On 08/01/2008 15:15, Brian Mathis wrote: >> From: Jack Bailey <jack at internetguy.net> >> Date: Jan 8, 2008 3:17 AM >> To: centos at centos.org >> >> Hello All, >> >> Consider a CentOS-5.1 Xen server (2.6.18-53.1.4.el5xen) hosting two >> domains running CentOS-5.1 (2.6.18-53.1.4.el5). One domain has a fairly >> accurate clock, the other domain has a clock that gains ungodly amounts >> of time, roughly one minute every two or three minutes. For a fix, one >> suggestion is to run this command in DomU: >> >> echo 1 > /proc/sys/xen/independent_wallclock >> >> This didn't change anything. As an experiment, I wrote a script to call >> ntpd -q, sleep 60, and repeat indefinitely. Here are a couple of >> snippets of output: >> >> goodclock# ksh ./xenclockdrift >> ntpd: time slew +0.001211s >> ntpd: time slew +0.001200s >> ntpd: time slew +0.001855s >> ntpd: time slew +0.001532s >> ntpd: time slew +0.001603s >> ntpd: time slew +0.001320s >> ntpd: time slew +0.001931s >> >> badclock# ksh ./xenclockdrift >> ntpd: time slew -0.000193s >> ntpd: time set -57.356377s >> ntpd: time slew +0.002352s >> ntpd: time slew +0.003018s >> ntpd: time set -57.417488s >> ntpd: time slew +0.012089s >> ntpd: time slew -0.000985s >> >> These domains are fully virtualized and set up identically, except >> "badclock" is allocated two processors versus one processor for >> "goodclock". DomU's clock is running normally. >> >> Anyone know what's going or know how to fix it? >> Jack >> > > > I'm not sure with Xen, but on VMWare one should not be using NTP at > all. Time syncing should be done with the vmware tools and the host, > with NTP only running on the host (not the guests). Using NTP on a > vmware system will result in similar behavior to what you are seeing > here. At least, that is the theory. Empirically, using the vmware tools time sync feature will push a slow VM's clock forwards, but it won't push a fast clock backwards. I'm yet to see a "best practice" for ensuring proper time synchronisation within VMware VMs, but for now, NTP seems the best option. cheers Luke