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What is the principle (process) of Hyper-V merge snapshot (differential disk)?

    Question

  • As the title, Hyper-V is creating multiple checkpoints (such as T1, T2 and T3). When I delete the T2 checkpoint, how is the difference disc (avhdx) generated at T2 merged? Even delete all checkpoints? How is it merged? In the process of merging, how do Hyper-V, VSS, etc. work?
    Thursday, August 9, 2018 8:04 AM

Answers

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  • "Hyper-V is creating multiple checkpoints"

    Hyper-V does not create multiple checkpoints on its own.  You are doing something to create those checkpoints. 

    The .avdx file contains the blocks of the .vhdx file that have been changed.  So when a merge occurs, it writes the changes found in the .avdx file into the .vhdx file so that the .vhdx file now looks as though a checkpoint was never taken.  

    Maybe an article like https://www.altaro.com/hyper-v/how-to-merge-hyper-v-checkpoints/ will help you understand things better.


    tim


    Thursday, August 9, 2018 12:11 PM
  • Hi,

    I agree with Tim.

    There is an official document about merging avhdx ,please refer to it.

    https://social.technet.microsoft.com/wiki/contents/articles/6257.manually-merge-avhd-to-vhd-in-hyper-v.aspx

    https://blogs.msdn.microsoft.com/virtual_pc_guy/2009/04/13/where-are-my-snapshot-files-hyper-v/


    Best Regards,
    Frank


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    Friday, August 10, 2018 3:09 AM

  • 我很感激看到你的回复。我的具体情况是:有些人使用C ++封装VSS接口以进行虚拟机备份和恢复。在时间T1,在我的控制台启动虚拟机的备份任务后,可以在Hyper-V管理器中看到:在虚拟机下生成恢复类型检查点(还将生成快照配置文件(xml)和差异磁盘(avhdx))。同样,在时间T2,我再次启动备份任务,并在Hyper-V管理器中生成恢复类型检查点(xml和avhdx)。不同之处在于,在时间T2生成的差异磁盘将合并到T1时间,因此只会删除一个检查点(在时间T2生成avhdx和xml删除)。因此我很想知道,手动在Hyper-V管理中创建的检查点和调用VSS创建的时间的过程到底有什么不同?为什么我这里产生的检查点都是recovery类型的?
    Monday, August 13, 2018 2:25 AM
  • I am very grateful to see your reply. My specific situation here is this: Some people use C++ to encapsulate the VSS interface for virtual machine backup and recovery. At time T1, after my console initiates a backup task for a virtual machine, it can be seen in the Hyper-V Manager that a recovery type checkpoint is generated under the virtual machine (the snapshot.xml will also be generated and difference disk(avhdx)). Similarly, at time T2, I initiate a backup task again, and a recovery type checkpoint (xml and avhdx) is generated in the Hyper-V Manager. The difference is that the difference disk generated at time T2 will be merged into T1 time, so only one checkpoint will be dropped (avhdx and xml deletion generated at time T2)

    So I would like to know how the process of manually creating a checkpoint in Hyper-V management and the time of calling VSS creation is different. Why are the checkpoints I generate here all of the recovery type?

    Monday, August 13, 2018 2:40 AM
  • Hi,

    There is a similar question with you,please refer to it.

    https://social.technet.microsoft.com/Forums/ie/en-US/39d8b7f7-8efa-48e5-8750-869d2ca8bf70/what-is-the-hyperv-recovery-snapshot-type?forum=winserverhyperv

    Best Regards,
    Frank


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    Monday, August 13, 2018 2:48 AM
  • yeah,3q
    Tuesday, August 14, 2018 8:34 AM
  • Hi,
    Just checking in to see if the information provided was helpful. Please let us know if you would like further assistance.

    Best Regards,

    Frank

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    Wednesday, August 15, 2018 6:11 AM
  • Yeah,The type of checkpoint is recovery, in the other word, it is a broken checkpoint. However, we do not care the type of checkpoint in actual use. The virtual machines' data has been copied to other storage media.

    However, there have snapshot remains in Hyper-V manager, which can only be deleted with PowerShell command. If I delete the snapshot, I have no way to incremental or differential backups of virtual machine.

    My company's products are mainly big data storage.

    Wednesday, August 15, 2018 7:13 AM
  • Hi,

    In general,the third-party backup software led to display the checkpoint which couldn't delete.I suppose you backup your OS via windows backup(WSB)

    https://social.technet.microsoft.com/Forums/forefront/en-US/fed74c7e-d9da-46c1-ae4f-42a80e760e57/cannot-delete-checkpoint-after-manually-merging-avhdxfiles?forum=winserverhyperv

    Best Regards,
    Frank


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    • Marked as answer by Chaplin-Wong Thursday, August 16, 2018 2:50 AM
    Wednesday, August 15, 2018 7:32 AM
  • Hi Frank, Thank you ! Maybe, I could not use WSB or DPM, The third-backup software be provided by us.
    Wednesday, August 15, 2018 8:20 AM