Crash-Consistent Backup

Crash-consistent backup is Veeam’s proprietary method of creating crash-consistent VM images. A crash-consistent image can be compared to the state of a VM that has been manually reset. Unlike offline backup, crash-consistent backup does not require any downtime of a VM.

Important

Crash-consistent backup does not preserve the data integrity of open files of transactional applications on the VM guest OS and may result in data loss.

The procedure of crash-consistent backup depends on the Microsoft Hyper-V Server version:

Microsoft Hyper-V Server 2012 R2 and earlier
Microsoft Hyper-V Server 2016 and later

Microsoft Hyper-V Server 2012 R2 and Earlier

Crash-consistent backup is performed in the following way:

Veeam Backup & Replication interacts with the Hyper-V host VSS Services and requests a backup of a specific VM.
The Hyper-V host VSS Writer notifies the VSS provider that volume snapshots can be taken.
The Hyper-V host VSS provider creates a snapshot of the requested volume.
The volume snapshot is presented to Veeam Backup & Replication. Veeam Backup & Replication reads VM files from the volume snapshot using one of two backup modes — on-host backup or off-host backup. After the backup is completed, the snapshot is deleted.

Microsoft Hyper-V Server 2016 and Later

Crash-consistent backup of VMs on Microsoft Hyper-V 2016 and later versions relies on standard checkpoints. Crash-consistent backup is performed in the following way:

Veeam Backup & Replication requests a standard checkpoint of a specific VM.
Further activities depend on the backup mode:

In the on-host backup mode, Veeam Backup & Replication reads data from VM disks in the read-only state. After the VM processing is complete, the standard checkpoint is merged with the original VM.
In the off-host backup mode, the Microsoft Hyper-V host VSS provider takes a snapshot of the volume on which VM disks are located.

The volume snapshot is mounted to the off-host backup proxy and presented to Veeam Backup & Replication. Veeam Backup & Replication reads VM data from the volume snapshot. After the backup job is complete, the volume snapshot and checkpoint are deleted.