Problem Statement

What is the best way to keep time synchronized within virtual machine guest operating systems?

Assumptions

1. ESXi hosts are using an accurate and reliable NTP server
2. A level of CPU overcommitment exists in the vSphere cluster

Motivation

1. Prevent the unlikely but possible event of CPU over commitment introducing time drift into guest operating systems

Architectural Decision

Do not use VMware Tools for Time Synchronization Source for Virtual Machines and Guest operating systems need to be configured to use an NTP server

Justification

1. Excessive overcommitment can cause timekeeping drift at rates that are uncorrectable by time synchronization utilities
2. This ensures time within virtual machines is not impacted by time drift in the event of CPU overcommitment
3. Ensure time will be consistent and provided by a central source for all virtual machines
4. NTP is a industry standard method of maintaining accurate time
5. Simplifies the process of maintaining time
6. Aviods the potential issue where Time runs too fast in a Windows virtual machine when the Multimedia Timer interface is usedSee VMware KB 1005953

Implications

1. Any/all templates need to be configured to use an NTP server within the guest operating system
2. All existing servers will need to be updated to use an NTP server within the guest operating system if they currently rely on the hypervisor (VMware Tools) for time

Alternatives

1. Use VMware Tools for time synchronization