Обсуждение: What does PostgreSQL do when time goes backward?
How does PostgreSQL react to time being stepped at bootup? My Chrony NTP package might cause it to do so on rare occasions when the hardware clock is way off. This would only happen during bootup. -- John Hasler jhasler@newsguy.com Elmwood, WI USA
John Hasler wrote: > How does PostgreSQL react to time being stepped at bootup? My Chrony > NTP package might cause it to do so on rare occasions when the hardware > clock is way off. This would only happen during bootup. My ntp client changes clock (by small amount) at any time: Jul 25 05:29:38 bax ntpd[10269]: adjusting local clock by 0.098724s Jul 25 05:31:43 bax ntpd[10269]: adjusting local clock by 0.038991s Jul 25 06:13:38 bax ntpd[10269]: adjusting local clock by -0.037131s Jul 25 15:01:52 bax ntpd[10269]: adjusting local clock by -0.112429s
On Wed, Aug 04, 2010 at 07:20:31AM -0400, Frank Bax wrote: > John Hasler wrote: >> How does PostgreSQL react to time being stepped at bootup? My Chrony >> NTP package might cause it to do so on rare occasions when the hardware >> clock is way off. This would only happen during bootup. > > > My ntp client changes clock (by small amount) at any time: > > Jul 25 05:29:38 bax ntpd[10269]: adjusting local clock by 0.098724s > Jul 25 05:31:43 bax ntpd[10269]: adjusting local clock by 0.038991s > Jul 25 06:13:38 bax ntpd[10269]: adjusting local clock by -0.037131s > Jul 25 15:01:52 bax ntpd[10269]: adjusting local clock by -0.112429s > PostgreSQL does not use system time to track transactions so you should be good. Also, these types of clock changes by ntpd use the adjtime() system call which either slows or speeds the system clock to make the adjustment over a period of time so it should be minimally disruptive. These do seem to be larger values than you might expect from a clock conditioned with ntpd. Is it a VM or is there something going on that would stop or suspend your system? Cheers, Ken
I wrote: > How does PostgreSQL react to time being stepped at bootup? My Chrony > NTP package might cause it to do so on rare occasions when the > hardware clock is way off. This would only happen during bootup. Ken writes: > PostgreSQL does not use system time to track transactions so you > should be good. Thank you. > Also, these types of clock changes by ntpd use the adjtime() system > call which either slows or speeds the system clock to make the > adjustment over a period of time so it should be minimally disruptive. This is about Chrony <http://www.chrony.tuxfamily.org>, an alternative ntp implementation. In any case, both chronyd and ntpd can step the clock (possibly backwards) at bootup under some rare circumstances. Frank writes: > My ntp client changes clock (by small amount) at any time: > Jul 25 05:29:38 bax ntpd[10269]: adjusting local clock by 0.098724s > Jul 25 05:31:43 bax ntpd[10269]: adjusting local clock by 0.038991s > Jul 25 06:13:38 bax ntpd[10269]: adjusting local clock by -0.037131s > Jul 25 15:01:52 bax ntpd[10269]: adjusting local clock by -0.112429s Ken writes: > These do seem to be larger values than you might expect from a clock > conditioned with ntpd. Is it a VM or is there something going on that > would stop or suspend your system? There is certainly something wrong there. -- John Hasler jhasler@newsguy.com Elmwood, WI USA
John Hasler wrote: > Frank writes: >> My ntp client changes clock (by small amount) at any time: >> Jul 25 05:29:38 bax ntpd[10269]: adjusting local clock by 0.098724s >> Jul 25 05:31:43 bax ntpd[10269]: adjusting local clock by 0.038991s >> Jul 25 06:13:38 bax ntpd[10269]: adjusting local clock by -0.037131s >> Jul 25 15:01:52 bax ntpd[10269]: adjusting local clock by -0.112429s > > Ken writes: >> These do seem to be larger values than you might expect from a clock >> conditioned with ntpd. Is it a VM or is there something going on that >> would stop or suspend your system? > John writes: > There is certainly something wrong there. I saw very bad clock performance on one Linux box I had (dual-single core AMD cpus, no VMs), even with NTP, until I changed the clocksource kernel parameter to hpet. Unfortunately (or fortunately) I no longer have that box. -- Steve Wampler -- swampler@noao.edu The gods that smiled on your birth are now laughing out loud.
