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Q. How does a Volume Shadow Copy Service (VSS) snapshot backup of a Hyper-V server work with guest OSs?
John Savill
June 23, 2009
3 Min Read
A. Hyper-V has a VSS writer that's registered with the backup software on the server. When you perform a VSS backup on the Hyper-V server, something very cool happens in the guest OSs, assuming you have Hyper-V Integration Services installed and the backup service enabled.
The backup software (the VSS requestor) on the Hyper-V server requests a VSS snapshot. The VSS requestor lists the VSS writers on the system to figure out what data the VSS writer can back up. The Hyper-V VSS writer, in conjunction with the VSS Coordination Service, forwards the VSS snapshot request to each guest OS via the backup integration service.
Each guest OS thinks it's receiving a native VSS request. Each one proceeds to notify all VSS writers on the guest OS to prepare for a snapshot.
Each VSS writer in the guest OSs writes any information to disk that relates to its service, such as Microsoft Exchange or SQL Server. The guest OS VSS writers notify the VSS coordinator that they're ready for a snapshot and tell it which data to back up. The part about which data to back up is ignored, however, because you'll be backing up the entire Virtual Hard Disk (VHD) from the Hyper-V host.
The backup integration service for each VM tells Hyper-V's VSS writer it's ready for a snapshot to be taken. The Hyper-V VSS writer notifies the backup application via the VSS coordinator that it is ready for a snapshot.
The backup software takes a VSS snapshot of the host file system that contains the virtual configuration files and the VHDs. All data on the VHDs is consistent because of the VSS request being passed into the VMs. Once the snapshot is taken, the VSS writer notifies the guests that the snapshot is complete and they continue their normal processing.
As you can see, you can take backups from the Hyper-V host and still take full advantage of all the backup integrity provided by the VSS writers within the guest OSs to make sure the content on disk is complete.
It's very important that when you perform the backup, all disks that contain VM configuration files and VHDs are selected as part of the backup.
You should note that only VHD content will be backed up using this method. If a VM has pass through or iSCSI storage storage connected through the guest OS iSCSI initiator, that content won't be backed up through a backup at the Hyper-V server level through the Hyper-V VSS Writer.
The scenario above describes an online backup, also known as child VM snapshot, where the guest OS meets the following requirements:
Integration services is installed, with the backup integration service enabled
It supports VSS
It uses NTFS file systems with basic disks (not dynamic)
If you have guest OSs that use dynamic disks, use non-NTFS partitions, don't have integration services installed, or don't have the backup integration service enabled, or if you're using an OS that isn't supported, such as Windows 2000, then an offline backup will be taken of the VM. This backup is also known as a saved state backup, because VMs that can't support an online backup are placed into a saved state during the VSS snapshot, so there's a period of downtime for the VM during the backup. OSs that have to use saved state include Windows XP, Windows 2000, Windows NT 4, and Linux OSs. Windows Server 2008, Windows Vista, Windows Server 2003, and later OSs all support the online backup method without any VM downtime.
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