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pages/GuestNTP.md

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+# Setting Up NTP in a Guest
+
+I read on the Internet, so it must be true, that a kvm guest defaults to a clock source of "kvm-clock", which sets the guest's TSC clock to the host's.  But this is at boot, and the guest then runs a virtual TSC from boot on out.  This can yield poor results.  To see what clocking is being used,
+
+```
+$ cat /sys/devices/system/clocksource/clocksource0/current_clocksource
+```
+
+which, in the default configuration, yields "kvm-clock", meaning that the guest's TSC was set to the host's TSC when the guest booted, and has been drifting on the virtual sea ever since.  This is not a good thing.
+
+So, setting up ntpd is advised, at least if you want better than the average time source.
+
+First, you need to install NTP, which also starts it
+
+```
+$ apt-get install ntp
+```
+
+Though not absolutely necessary, I also like to have a known stratum-1 in the configuration,  so I add the following line to /etc/ntp.conf
+
+```
+server my.strat.1
+```
+
+You then have to whack the NTP daemon so it sees the configuration change
+
+```
+$ service ntp restart
+```
+
+Wait a minute and then check that ntpd is OK
+
+```
+$ ntpq -p
+```
+
+There is one remaining problem.  the ntp daemon is setting the guest TSC which conflicts with the  kvm-clock syncing the TSC off the host's clock.  To see what clocking is being used, try
+
+```
+$ cat /sys/devices/system/clocksource/clocksource0/current_clocksource
+```
+
+which should yield the default "kvm-clock".  As you do not want this, to disable kvm-clock, you have to add
+
+```
+no-kvmclock
+```
+
+to the kernel grub invocation parameter list.  After rebooting with the kernel config change
+
+```
+$ cat /sys/devices/system/clocksource/clocksource0/current_clocksource
+```
+
+should show "tsc" as opposed to "kvm-clock".  This is saying that the  guest is using its internal TSC, a virtual hardware clock, which we have arranged to maintained by the ntpd daemon in the guest.