk blocks (in that order).

In all three cases, the code calls (either directly or indirectly) bwrite(),
the exact operation of which (wait for completion, or merely schedule the
operation) on any given buffer depends on the flag bits on that buffer.

At least one of the cases (the third), it sets the 'ASYNC' bit on the buffer,
i.e. it doesn't wait for the I/O to complete, merely schedules it. For the
first two, though, it looks like it probably waits.

    > so the second sync was just a time-filling no-op. If all the disks were
    > in view, it probably sufficed just to watch them until all the lights
    > ... had stopped blinking.

Yes. If the system is single-user, and you say 'sync', if you wait a bit for
the I/O to complete, any later syncs won't actually do anything.

I don't know of any programmatic way to make sure that all the disk I/O has
completed (although obviously one could be written); even the 'unmount' call
doesn't check to make sure all the I/O is completed (it just calls update()).
Watching the lights was as good as anything.

    > I usually typed sync three or four times myself.

I usually just type it once, wait a moment, and then halt the machine. I've
never experienced disk corruption from so doing.

With modern ginormous disk caches, you might have to wait more than a moment,
but we're talking older machines here...

	Noel