Gene Heskett wrote:
Big reason #2: ntpdate doesn't guarantee a monotonically increasing system time (translation: it may set the clock backwards). Unix systems don't like that.Typically a crash recovery reboot will do exactly that when you keep the hardware clock on UT. In that event, the normal shutdown does not have a chance to reset the clock to local time, and hence is off by many hours on the reboot. With ntpdate doing the crash re-setting, my clocks get set backwards by 5 hours, but other than whats been logged previously in this boot proceedure, everything is still future tense to the processes that will run once booted.However, other than trying to make sense of the system logs, and an occasional squawk from smartd which is started before ntpd for some reason here, this has been a zero problem problem. Smartd only fuss's once, resetting itself to the new time on the next iteration anyway.
The problem manifests itself when run as a cron job instead of at boot. If the clock is running fast, ntpdate will have to set it back for it to have the right time. That can mess up various things, such as strange errors while running make.
Big reason #3: ntpd only queries remote servers when it believes thelocal time might have drifted by more than an acceptable amount. If you have a very stable clock, then it will ask for the timefairly rarely. A cron job knows nothing about this: it asks once per configured time unit, whether the local clock is likely to be dead accurate or has already slipped far out of sync. Big reason #4: if everyone put "0 * * * * ntpdate ntp.example.com"in their crontab, then once an hour ntp.example.com gets hammered. This also means that those answers are likely to be far moredelayed (and therefore inaccurate) then they are during the rest of the hour. Besides, the ntp.example.com admins would hate you.No one who actually thinks about such nuances schedules such important stuff at the top of the hour, every hour. And that tends to leave a slack spot at the top of the hour for those that don't think, but just do it. I expect its probably an mrtg trackable peak though.
Isn't it a little too optimistic thinking that most people will care enough and actually think about it?
-- "Codito ergo sum" Roel Schroeven