Discussion:
Time difference in operating system
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Jake Anderson
2018-07-05 07:36:49 UTC
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Hi

I understand that the operating system timing takes from HMC.

Still we find a few seconds of difference in Operating system.

Does it takes time to percolate and get in sync ?

Jake

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R.S.
2018-07-05 15:14:58 UTC
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Post by Jake Anderson
Hi
I understand that the operating system timing takes from HMC.
Still we find a few seconds of difference in Operating system.
Does it takes time to percolate and get in sync ?
Your assumption is not true.
If you don't have STP, your time is just set by operator command. For
STP enablement *and* NTP connection you time is set (during POR) by
SE/HMC and synchronized with external time source.
--
Radoslaw Skorupka
Lodz, Poland




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Joel C. Ewing
2018-07-06 04:46:11 UTC
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Post by R.S.
Post by Jake Anderson
Hi
I understand that the operating system timing takes from HMC.
Still we find a few seconds of difference in Operating system.
Does it takes time to percolate and get in sync ?
Your assumption is not true.
If you don't have STP, your time is just set by operator command. For
STP enablement *and* NTP connection you time is set (during POR) by
SE/HMC and synchronized with external time source.
Perhaps things have changed in the last several years, but it used to be
that without an STP the Processor TOD clock was still  set from the HMC
clock at a POR, and the LPAR "virtual" TOD clock was then initialized
from the processor TOD clock when the LPAR was activated.  Depending on
CLOCKnn PARMLIB member, the Operator might have an option to adjust the
clock at IPL and that would also change the LPAR TOD clock.  Changing
the local clock time after z/OS is initialized only changes the offset
of local time from the LPAR TOD clock, not the LPAR TOD clock itself. 

In practice, we always reset the HMC clock as close to real time as
possible before a planned POR and never needed to fine-tune the clock
value from the z/OS console unless we were doing a DR exercise for a
different date/time than real time.  All LPAR TOD clocks increment in
sync, but if two LPARs on the same processor had their LPAR TOD clocks
set manually to slightly different times at IPL rather than being
controlled by an STP, they would continue to have the same offset
indefinitely.  The dynamic correction of TOD times only occurs when an
STP is in control of the LPAR TOD clocks.   Changes to the HMC clock
after POR and LPAR activation have no effect on LPAR TOD values..
    Joel C. Ewing
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Joel C. Ewing, Bentonville, AR ***@acm.org

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