Offline backup (or saved state backup) is a native Microsoft Hyper-V approach to quiesce VMs before taking a volume snapshot. This type of backup requires some downtime of a VM. When a VM is backed up, the Microsoft Hyper-V VSS Writer forces the VM into the saved state (hibernates it) to create a stable system image.
Offline backup is not applicable to VMs registered on Microsoft Hyper-V Server 2016 or later.
Offline backup is performed in the following way:
- Veeam Backup & Replication interacts with the Microsoft Hyper-V host VSS Services and requests backup of a specific VM.
- The Microsoft Hyper-V host VSS Writer forces a VM into the saved state for several seconds. The VM OS hibernates, and the content of the system memory and CPU is written to a dump file.
- The Hyper-V host VSS provider takes a snapshot of a volume on which VM disks are located. The VM returns to the normal state.
- The volume snapshot is presented to Veeam Backup & Replication. Veeam Backup & Replication reads VM data from the volume snapshot in one of two backup modes — on-host backup or off-host backup. After the backup is complete, the volume snapshot is deleted.
In the production environment, offline backup may be inappropriate. It implies VM downtime and does not produce transactionally consistent backups and replicas as data on the VM is not quiesced before a volume snapshot is taken. As an alternative to offline backup, Veeam Backup & Replication offers the crash-consistent backup method for those cases when online backup cannot be used, and offline backup is inappropriate.