When you need to create a point-in-time backup of a CSV volume, you must create a stable environment. Conducting backups of a CSV volume requires special considerations and new APIs that backup applications must use to work in a CSV environment. When a backup of a CSV volume is initiated on a Hyper-V Cluster node, that particular CSV volume must be locally mounted on (or moved to) the machine running the backup. In addition, all remote application disk I/O to and from the CSV volume is re-routed over Server Message Block for the duration of the backup, instead of using direct I/O. Backup applications that haven't been modified to use the new VSS API will not be able to safely backup a CSV volume. Microsoft is working with the ISV and independent hardware vendor communities to evangelize the changes required for CSV Backup/Restore. You should verify with your backup vendor that they explicitly support CSV by using the new PrepareVolumeForSnapshotSet API call.
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