John Hasler wrote: > Frank writes: >> My ntp client changes clock (by small amount) at any time: >> Jul 25 05:29:38 bax ntpd[10269]: adjusting local clock by 0.098724s >> Jul 25 05:31:43 bax ntpd[10269]: adjusting local clock by 0.038991s >> Jul 25 06:13:38 bax ntpd[10269]: adjusting local clock by -0.037131s >> Jul 25 15:01:52 bax ntpd[10269]: adjusting local clock by -0.112429s > > Ken writes: >> These do seem to be larger values than you might expect from a clock >> conditioned with ntpd. Is it a VM or is there something going on that >> would stop or suspend your system? > > There is certainly something wrong there. System is not a VM; it runs 24/7 hosting for a few simple domains. It runs OpenBSD, not Linux. I believe OpenBSD has it's own ntpd implementation. I read once that the amount of time mentioned is how much time clock needs to be changed, not how much the clock will actually be changed. I know nothing about ntpd internals; thanks for letting me hijack this thread. The log messages may be out-of-context, since I excluded frequency changes. Still something wrong? Jul 25 00:13:25 bax ntpd[10269]: adjusting local clock by -0.038453s Jul 25 00:42:25 bax ntpd[10269]: adjusting clock frequency by -10.296223 to -20.774345ppm Jul 25 02:38:31 bax ntpd[10269]: adjusting local clock by 0.039182s Jul 25 04:56:29 bax ntpd[10269]: adjusting clock frequency by 3.332808 to -17.441537ppm Jul 25 05:29:38 bax ntpd[10269]: adjusting local clock by 0.098724s Jul 25 05:31:43 bax ntpd[10269]: adjusting local clock by 0.038991s Jul 25 06:13:38 bax ntpd[10269]: adjusting local clock by -0.037131s Jul 25 08:26:12 bax ntpd[10269]: adjusting clock frequency by -5.876526 to -23.318063ppm Jul 25 11:07:08 bax ntpd[10269]: adjusting clock frequency by 16.235236 to -7.082827ppm Jul 25 15:01:52 bax ntpd[10269]: adjusting local clock by -0.112429s Jul 25 15:38:06 bax ntpd[10269]: adjusting clock frequency by -9.112840 to -16.195667ppm Jul 25 20:42:03 bax ntpd[10269]: adjusting clock frequency by 1.838351 to -14.357316ppm Jul 26 05:57:41 bax ntpd[10269]: adjusting clock frequency by -0.485347 to -14.842663ppm Jul 26 15:48:01 bax ntpd[10269]: adjusting clock frequency by -0.130635 to -14.973298ppm Jul 26 23:51:48 bax ntpd[10269]: adjusting clock frequency by -0.632588 to -15.605885ppm Jul 27 09:14:44 bax ntpd[10269]: adjusting clock frequency by 0.761208 to -14.844677ppm Jul 27 11:33:29 bax ntpd[5857]: adjusting local clock by 0.332560s Jul 27 11:57:09 bax ntpd[5857]: adjusting clock frequency by -0.180636 to -15.025316ppm Jul 27 12:16:16 bax ntpd[5857]: adjusting clock frequency by 0.153771 to -14.871545ppm Jul 27 12:38:36 bax ntpd[5857]: adjusting clock frequency by -0.314905 to -15.186450ppm Jul 27 22:30:47 bax ntpd[5857]: adjusting clock frequency by -0.058928 to -15.245378ppm Jul 28 22:00:57 bax ntpd[5857]: adjusting clock frequency by 0.508728 to -14.750045ppm Jul 29 07:09:05 bax ntpd[5857]: adjusting clock frequency by 0.056698 to -14.693347ppm Jul 29 21:25:01 bax ntpd[5857]: adjusting clock frequency by -0.233626 to -14.926974ppm Jul 30 06:28:33 bax ntpd[5857]: adjusting clock frequency by -0.255362 to -15.182336ppm Jul 30 15:45:17 bax ntpd[5857]: adjusting clock frequency by 0.291147 to -14.891189ppm Jul 31 11:57:51 bax ntpd[5857]: adjusting clock frequency by 0.198529 to -14.725782ppm Aug 1 08:47:12 bax ntpd[5857]: adjusting clock frequency by -0.063213 to -14.806622ppm Aug 2 12:19:10 bax ntpd[5857]: adjusting clock frequency by 0.096080 to -14.759072ppm Aug 3 01:30:19 bax ntpd[5857]: adjusting clock frequency by -0.353416 to -15.112489ppm Aug 3 08:10:34 bax ntpd[5857]: adjusting clock frequency by 0.251130 to -14.861359ppm Aug 3 20:29:11 bax ntpd[5857]: adjusting clock frequency by -0.105335 to -14.966694ppm Aug 4 07:22:22 bax ntpd[27189]: adjusting local clock by -0.097095s Aug 4 07:49:26 bax ntpd[27189]: adjusting clock frequency by -2.970136 to -17.898726ppm Aug 4 08:04:04 bax ntpd[27189]: adjusting clock frequency by 2.495076 to -15.403651ppm