Xen Problem

XenServer is headache http://sysconfig.org.uk/2011/01/so-long-xenserver/

- clocksource problem https://wiki.debian.org/Xen/Clocksource

When running no-citrix kernel, sometime running to clocksource problem.
The clocksource problem appears after you suspend a Linux host and the kernel in the virtual machine starts spewing this:

Mar  5 09:24:17 co kernel: [461562.007153] clocksource/0: Time went backwards: ret=f03d318c7db9 delta=-200458290723043 shadow=f03d1d566f4a offset=14

It turns out that the Citrix kernel uses jiffies as the clocksource per default, while Debian uses the xen clocksource.
So by just running this on the domU fixes the problem:

echo "jiffies"> /sys/devices/system/clocksource/clocksource0/current_clocksource

There is no need to decouple the clock from the host. To make this change permanent, you need to addclocksource=jiffies to the bootparameters of your domU kernel.

For fully virtualized (HVM) instances
To find your current clocksource

cat /sys/devices/system/clocksource/clocksource0/current_clocksource

To see available clocksources

cat /sys/devices/system/clocksource/clocksource0/available_clocksource

For para-virtualized (PV) instances,

echo 1 > /proc/sys/xen/independent_wallclock

In order to ensure the setting sticks on boot, you should also issue a:

echo "xen.independent_wallclock = 1" >> /etc/sysctl.conf