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The inner workings of agentless backups vary from one vendor to the next. In the case of an agentless backup against a virtualization host, the first step is that the software queries the server in order to determine which resources exist and need to be backed up. In the case of a Hyper-V server, for example, a backup application could theoretically use PowerShell cmdlets and WMI calls to query the server's status.
The next step in the process usually involves creating a shadow copy of the virtual hard disks. Again, the exact method for doing so varies. In the case of a Hyper-V host, this process might make use of the default provider or it may use a VSS provider that has been supplied by the storage vendor.
Creating a shadow copy of the virtual hard disk makes it possible to back up the virtual machine while it is running, but depending on which vendor's solution is being used, there might not be any application awareness at this point in the process. If that is the case, then the backup software will typically look inside of the virtual hard disk and parse the file system in an effort to determine which operating system and applications are in use within the virtual machine.
Then, the backup software will have to take action to prepare the applications to be backed up. This may involve using VSS or it could involve using proprietary code to place the application (as it exists within the shadow copy) into a consistent state that can be cleanly backed up.
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