How to Back-up your Virtual Machine?

A Virtual Machine (VM) is a software execution of a computing environment in which an operating system or program can be installed and run.

When working with technology we should be very careful about the data. We use secure methods such as virus guards firewalls to protect our data but still there is a risk from sudden crashes, virus/malware attacks. In such situation happens you will lose all the data or a part of it. Therefor we need to back up our data in regular basis even it if is a Virtual Machine either Physical machine. 

There are two types of Back up mechanism, one is called “Saved State” (default in Hyper-V) and the other one is “Child VM Snapshot”. In “Saved State”, snapshots are taken of the correct volumes, and the VM is returned to the default state.

In "Child VM Snapshot", use VSS inside the child VM to participate in the backup. And you should meet the following conditions in this state. 

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

Backup Integration Service will not be supported for Windows 2000.

• The Child VM should be in the processing state.

• The Snapshot File Location for the VM is set to be the same volume in the host OS as the Virtual Hard Disk files for the Virtual Machine.

• All volumes in the child VM not dynamic disks (they are basic disks).

• All disks in the child VM must use a file system that supports snapshots (example, NTFS).

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.

The following procedure describes how to back up VMs.

1. For each VM in the writer metadata, if the "Saved State" method is used, the VM is put into a saved state. For VMs using the "Child VM Snapshot" method, the Hyper-V Volume Shadow Copy Requestor Service in the child VM processes the backup as detailed in Overview of Processing a Backup Under VSS. All VSS events in the child VM occur during the host operating system processing of the PrepareForSnapshot event.

2. After all VMs have either been put in the saved state or had snapshots taken, the Hyper-V VSS writer returns from the PrepareForSnapshot event. No processing is done by the Hyper-V VSS writer during the Freeze and Thaw events.

3. When the Hyper-V VSS writer processes the PostSnapshot event, VMs that were backed up using the "Saved State" method and were put into a saved state by the Hyper-V VSS writer are returned to the state they were in before the backup started. For the VMs that were backed up using the "Child VM Snapshot" method, the host image of the VHD files that had the snapshots taken are rolled back to the snapshot taken during the processing of the PrepareForSnapshot event. This processing is done independently of the VSS writers in the child VMs so the snapshots taken must be auto-recoverable. (VSS_VOLSNAP_ATTR_NO_AUTORECOVERY is not set in the context.)

 

Note  

• Partial backups are not supported. If any VM fails to create a snapshot, no VMs will be backed up.

• Pass-through and iSCSI disks are not visible to the host operating system and therefore not backed up by the Hyper-V VSS writer. Backups of these volumes must be done entirely within the VM.