On Tue, Nov 8, 2011 at 6:59 PM, Dave Hart <h...@ntp.org> wrote: > > On Fri, Nov 4, 2011 at 23:27, Nathan Kitchen > <nkitc...@aristanetworks.com> wrote: > > I'm curious about some behavior that I'm observing on a host running > > ntpd as a client. As I understand it, configuring a local reference > > clock--either an undisciplined local clock or orphan mode--shouldn't > > help me, but I see different behavior when I do have one. In > > particular, when I'm synchronizing after correcting a very large > > offset, I synchronize about 2x faster in orphan mode than with no > > local clock, and with an undisciplined local clock I don't even fix > > the offset. > > > > I'm curious about whether this difference should be expected. > > > > I'm using the following configuration in all cases: > > > > driftfile /persist/local/ntp.drift > > server 172.22.22.50 iburst > > > > My three different configurations for local clocks are the following: > > > > 1. No additional commands > > > > 2. tos orphan 10 > > > > 3. server 127.127.1.0 > > fudge 127.127.1.0 stratum 10 > > > > In all three cases, my test has these steps: > > > > 1. Stop ntpd. > > 2. Set the clock to 2000-1-1 00:00:00 (that is, more than 10 years ago). > > 3. Run ntpd -g. > > 4. Check that the 11-year offset is corrected. > > 5. Wait for synchronization to the time server. > > > > With either configuration #1 (no local clock) or #2 (orphan mode), the > > offset is corrected quickly: 4 and 13 seconds, respectively. With > > configuration #3 (undisciplined local clock), it fails to be corrected > > within 60 seconds. > > > > After the offset is corrected, configuration #1 takes 921 seconds to > > synchronize to the server. Configuration #2 takes 472. > > > > Can you provide any insight? > > And in a later message, Nathan points out he's testing ntpd 4.2.6p1 on > 64-bit Linux 2.6.32.28. > > I agree that I would not expect to see any difference in behavior > between LAN source only (#1) and LAN source + orphan mode (#2). I am > not surprised to see difference in behavior with 4.2.6 between either > of those and using a LAN source and the LOCAL driver (#3) as that > version is prone to switch to the LOCAL driver more often than is > helpful. If you stick with that version of ntpd, I would advise > avoiding the LOCAL driver, and I would also wonder if even orphan mode > is helpful in this case. Why not use config #1, what problem is LOCAL > or orphan mode solving?
It doesn't exactly solve a problem, but faster synchronization is nice to have, and it does appear to have that effect. Thanks for the insight on the appropriate use of LOCAL and orphan mode. -- Nathan > > I suspect if you try the latest ntp-dev, you'll see more symmetric > behavior between the setup with a a network source as well as either > the undisciplined local clock or orphan mode (which I would not > describe as a local reference clock, though it does behave similarly > if the system becomes the orphan parent as if it selects the LOCAL > driver as the system peer). > > In the configuration using the LOCAL driver, after the clock is > stepped massively, I suspect the LOCAL driver is being selected as the > system peer before the network source has provided enough samples to > be selectable (to have a low-enough dispersion). It is normal and > expected that reference clocks are selectable after a single poll, > rather than the several required for a network source. Once it is > selected, it continues to appear to be a better source than the > network source, even after many polls, thanks to the 0 delay and root > dispersion of reference clocks. This behavior is why this > list/newsgroup is full of advice to avoid using the LOCAL driver, and > why in 4.2.7 ntpd now refuses to use the LOCAL driver (or ACTS modem > driver lacking prefer, or to become orphan parent) until after a > holdoff period of 300s default (tos orphanwait), so that network > sources have a chance to become selectable before backup sources are > engaged. > > Without understanding why you are using LOCAL or orphan mode, it's > hard to say much more. Between the two, I think you'll find orphan > mode works better unless you actually have some other mechanism > disciplining the system clock and ntpd is there solely to serve that > time, in which case LOCAL should be the only source in ntp.conf and be > marked "prefer". > > Cheers, > Dave Hart _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
