Enabling Slew Mode for NTP

Products

• VMware vCenter Server

Issue/Introduction

This article provides the steps to enable slew mode for NTP. Slew mode may be required if a VMware product is affected by the leap second change.

A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC) in order to correspond to the mean solar time.

An adjustment of +1 second is scheduled for 23:59 UTC on the 31st December 2016.

Systems configured as Network Time Protocol (NTP) clients will receive a NTP packet with a leap second flag on the day of the adjustment.

Note: ESXi/ESX are unaffected by the leap second change. For more information, see VMware ESXi and Leap Seconds (2121190).

For more information, see Support for Leap Seconds in VMware Products (2147498).

Resolution

Switch the ntp daemon to slew mode 24 hours prior to the leap second, continue in slew mode for 48 hours afterwards and then revert to normal operation.

To switch the ntp daemon to slew mode:

1. Open a console or SSH session to the virtual appliance.
2. Open the /etc/sysconfig/ntp configuration file using a text editor.
3. Find the NTPD_OPTIONS setting. It looks similar to:
   
   NTPD_OPTIONS="-g -u ntp:ntp"
    
4. Change the configuration parameter to add the -x switch.
   
   NTPD_OPTIONS="-x -g -u ntp:ntp"
    
5. Save and close the file.
6. Restart the ntp service by running this command:
   
   service ntp restart

When the leap second window is over, the NTP mode can be safely changed back again.

To switch the ntp daemon back to regular operation after the leap second:

1. Open a console or SSH session to the virtual appliance.
2. Open the /etc/sysconfig/ntp configuration file using a text editor.
3. Find the NTPD_OPTIONS setting. It looks similar to:
   
   NTPD_OPTIONS="-x -g -u ntp:ntp"
    
4. Change the configuration parameter to remove the -x switch.
   
   NTPD_OPTIONS="-g -u ntp:ntp"
    
5. Save and close the file.
6. Restart the ntp service by running this command:
   
   service ntp restart

Additional Information

VMware ESXi and leap seconds
为 NTP 启用转换模式
NTP のスルー モードの有効化
Support for Leap Seconds in VMware Products