Timekeeping best practices for Windows, including NTP

Products

• VMware Desktop Hypervisor
• VMware vSphere ESXi

Issue/Introduction

This article provides best practices for achieving accurate timekeeping in Windows guest operating systems. These recommendations include a suggested configuration for time synchronization in the guest and on the host.

Resolution

For achieving accurate timekeeping in Windows guest operating systems, there are two main issues to consider: correctly configuring time synchronization and avoiding excessive CPU and memory overcommitment. Time synchronization utilities are necessary to correct time drift introduced by hardware time drift and guest operating system timekeeping imprecision. Excessive overcommitment can cause timekeeping drift at rates that are uncorrectable by time synchronization utilities. This best practices document covers time synchronization recommendations.

Time Synchronization

Use either w32time or NTP as the primary time synchronization utility.

Note: w32time is the time synchronization utility that ships with Windows. NTP (the Network Time Protocol daemon) is available for Windows through a variety of third-party ports to Windows.

 

Configuring w32time

When using w32time, there are a number of configuration parameters that can be changed. The table below describes the relevant parameters and gives a recommended value. All the parameters are stored in the registry. Some of them can also be modified through the w32tm utility instead of directly editing the registry. This best practices guide covers running w32time in NTP mode. w32time can also use the windows domain hierarchy as time servers, which is not covered in this best practices guide.

After changing w32time's settings it is necessary to restart w32time. Either reboot the virtual machine, run net stop w32time && net start w32time from the command line, or stop and start the w32time service. After restarting the w32time service, run the command w32tm /resync to force w32time to resync the time.

 

 

Key	Details
HKEY_LOCAL_MACHINE\ SYSTEM\CurrentControlSet\ Services\W32Time\ TimeProviders\NtpClient\ SpecialPollInterval	Recommended Value: 900 Type: REG_DWORD Description: This parameter controls how often w32time will poll the time server to check whether time on the client needs to be corrected. The parameter is specified as number of seconds to wait between polling. The recommended value of 900 specifies that the time server should be polled once every 15 minutes.
HKEY_LOCAL_MACHINE\ SYSTEM\CurrentControlSet\ Services\W32Time\Parameters\ NtpServer	Recommended Value: pool.ntp.org,0x1 pool.ntp.org,0x1 pool.ntp.org,0x1 Type: REG_SZ Description: This parameter specifies the time servers to use. It is specified as a string of space separated servers. Specifying ",0x1" after the server name indicates that the server should be contacted at the frequency specified by the SpecialPollInterval setting. Note: Modify the recommended value to point to the ntp servers available in the environment.   w32tm Command: w32tm /config "/manualpeerlist: pool.ntp.org,0x1 pool.ntp.org,0x1 pool.ntp.org,0x1" Note: Modify the command to use the ntp servers available in the environment.
HKEY_LOCAL_MACHINE\ SYSTEM\CurrentControlSet\ Services\W32Time\Parameters\ Type	Recommended Value: NTP   Type: REG_SZ   Description: This parameter specifies the mode that w32time should use. A value of NT5DS indicates that w32time should use the Windows domain hierarchy as its time servers rather than the NTP servers specified in the NtpServer key.     w32tm Command: w32tm /config /syncfromflags:   MANUAL

 

VMware Tools Time Synchronization and Configuration

When using w32time or NTP in the guest, disable VMware Tools periodic time synchronization.

To disable VMware Tools periodic time sync, use one of these options:

• Set tools.syncTime = "0" in the configuration file ( .vmx file) of the virtual machine.
  
  or
   
• Deselect Time synchronization between the virtual machine and the host operating system in the VMware Tools toolbox GUI of the guest operating system.
  
  or
   
• Run the VMwareService.exe -cmd "vmx.set_option synctime 1 0" command in the guest operating system. VMwareService.exe is typically installed in C:\Program Files\VMware\VMware Tools.
  
  or
   
• Run the vmtoolsd.exe timesync {enable|disable} command from the guest operating system and located at c:\Program Files\VMware\VMware Tools\vmtoolsd.

These options do not disable one-time synchronizations done by VMware Tools for events such as tools startup, taking a snapshot, resuming from a snapshot, resuming from suspend, or vMotion. These events synchronize time in the guest operating system with time in the host operating system even if VMware Tools periodic time sync is disabled, so it is important to ensure that the host operating system's time is correct. For more information about ESX 4/ESXi 4 or Workstation 7, see Timekeeping in VMware Virtual Machines.

To ensure the host operating system’s time is correct for VMware ACE, VMware Fusion, VMware GSX Server, VMware Player, VMware Server, and VMware Workstation run time synchronization software such as NTP or w32time in the host. For VMware ESX, run NTP in the service console. For VMware ESXi, run NTP on the VMkernel.

 

Additional Information

 

Note :

• If the windows virtual machine is not on the domain, run this command to ensure that the Windows and the Domain Controller time are in sync after a reboot:
  
  sc triggerinfo w32time start/networkon stop/networkoff
   
• Using Windows Group Policy to set the NTP settings for the virtual machines decreases the administration required for time synchronization.
  
  For information regarding setting up an NTP server on an ESX host, see Configure Network Time Protocol (NTP)
  

   

  For more information on configuring an authoritative time server in Windows, see Microsoft Knowledge Base article How to configure an authoritative time server in Windows Server
  
  For information on Linux systems, see Timekeeping best practices for Linux guests.