converted to 1.6 markup
added link to tool chrony
|Deletions are marked like this.||Additions are marked like this.|
|Line 13:||Line 13:|
|1. If ntpd doesn't work well (e.g. a bad network connection), you can use [[http://cr.yp.to/clockspeed.html|clockspeed]] as well.||1. If ntpd doesn't work well (e.g. a bad network connection), you can use [[http://cr.yp.to/clockspeed.html|clockspeed]] or [[http://chrony.sunsite.dk/|chrony]] as well.|
Time moved backwards error
Dovecot isn't very forgiving if your system's time moves backwards. There are usually two possibilities why it's moving backwards:
- You're running ntpdate periodically. This isn't a good idea.
- You're using some kind of a virtual server and you haven't configured it right (or it's buggy).
Moving time backwards might cause various problems (see below), so for now Dovecot doesn't even try to handle the situation. Perhaps in some future version.
There are two choices for synchronizing your clock:
Use ntpd. It periodically checks the current time from NTP server and slows down or speeds up the clock if necessary. Unlike ntpdate, it doesn't just move the time forwards or backwards (unless the difference is large).
- If the time difference is too large for ntpd and it "steps", then use "-x" as a command line option for ntpd or use "tinker step 0" in /etc/ntp.conf.
If all else fails, you can just go and remove the error checking code from src/lib/ioloop.c. It's unlikely that anything will break badly, but you might get some errors logged once in a while.
In some systems ntpd/ntpdate is run at boot, but only after Dovecot has started. That can cause Dovecot to die immediately. If you have this problem, fix your init scripts to run ntpd/ntpdate first, before starting Dovecot. And again, strongly consider running ntp-wait before starting Dovecot.
With Xen you should run ntpd only in dom0. Other domains should synchronize time automatically (see this Xen FAQ).
Time moved backwards by 4398 seconds? Buggy kernel/hardware.
What about Daylight Saving/Summer time?
On Unix-like systems, time is stored internally as the number of seconds since January 1, 1970, 00:00:00 UTC (see Unix_time on Wikipedia); concepts such as time zones and daylight saving time are applied in user space by the C library, and will normally not have an impact on Dovecot's behavior.
Why not just..?
It's not as simple as fixing timeout handling to work correctly. Even a full Dovecot restart doesn't necessarily help, because some files' timestamps can be in future. To make Dovecot correctly handle time moving backwards would require verifying every single timestamp comparison code to make sure that it behaves correctly with future timestamps or with the current timestamp suddenly shrinking instead of growing. Perhaps all this will be made to work with future Dovecot releases, but why bother when you can just fix the real problem, which might fix some other random software failures as well?