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Correcting Clock Drift On A CentOS VM Under Hyper-V On Server 2008 R2

Correcting Linux Clock Drift on Hyper-V on Server 2008 R2

How to correct clock drift on a CentOS 5 virtual machine guest running on a Server 2008 R2 Hyper-V host.

I recently had a problem where my CentOS 5 virtual machine running on Hyper-V on Server 2008 R2 was gaining time quite dramatically.
Initially, I turned off “Time synchronization” in the Integration Services section of the Hyper-V settings for the CentOS virtual machine. This reduced the clock drift from more than a minute per minute, down to only seconds per minute, however this is still far too inaccurate for NTP or similar to correct.
Here is what I did to correct the clock inaccuracy problem:

1. Modify the kernel boot options by editing /boot/grub/grub.conf and adding the line “divider=10 clocksource=acpi_pm” after the appropriate kernel line, as below. Clearly, this will vary depending on your exact install.:

   # grub.conf generated by anaconda
   #
   # Note that you do not have to rerun grub after making changes to this file
   # NOTICE:  You have a /boot partition.  This means that
   #          all kernel and initrd paths are relative to /boot/, eg.
   #          root (hd0,0)
   #          kernel /vmlinuz-version ro root=/dev/VolGroup00/LogVol00
   #          initrd /initrd-version.img
   #boot=/dev/hda
   default=0
   timeout=5
   splashimage=(hd0,0)/grub/splash.xpm.gz
   hiddenmenu
   title CentOS (2.6.18-164.6.1.el5)
   	root (hd0,0)
   	kernel /vmlinuz-2.6.18-164.6.1.el5 ro root=/dev/VolGroup00/LogVol00 hda=noprobe hdb=noprobe divider=10 clocksource=acpi_pm
   	initrd /initrd-2.6.18-164.6.1.el5.img
   title CentOS (2.6.18-164.el5)
   	root (hd0,0)
   	kernel /vmlinuz-2.6.18-164.el5 ro root=/dev/VolGroup00/LogVol00
   	initrd /initrd-2.6.18-164.el5.img
   

2. Configure ntpd, as below:
   1. The following is a sample /etc/ntp.conf. Note that you should normally change 0.vmware.pool.ntp.org to something specific to your country, for example, 0.au.ntp.pool.org, and the same for the other ntp servers here, and in /etc/ntp/step-tickers in item 2 below. See www.pool.ntp.org/en for more information on the NTP pool and which servers you should use.

      tinker panic 0
      restrict 127.0.0.1
      restrict default kod nomodify notrap
      server 0.vmware.pool.ntp.org
      server 1.vmware.pool.ntp.org
      server 2.vmware.pool.ntp.org
      driftfile /var/lib/ntp/drift

   2. The following is a sample /etc/ntp/step-tickers:

      0.vmware.pool.ntp.org
      1.vmware.pool.ntp.org

      The configuration directive tinker panic 0 instructs NTP not to give up if it sees a large jump in time. This is important for coping with large time drifts and also resuming virtual machines from their suspended state. Note: The directive tinker panic 0 must be at the top of the ntp.conf file.
   3. It is also important not to use the local clock as a time source, often referred to as the Undisciplined Local Clock. NTP has a tendency to fall back to this in preference to the remote servers when there is a large amount of time drift. An example of such a configuration is:

      server 127.127.1.0
      fudge 127.127.1.0 stratum 10

      Comment out both lines.
3. Synchronised the time manually:

   ntpd -q

4. Set the hardware clock to the newly synchronised time:

   hwclock --systohc

5. Set up ntpd to automatically run and synchronise the time, as below:

   chkconfig ntpd on

6. Reboot the machine. I have not had any further time issues.

More Information

For more information about:

• Windows Server 2008 R2, see microsoft.com/windowsserver2008/en/us/default.aspx
• Hyper-V, see en.wikipedia.org/wiki/Hyper-V
• CentOS, see www.centos.org
• Adding “divider=10 clocksource=acpi_pm”, and configuring ntpd for use on virtual machines, see kb.vmware.com/kb/1006427
• ntpd, see linux.die.net/man/1/ntpd
• hwclock, see linux.die.net/man/8/hwclock
• pool.ntp.org, see www.pool.ntp.org/en

Updated about 8 mths, 18 days ago (December 18, 2009). Know a better answer? Let us know!

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