[CentOS] Why the command 'service ntpd stop' cause the time reversed?

[John R Pierce]

On 2/11/2015 5:53 PM, David chen wrote:
> A shell script is deployed to synchronize time, the script is invoked hourly by crontab, and its content is as follows:
> #!/bin/bash
> service ntpd stop
> ntpdate 192.168.0.191 #it's a valid ntpd server in LAN
> service ntpd start
> chkconfig ntpd on



huh?   ntpd synchronizes time continuously with the given reference 
servers.  why in heck would you stop it to set the clock then restart it ?!?

$ uptime
   6:37pm  up 422 days, 19:38,  1 user,  load average: 0.00, 0.04, 0.06
$ ntptrace
localhost: stratum 3, offset 0.000015, synch distance 0.05679
enigma.wiredgoats.com: stratum 2, offset 0.000249, synch distance 0.04080
lantime.sonic.net: stratum 1, offset -0.000116, synch distance 0.00044, 
refid 'PPS'

ntpd hasn't been stopped or restarted since that server last booted over 
a year ago...  my time is within 0.000116 seconds, thats 116 
microseconds, of the reference server.    ntpd works by incrementally 
fine tuning the speed the system clock runs at, using phase lock loop 
techniques, so the clock remains monotonic (always increasing) and 
stable.    running a hard time set program can cause the clock to go 
BACKWARDS if its running fast.

if ntpd isn't working for you properly, you should investigate what ntp 
servers you're using in /etc/ntp.conf



-- 
john r pierce                                      37N 122W
somewhere on the middle of the left coast