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:

 1. You're running 'ntpdate' periodically. This isn't a good idea.
 2. 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 Dovecot
versions older than v2.0 don't even try to handle the situation.

Time synchronization
--------------------

There are two choices for synchronizing your clock:

 1. Use ntpd [http://www.ntp.org/]. 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'.
        * This shows up in logs as: 'ntpd[17697]: time reset -2.075483 s'
 2. If ntpd doesn't work well (e.g. a bad network connection), you can use
    clockspeed [http://cr.yp.to/clockspeed.html] or chrony
    [http://chrony.sunsite.dk/] as well.

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. 
Also, seriously consider running ntp-wait before starting Dovecot.

Bugs/Issues
-----------

 * With Xen you should run ntpd only in dom0. Other domains should synchronize
   time automatically (see this Xen FAQ [http://xen.epiuse.com/xen-faq.txt] and
   this thread [http://dovecot.org/list/dovecot/2009-October/043301.html]).
 * With Vmware you follow the guidlines at the Vmware knowledge base
   [http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1006427].
 * Time moved backwards by 4398 seconds
   [http://www.dovecot.org/list/dovecot/2008-June/031548.html]? 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 [WikiPedia: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.

Dovecot shouldn't just die!
---------------------------

Dovecot v2.0 finally tries to handle this a bit more gracefully. Its behavior
when time moves backwards is:

 * Existing imap and pop3 processes either sleep or die, just like with older
   versions
 * Master process stops creating new processes until either the original time
   is reached, or after a maximum wait of 3 minutes.
 * Other processes log a warning, but do nothing else.
 * Timeouts are updated so that the timeout is executed approximately at the
   original intended time.

Dovecot v2.0 also notices when time unexpectedly jumps forwards. In that
situation it logs a warning and also updates timeouts.

The reason why imap/pop3 processes get killed and new ones can't be created for
a while is to avoid problems related to timestamps. Some issues are:

 * Uniqueness of Maildir filenames and dbox global unique identifiers relies on
   a growing timestamp
 * Dotlock files' staleness is detected by looking at its mtime.
 * Timestamps are stored internally all around in memory (as well as in index
   files) and compared to current time. Those checks may or may not be buggy if
   current time shrinks.

While killing mail processes doesn't fully solve any of those issues, they're
at least less likely to happen then.

(This file was created from the wiki on 2016-03-16 04:44)
