xen

Xen 3.0 Time Drift Bug

In Xen the user VPS' clocks are tied to the host server by default. This is typically what you'd want. Since we run ntp on the host servers to keep their time accurate.

e.g.:

# VPS time is locked to the host server's time and cannot be changed.
# date --set 20050101
Sat Jan  1 00:00:00 EST 2005
# date
Sun Mar 26 03:04:05 EST 2006

Host Server NTP Not Working

In the past we have seen a few host servers with mis-configured NTP settings. In this case you may see time drift. And if so pop in a ticket and we can fix the NTP setup. Hopefully this should not be an issue any more.

Xen 3.0.0 and Xen 3.0.1 Time Drift

Some early versions of Xen 3.0 (3.0.0 and 3.0.1) have this problem (but not Xen 2.0.x or Xen 3.0.2 and later). We have very few (any) servers still running this version. If your VPS kernel is 2.6.16 or later then the problem does not affect you.

If the problem _does_ affect you then you may want to a time on your VPS independent of the host server.

To do that run:

# set independent_wallclock
# echo 1 > /proc/sys/xen/independent_wallclock

(You can put that in your startup scripts if you wish. e.g. at /etc/rc.local on redhat distros).

The VPS date can now be set.

# now the date is set as expected
# date --set 20050101
Sat Jan  1 00:00:00 EST 2005
# date
Sat Jan  1 00:00:01 EST 2005
# e.g. so you can use ntpdate
# ntpdate 0.pool.ntp.org
26 Mar 03:17:43 ntpdate[1614]: step time server 81.171.100.41 offset 38805449.236315 sec
# date
Sun Mar 26 03:17:46 EST 2006

NBD Errors

You may see something like this on bootup:

Apr 22 11:26:36 host kernel: nbd0: Attempted send on closed socket
Apr 22 11:26:36 host kernel: end_request: I/O error, dev nbd0, sector 0
Apr 22 11:26:36 host kernel: nbd1: Attempted send on closed socket

These messages relate to network block device kernel code. They appear even when you are not running anything that uses nbds. This message is completely ignorable (and I think it likely a bug that it even appears).