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On my server I have several VMs on drive D:. I'm using Windows Server Backup 2012 to back them up.

When I'm asked to select the items to backup, what's the difference between choosing the entries from the Hyper-V tree vs. choosing the partition, where the VMs are located on, itself?

Select items to backup in Windows Server Backup 2012

I found out that when I leave drive D: unticked, the backup will fail on most days (but not always).

So, is there a downside when I don't select the Hyper-V tree but just drive D: instead? Since all VHD(X) files and all settings are stored on drive D:, this shouldn't make a difference, should it?

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I would think... that one method uses APIs to intelligent freeze virtual machines so that their VSS snapshots of the VHD files are crash consistent, while the other just does a block level snapshot. –  SpacemanSpiff Mar 4 '13 at 3:20

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up vote 4 down vote accepted

Backing up the D: drive simply attempts to back up all the files on the D: drive.

The Hyper-V specific methods of backing up VMs use the Volume Shadowcopy Service and VSS requestors and writers to allow the guest OS to intelligently participate in its own backup process. The success of this method is highly contingent upon the Integration Services/tools being properly installed and enabled on the guest OS.

If you do not backup the VMs through Hyper-V, another way to get good backups of your VMs is to simply set up Windows Server Backup on each one of your guest VMs themselves.

You will not get good backups of your VMs if you simply copy the VHD[X] files while the VM is running, because the VMs are in a volatile state, with data in RAM, etc.

As for why the backup is failing when set to a certain configuration, we would need all the relevant events and errors.

Backing Up Virtual Machines

Hyper-V uses one of two mechanisms to back up each VM. The default backup mechanism is called the "Saved State" method, where the VM is put into a saved state during the processing of the PrepareForSnapshot event, snapshots are taken of the appropriate volumes, and the VM is returned to the previous state during the processing of the PostSnapshot event.

The other backup mechanism is called the "Child VM Snapshot" method, which uses VSS inside the child VM to participate in the backup. For the "Child VM Snapshot" method to be supported, all of the following conditions must be met:

Backup (volume snapshot) Integration Service is installed and running in the child VM. The service name is "Hyper-V Volume Shadow Copy Requestor".

Windows 2000: Backup Integration Service is not supported.

The child VM must be in the running state. The Snapshot File Location for the VM is set to be the same volume in the host operating system as the VHD files for the VM. All volumes in the child VM are basic disks and there are no dynamic disks. All disks in the child VM must use a file system that supports snapshots (for example, NTFS).

In general, the process for backing up VMs is the same as described in Overview of Processing a Backup Under VSS. The unique behavior happens when the Hyper-V VSS writer (part of the "Hyper-V Virtual Machine Management" service) processes the PrepareForSnapshot event. If the backup was done using the "Child VM Snapshot" method, there is additional processing done but it is not visible to the child VM.

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+1 Also, just backing up the VHD files might get you the drives, but not the configuration of the VM (CPUs, RAM, NICs/MACs, etc). –  Chris S Mar 4 '13 at 14:05

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