Joe Harvell wrote:
David L. Mills wrote:
<snip>
5. If for some reason the server(s) are not reachable at startup and
the applications must start, then I would assume the applications
would fail, since the time is not synchronized. If the applications
use the NTP system primatives, the synchronization condition is
readily apparent in the return code. Since they can't run anyway,
there is no harm in stepping the clock, no matter what the initial
offset. Forcing a slew in this case would seem highly undesirable,
unless the application can tolerate large differences between clocks
and, in that case, using ntpd is probably a poor choice in the first
place.
I agree that the condition of no time servers reachable on startup is
the most common case where a large offset will eventually be observed.
I agree that the application should detect this and fail before starting
up. I am concerned about clock and network failure scenarios that cause
an NTP client to see two different NTP servers with very different times.
This actually happened in a testbed for our application. NTP stats show
that over the course of 22 days, the offsets of two configured NTP
servers (both ours) serving one of our NTP clients started diverging up
to a maximum distance of 800 seconds. During this time, our NTP client
stepped its clock forward 940 times and backwards 803 times, with
increasing magnitudes up to ~400 seconds. The problem went away when
someone "added an IP address to the configuration of one of the NTP
servers." (I am still trying to determine exactly what happened). The
ntp.conf files of the NTP client, the stats, and a nice graph of the
offsets is found at http://dingo.dogpad.net/ntpProblem/.
I concede that only having 2 NTP servers for our host made this problem
more likely to occur. But considering the mayhem caused by jerking the
clock back and forth every 15 minues for 22 days, I think it is worth
investigating whether to eliminate stepping altogether.
Why didn't anyone notice the problem for 22 days? If, indeed, it caused
mayhem, why was it allowed to continue for so long?
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