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Bare metal restore - system time

    Question

  • I recently did a restore from an old metal backup and everything went well. Just one very interesting thing that I can't understand.

    - Let's assume the backup was made Jan/1/2015

    - I set the time/date on the machine used for restore back in time to be Jan/2/2015, one day after the backup date

    - The machine is not sync'ing time with host (it's a vm)

    - The machine resides on an isolated network so it's on its own

    Once the restore completed and system restarted, the OS time is set to current time! I just can't understand how it knows the current time, and what's the purpose for AD or OS to set the time forward (it must be designed that way?).

    Friday, February 3, 2017 6:27 PM

Answers

  • Hi,

    I found this:

    Unfortunately virtual machines do not have any “batteries”.  When a virtual machine is turned off there is no component that keeps track of time for it.  Instead – whenever you start a virtual machine we take the time from the management operating system and put this into the real-time clock of the virtual machine.

    This is done without the use of the Hyper-V time synchronization integration servers (it happens long before the integration services have loaded). 

    https://blogs.msdn.microsoft.com/virtual_pc_guy/2010/11/19/time-synchronization-in-hyper-v/

    Also, look for event id 35/37 in system-time service. That will tell what time source is used.


    Best Regards,

    Jesper Vindum, Denmark

    Systems Administrator

    Help the forum: Monitor(alert) your threads and vote helpful replies or mark them as answer, if it helps solving your problem.

    • Marked as answer by strongline Monday, February 6, 2017 3:28 PM
    Sunday, February 5, 2017 6:06 AM

All replies


  • I just can't understand how it knows the current time, and what's the purpose for AD or OS to set the time forward (it must be designed that way?).

    The BIOS I guess. When you restore a server you do not restore the BIOS. It is all there, sitting once the restore is completed and will trigger it's change to the machine. 

    Mahdi Tehrani   |     |   www.mahditehrani.ir
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    Sunday, February 5, 2017 4:11 AM
    Moderator
  • Hi,

    I found this:

    Unfortunately virtual machines do not have any “batteries”.  When a virtual machine is turned off there is no component that keeps track of time for it.  Instead – whenever you start a virtual machine we take the time from the management operating system and put this into the real-time clock of the virtual machine.

    This is done without the use of the Hyper-V time synchronization integration servers (it happens long before the integration services have loaded). 

    https://blogs.msdn.microsoft.com/virtual_pc_guy/2010/11/19/time-synchronization-in-hyper-v/

    Also, look for event id 35/37 in system-time service. That will tell what time source is used.


    Best Regards,

    Jesper Vindum, Denmark

    Systems Administrator

    Help the forum: Monitor(alert) your threads and vote helpful replies or mark them as answer, if it helps solving your problem.

    • Marked as answer by strongline Monday, February 6, 2017 3:28 PM
    Sunday, February 5, 2017 6:06 AM
  • Thanks Jesper - although I was not using hyper-v, but what you quoted makes sense. I assume same logical can be applied to vmware too.

    Edit: when I gave it second thought, can't the host keep track of time - on behalf of guest BIOS - for individual guests? It shouldn't be too difficult from a programmer's point of view. When a guest is shut off, the host records its time, when the guest comes back online, host set the time again in guest BIOS before the guest is actually powered up (last recorded time plus the duration of off time).

    • Edited by strongline Monday, February 6, 2017 3:37 PM
    Monday, February 6, 2017 3:30 PM