[ntp:questions] Question regarding local clock

unruh unruh at invalid.ca
Wed Oct 10 20:26:25 UTC 2012

On 2012-10-09, Joachim Larsson <joachim.larsson at ericsson.com> wrote:
> Hello all,
> In our environment we have both rhel5 and rhel6, the ntp package in rhel5 specifies a local clock being used;
Don't. Do NOT use a local clock ever.

> # Undisciplined Local Clock. This is a fake driver intended for backup
> # and when no outside source of synchronized time is available.
> server     # local clock
> fudge stratum 10
> This section is commented out in rhel6, but our IT policy requires me to make extensive research and/or testing before even fixing a typo.
> I have read the documentation regarding the LCL, and i understand what it could be used for. I also understand that Orphan Mode is preferred over using a LCL.
> However, i could not find any documentation regarding the impact systemwise when using LCL, together with a external source.
> For example, will there be extra cpu-cycles wasted, memory used, network activity, interrupts etc etc that potentially could lock up ntp, or worse, the entire machine?

No, none of those. But what could happen is that the system looses all
external sources of time, but ntpd keeps wandering happily into the
wilderness with its time become more and more inaccurate with no warning
at all, because it thinks its time is exactly correct ( after all its
error when it compares the local system clock to itself is always zero.
It is always exactly synchronized with itself, no matter how badly off
itself is. 

> I want to disable this, as per the recomendations, but i need to find out impact before doing this.

The advantage is that if someone else uses this system as a server, when
this system looses all contact with the true time, that other system
will not think that this system is still giving true time. 
Note that even if the time, due to some bug, jumps by 17 days, ntpd will
still think it is perfectly on time if you have the local time set up
and external sources are not there. 

> Thanks in advance!
> Sincerely,
> Joachim Larsson

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