[CentOS] Updating hardware clock from cron

Denniston, Todd A CIV NAVSURFWARCENDIV Crane todd.denniston at navy.mil
Fri Mar 4 20:04:56 UTC 2011

> -----Original Message-----
> From: centos-bounces at centos.org [mailto:centos-bounces at centos.org] On
> Behalf Of Kenneth Porter
> Sent: Friday, March 04, 2011 14:15
> To: CentOS mailing list
> Subject: [CentOS] Updating hardware clock from cron
> Is there a package to do this?
> Normally the hardware clock is set during shutdown if one is running
> ntpd.

hwclock --systohc is only called at start time (in
/etc/rc.d/init.d/ntpd), and only if ntpdate got a good time, which is a
good thing.

> But if a long-running server shuts down unexpectedly, this isn't done,
> and
> the hardware clock might be off by a lot when it comes back up. 

Not if you are running ntp and it was able to sync, because ntpd
activates a mode in the kernel that sets the hwclock every 11 minutes
when ntp declares it got synced.

If your hwclock is off by a lot when it comes up I believe it is from
one of the following:
A) bad cmos battery.
B) poor cmos clock
C) confusing info in /etc/adjtime due to using both hwclock --adjust [at
boot] and ntp (long story, but it is due to both tweaking the clock
without coordination between them).
D) booting a different OS with different ideas of timezones.
E) manual tweaking of time via bios.

> So setting
> it periodically from a cron job could be useful.
> What do others do? Adding a one liner to /etc/cron.daily that invokes
> /etc/rc.d/init.d/ntpd would do it but it seems heavyweight to restart
> ntpd.
> Alas, the script doesn't export just the sync_hwclock function.

Understand `hwclock --systohc` should _only_ be called when the admin
knows a good system time was *_JUST_* set from a good source, i.e.,
following a successful call to ntpdate or the admin setting the systime
with date.

On some systems that do NOT have ntpd service available (not on a
network with a time server), I will have them do a sequence of
hwclock --hctosys
hwclock --adjust
hwclock --hctosys
in /etc/rc.d/rc.sysinit where it already does the hwclock --hctosys so
that once I have set the time from a known source a few times the box
will reasonably self correct time on boot, I will even on some of those
systems have that sequence in a cronjob that gets run once a week.  If
this is on a network, I have the box that is doing this serve ntp (via
local clock) to the rest of the network, so they drift together.

Reminder: `hwclock --systohc` should _only_ be called when the admin
knows system time was JUST set from a good source, i.e., NOT from a cron
job that is not also making sure the ntpdate worked.

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