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How Time Synchronization works in Hyper-V

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Ben Armstrong explains in dep How Time Synchronization works in Hyper-V.

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.

UPDATE 11/22: One thing that you should be aware of here: the rate at which the time in a virtual machine drifts is affected by the total system load of the Hyper-V server.  More virtual machines doing more stuff means time drifts faster.

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:   These time samples are correctly adjusted for any time zone difference between the management operating system and the guest operating system.

Problem #2 – Saved virtual machines / snapshots have the wrong time when they are restored.

When we restore a virtual machines from a saved state or from a snapshot we put back together the memory and run state of the guest operating system to exactly match what it was when the saved state / snapshot was taken.  This includes the time calculated by the guest operating system.  So if the snapshot was taken one month ago – the time and date will report that it is still one month ago.

Interestingly enough, at this point in time we will be reporting the correct (with some caveats) time in the systems RTC.  But unfortunately the guest operating system has no idea that anything significant has happened – so it does not know to go and check the RTC and instead continues with its own internally calculated time.

To deal with this the Hyper-V time synchronization integration service detects whenever it has come back from a saved state or snapshot, and corrects the time.  It does this by issuing a time change request through the normal user mode interfaces provided by Windows.  The effect of this is that it looks just like the user sat down and changed the time manually.  This method also correctly adjusts for time zone differences between the management operating system and the guest operating system.

Read more here 🙂

Written by IT Core

November 23, 2010 at 9:25 PM

Posted in Documentation, Tools, Virtualization

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Windows Storage Server 2008 R2 Documentation

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From Windows Storage Server blog here’s a list of useful documentation references: 

Here are some relevant references to Windows Storage Server-centric content. Keep checking back for more links. 
Windows Server 2008 R2
New Technology:
New in Windows File Services:

Storage Technology Facts
Default cluster size for NTFS, FAT, and exFAT (256 TB NTFS limit) –
Large Logical Unit Support and Windows Server 2003 SP1 (GPT): and
How NTFS works (256 TB limit) :
Reviewing Storage Limits:
Microsoft Storage: Fact and Fiction –
How to calculate the LUN limit per HBA –
How dynamic disks and volumes work –
How basic disks and volumes work –
Windows Storage Server Whitepapers
Windows Server 2008 R2 Whitepapers
Improve your understanding and get more in-depth information about Windows Server 2008 R2 in these whitepapers:

Performance Tuning Guidelines
This guide describes important tuning parameters and settings that you can adjust to improve the performance and energy efficiency of the Windows Server 2008 R2 operating system. This guide describes each setting and its potential effect to help you make an informed decision about its relevance to your system, workload, and performance goals.

NFS Account Mapping Whitepaper
This paper covers Network File System (NFS) account mapping and the deployment in Windows Server 2008 R2. NFS is a network file sharing protocol that allows remote access to files over a network. NFS implementations include an NFS server component, which enables the sharing of files for use by other networked computers, and an NFS client component, which enables computers to access files shared by NFS servers. The Services for NFS role service in Windows Server provides the ability to function as an NFS server. Windows and UNIX operating systems use different account and security systems. Windows operating systems represent users and groups with a unique security identifier (SID), while UNIX operating systems represent users with user identifiers (UIDs) and group identifiers (GIDs). Account mapping is the process of correlating the UNIX UIDs and GIDs to corresponding Windows user and group SIDs.

Operational TCO Comparison:
Windows Server 2008 File Services vs. Dedicated Storage System Vendors  (Done by the Edison Group)

Windows Storage Server 2008 R2 
TechNet Overview:
Getting Started Guide:
Known Issues List:
OEM Deployment Guide: Download the guide here.
OEM Partners:

Find out more about Specialized Server Solutions and how to become an OEM Partner.

MSDN and TechNet Download:
Embedded OEM Trial Software:  The Embedded Server Evaluation Website now has the Windows Storage Server evaluation package available for IHVs, OEMs, ISVs, consultants and VARs to evaluate and test the product. The download is available after a quick registration page.  

Written by IT Core

November 23, 2010 at 12:59 PM

System Center Data Protection Manager 2010 Documentation

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Written by IT Core

September 21, 2010 at 8:24 PM

Documentation for Dynamic Memory Resources

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From Virtual PC Guy’s Blog, here’s a documentation list for Dynamic Memory included in in the Windows Server 2008 R2 SP1 beta release:

Dynamic Memory Technical Overview whitepaper
This paper talks about what dynamic memory is, what it does and how it does it.  It is a good read for those curious to understand what is happening under the covers.

Hyper-V Dynamic Memory Evaluation Guide
This article steps you through the process of setting up SP1 and enabling dynamic memory.  It also gives you some good tips on configuration and troubleshooting.

Windows Server 2008 R2 Service Pack 1 – TechNet Center
For all things “Windows Server 2008 R2 Service Pack 1” related – the TechNet Center has you covered.

Hyper-V: Dynamic Memory Survival Guide
The TechNet Wiki also has a “Dynamic Memory Survival Guide” article with some extra links.

Thanks Ben 🙂

Written by IT Core

July 15, 2010 at 12:53 AM

Hyper-V cluster limits were extended to 384 virtual machines per host

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As you may know until now only 64 VM’s were supported on a clustered Hyper-V host. Microsoft changed the support limits of virtual machines running in a clustered environment to 1000 VM’s per cluster with a maximum of 384 VM’s per node. This means that clusters with 2 nodes will be able to run a maximum of 2 x 384 = 768 VMs. With a cluster with 3 Nodes you should be able to run 333 VMs per node. In a cluster with the maximum nodes allowed (16 physical nodes), you should be able to run 62 VMs per node…


Requirements and Limits for Virtual Machines and Hyper-V in Windows Server 2008 R2


Written by IT Core

June 15, 2010 at 8:50 PM

Posted in Documentation, News, Virtualization

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TechEd 2010 Virtualization Sessions

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Here’s some interesting sessions for virtualization from TechEd.

Networking and Windows Server 2008 R2 Hyper-V: Deployment Considerations


Microsoft System Center Virtual Machine Manager 2008 R2: Advanced Virtualization Management

The Microsoft System Center Operations Manager Top 20 Must-Have Customizations

Microsoft System Center Operations Manager and Virtual Machine Manager: Monitoring the Service Stack

See the Largest Mission Critical Deployment of Microsoft SQL Server around the World

Check the Latest Videos from TechEd North America

Have Fun 😀

Hyper-V and Dynamic Memory in Depth by Benjamin Armstrong

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For those of you that read/view the “Dynamic Memory in Hyper-V on Windows Server 2008 R2 SP1” now you can also see in TechEd – Hyper-V and Dynamic Memory in Depth by Benjamin Armstrong.

Dynamic memory is a new feature of Hyper-V coming to Windows Server 2008 R2 SP1. Come to learn how Dynamic Memory enables Hyper-V to more efficiently utilize system resource, and how this can benefit your environment. Also learn how Dynamic Memory works under the covers and what you should be preparing for with the release of Windows Server 2008 R2 SP1.

Written by IT Core

June 10, 2010 at 8:50 PM

Posted in Documentation, Videos, Virtualization

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