The Timekeeping in virtual machines whitepaper has been updated to reflect the enhancements in VMware vSphere 4 and VMware Workstation 7.0 versions. In case you are wondering, why I am posting this under virtualization management, here are couple pointers:
- VMware Tools include the timekeeping daemon responsible for periodic clock synchronization, and several virtualization management solutions have checks and policies for VMware Tools configuration and compliance.
- Time and performance measurements inside virtual machines has always been a topic of discussion, especially management solutions that rely on accuracy of guest timekeeping.
To highlight couple enhancements mentioned in this updated whitepaper:
- In VMware Workstation 6.5 and earlier and in ESX/ESXi 4.0 and earlier, VMware Tools periodic clock synchronization does not correct the guest clock if it gets ahead of real time (except in the case of NetWare guest operating systems). Starting in VMware Workstation 7.0 and vSphere 4.0, VMware Tools periodic clock synchronization has been improved to address this issue. It does that by making the guest clock run more slowly than normal until host time catches up with it.
- Similarly, if the guest time is behind host time by less than 1 second, this improved VMware Tools will speed up the guest clock slightly until it catches up with host time.