Dovecot time Machine

05 January 2008

Dovecot time Machine

I have a XEN virtual machine which was complaining about time. Dovecot seemed to be the most vocal with errors like this in the logs:


dovecot: IMAP(charl): Time just moved backwards by 1 seconds. I'll sleep now until we're back in present. http://wiki.dovecot.org/TimeMovedBackwards


I run openntpd (OpenBSD NTP daemon) on the box but that was not seemingly keeping the date, well, up-to-date. Manually running ntpdate was also not providing the sync I sought and because this is a XEN box I have no access to hwclock.

Travelling back in time
A bit of digging on my provider's forums though showed that they were controlling the time syncing and had forgotten to turn the time sync back on after some troubleshooting they were doing.

That's all good and well but my box was still not syncing.

The Dovecot linked article in the log error message simply states:


With Xen you should run ntpd only in dom0. Other domains should synchronize time automatically (see this Xen FAQ).


XEN wisdom
The XEN FAQ has the following relevant things to say about time on a XEN box:


Q: My xen machines doesn't accept setting its time. Basically, it's stuck to RTC time. ntpdate, date, hwclock all "seem" to work, but they don't actually change the system time. The only way I have to change it right now is to change it in the BIOS.
A: Only affects 1.0. Fixed in newer versions.

Q: Where does a domain get its time from?
A: Briefly, Xen reads the RTC at start of day and by default will track that with the precision of the periodic timer crystal. Xen's estimate of the wall-clock time can only be updated by domain 0. If domain 0 runs ntpdate, ntpd, etc. then the synchronised time will automatically be pushed down to Xen every minute (and written to the RTC every 11 minutes, just as normal x86 Linux does). All other domains always track Xen's wall-clock time: setting the date, or running ntpd, on these domains will not affect their wall-clock time. Note that the wall-clock time exported by Xen is UTC --- all domains must have appropriate timezone handling (i.e. a correct /etc/localtime file).

Q: Is there is some cross-domain time synchronization : are they always in perfect sync, or should I run some kind of ntp in each subdomain ? Or only domain 0 would be enough ?
A:If you want each domain to keep its own time, there are two ways to cause a domain to run its wallclock independently from Xen:
1. Specify 'independent_wallclock' on the command line.
2. 'echo 1 >/proc/sys/xen/independent_wallclock'

To reenable tracking of Xen wallclock:
1. 'echo 0 >/proc/sys/xen/independent_wallclock'


"Shut it down, Shut it down forever!"
Following the FAQ I modified /proc/sys/xen/independent_wallclock and added it to /etc/sysctl.conf ("xen.independent_wallclock = 1") so that the change would survive a reboot.

I also changed my timezone to the same local as where I am working currently by doing the following:

$ sudo cp /usr/share/zoneinfo/Australia/Sydney /etc/localtime

Time marches on
openntpd now keeps time synced and Dovecot no longer complains about running in the future.