Time Synchronization in Hyper-V

There is a lot of confusion about how time synchronization works in Hyper-V – so I wanted to take the time to sit down and write up all the details. 

There are actually multiple problems that exist around keeping time inside of virtual machines – and Hyper-V tackles these problems in different ways.

Problem #1 – Running virtual machines lose track of time.

While all computers contain a hardware clock (called the RTC – or real-time clock) most operating systems do not rely on this clock.  Instead they read the time from this clock once (when they boot) and then they use their own internal routines to calculate how much time has passed.

The problem is that these internal routines make assumptions about how the underlying hardware behaves (how frequently interrupts are delivered, etc…) and these assumptions do not account for the fact that things are different inside a virtual machine.  The fact that multiple virtual machines need to be scheduled to run on the same physical hardware invariably results in minor differences in these underlying systems.  The net result of this is that time appears to drift inside of virtual machines.

In order to deal with time drift in a virtual machine – you need to have some process that regularly gets the real time from a trusted source and updates the time in a virtual machine.

Hyper-V provides the time synchronization integration services to do this for you.  The way it does this is by getting time readings from the management operating system and sending them over to the guest operating system.  Once inside the guest operating system – these time readings are then delivered to the Windows time keeping infrastructure in the form of an Windows time provider (you can read more about this here: http://msdn.microsoft.com/en-us/library/bb608215.aspx).   These time samples are correctly adjusted for any time zone difference between the management operating system and the guest operating system.

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http://blogs.msdn.com/b/virtual_pc_guy/archive/2010/11/19/time-synchronization-in-hyper-v.aspx