On Thursday 11 September 2008 10:33, Alan Altmark wrote:
>If you enable the external timer function of System z, it will syncronize.
> For large time deltas, an LPAR that supports STP or ETR will be notified.
> For small deltas, the LPARs will drift to the correct time. The clock
>will appear to run faster or slower as needed. We call this "TOD clock
>steering".
>
>When Linux is running on z/VM, it cannot receive the "time shift"
>notifications. This is because CP does not register with the hardware to
>receive them. (It's more complicated than that, really.)
>
>But if you get the box time in sync and keep it that way, and then
>deactivate/reactivate your VM LPAR, the VM and guest TOD clocks will
>remain in sync with the external time reference.
True, but Linux only examines the TOD clock at IPL, and uses a software clock
from then on. Unless the tickless-timer patch changed all that, that is. So
even if the TOD clock is in sync, Linux won't be tracking it. So you'd still
need to do a periodic "ntp -q -x" from cron, with ntpd configured to use the
local clock as its reference, to keep Linux in sync with the TOD clock. And
ntpd wakes up every second.
I may have found a solution for that problem, though. The NTP package also
contains a sntp (Simple NTP) program, which implements a subset of the NTP
and is supposed to be used as a client. It only wakes up every five hours.
Like ntpd, it uses adjtime() to adjust the software clock as needed, ensuring
that you don't jump backwards in time. To run it as a daemon, use a command
such as: "sntp -x -a ntp.example.com </dev/null >/dev/null 2>&1 &". The
undocumented -x option is what makes it run forever. Perhaps this is a
better tool than ntpd for the VM environment?
- MacK.
-----
Edmund R. MacKenty
Software Architect
Rocket Software, Inc.
Newton, MA USA
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