sync.8: Document that the "sync dance" is not a thing

People still believe that it is essential to run sync(8) a couple of
times before a reboot/halt. Document that this has not been necessary
for a long time now.

Reviewed by:	imp, bcr, Pau Amma <pauamma@gundo.com>
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D33233

(cherry picked from commit 177ba183076bd6fe6b29b1d657b954cca256506b)

1 files changed, 29 insertions, 1 deletions

diff --git a/bin/sync/sync.8 b/bin/sync/sync.8
index 732d813880e2..222e9edf59f9 100644
--- a/bin/sync/sync.8
+++ b/bin/sync/sync.8
@@ -28,7 +28,7 @@
 .\"
 .\"	@(#)sync.8	8.1 (Berkeley) 5/31/93
 .\"
-.Dd May 31, 1993
+.Dd April 25, 2024
 .Dt SYNC 8
 .Os
 .Sh NAME
@@ -71,3 +71,31 @@ A
 .Nm
 utility appeared in
 .At v4 .
+.Pp
+On systems older than
+.Bx 4.0 ,
+commands like
+.Xr reboot 8
+and
+.Xr halt 8
+were unavailable.
+The shutdown procedure involved running
+.Nm ,
+waiting for the lights to stop,
+and turning off the machine.
+.Pp
+Issuing three separate
+.Nm
+commands (one line each) was a placebo that would generally suffice in
+.At v7
+machines that were otherwise quiesced systems.
+It replaced the one-per-line
+.Nm
+as a substitute for waiting.
+.Pp
+.Bx 4.0
+introduced
+.Xr reboot 2
+and
+.Xr sync 2
+which rendered this trick obsolete.