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Timekeeping best practices for Windows, including NTP

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 of 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:
  1. pool.ntp.org,0x1
  2. pool.ntp.org,0x1
  3. 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 your environment.
 
w32tm Command:
w32tm /config "/manualpeerlist:
  1. pool.ntp.org,0x1
  2. pool.ntp.org,0x1
  3. pool.ntp.org,0x1"

    Note: Modify the command to use the ntp servers available in your 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.

Disabling Time Synchronization

The time synchronization checkbox controls only whether time is periodically resynchronized while the virtual machine is running. Even if this box is unselected, by default VMware Tools synchronizes the virtual machine's time after a few specific events that are likely to leave the time incorrect.

Note: Unlike the normal periodic time sync, which can move the guest clock only forward, these special time syncs can move the guest clock either forward or backward. If you want your guests to never sync backward in time, disable the options (by selecting "False" for each option).

  1. Select the virtual machine in the vSphere Web Client inventory and power it off.
  2. Right click the virtual machine and choose Edit Settings...
  3. Click the VM Options tab.
  4. Expand the Advanced option.
  5. Under Configuration Parameters click Edit Configuration.
  6. Click Add Row and add this information:
     
    Name Value
    tools.syncTime False
    time.synchronize.continue False
    time.synchronize.restore False
    time.synchronize.resume.disk False
    time.synchronize.shrink False
    time.synchronize.tools.startup False
    time.synchronize.tools.enable False
    time.synchronize.resume.host False
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