Messages in this thread |  | | Date | Thu, 1 Nov 2001 00:52:19 +0000 (GMT) | From | Riley Williams <> | Subject | Re: PROBLEM: Linux updates RTC secretly when clock synchronizes |
| |
Hi Kurt.
>>>> PROBLEM: Linux updates RTC secretly when clock synchronizes. >>>> >>>> When /usr/sbin/ntpd synchronizes the Linux kernel (or system) clock >>>> using the Network Time Protocol the kernel time is accurate to a >>>> few milliseconds. Linux then sets the Real Time (or Hardware or >>>> CMOS) Clock to this time at approximately 11 minute intervals. >>>> Typical RTCs drift less than 10 s/day so rebooting causes only >>>> millisecond errors.
>> When the machine reboots from a power interruption Internet >> connection may be unavailable (usual here). Ntpd can start >> synchronized to an RTC but time is inaccurate because hwclock has >> not adjusted it. When Internet connection reestablishes ntpd notices >> the time conflict and terminates. Human intervention or cron lines >> can run ntpdate and then restart ntpd. This results in time of low >> quality.
> I'm not really sure about this, but I think you need a *high* > offset, like in the order of an hour, before ntpd refused to adjust. > I know it happely does 5 minutes.
The offset needs to be sufficient to handle the case of the RTC being set to local time and the boot and first ntp sync occurring on opposite sides of a Daylight Savings Time change. A couple of timezones use a DST offset of 90 minutes, so if it's less than that, there will be problems.
>> There are lots of high quality and cost hardware solutions. The >> solution in my first post was a very small kernel patch to add an 11 >> minute update log so hwclock etc could adjust the time. Any comments >> on my patch?
> Note that hwclock does not adjust the clock if the error is smaller > than 1 second, or it already wrote to the RTC is the past 23 hours.
I knew about the "not less than 1 second" restriction, but not the "only once a day" restriction. Can you confirm that the latter restriction is indeed the case please?
Best wishes from Riley.
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