On Monday, 07/25/2016 at 11:36 GMT, WF Konynenberg <w...@konynenberg.org>
wrote:
> Yes, when your hardware clock is not managed by NTP you cannot rely on
it to accurately initialize
> the system clock on boot, so you should force an NTP clock sync ("jump")
on startup. Otherwise the
> NTP smooth clock adjustment could take quite a while to get the system
clock synced.
>
> Once the initial sync has happened, NTP should be able to keep the clock
within at most a few
> milliseconds accurate, even on a VM with some scheduling artifacts.
Linux on z does not alter the TOD clock after it's booted. NTP affects
only the offset from the TOD that the kernel maintains. Any app that
wants to know the time must ask the kernel. It can't just issue STCK(E).
In an LPAR, Linux will sync the LPAR TOD to the CPC TOD when it boots.
After that it depends on STP, and it really depends on having the proper
number of leap seconds configured in the CPC and in Linux.
We really need CP to virtualize STP (when real STP is being used e.g. with
NTP). That would allow Linux to always have the correct time without
running its own NTP client.
The problem today is that VM will not generate a sync check when the
difference between NTP and the TOD becomes large enough that it cannot be
steered out in a short period of time, such as when the external time
source is reconnected or when a leap second is added.
Alan Altmark
Senior Managing z/VM and Linux Consultant
Lab Services System z Delivery Practice
IBM Systems & Technology Group
ibm.com/systems/services/labservices
office: 607.429.3323
mobile; 607.321.7556
alan_altm...@us.ibm.com
IBM Endicott
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