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.
Crash-consistent backup does not preserve 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:
Crash-consistent backup is performed in the following way:
- Veeam Backup & Replication interacts with the Hyper-V host VSS Services and requests 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.
Crash-consistent backup of VMs on Microsoft Hyper-V 2016 and later versions relies on production checkpoints. Crash-consistent backup is performed in the following way:
- Veeam Backup & Replication requests a production checkpoint of a specific VM.
- A production checkpoint is created in the crash-consistent state. Every virtual disk of a VM receives a temporary AVHDX file. All new writes are redirected to temporary AVHDX files.
- Further activities depend on the backup mode:
- In the onhost backup mode, Veeam Backup & Replication reads data from VM disks in the read-only state. After the VM processing completes, the production checkpoint is merged with the original VM.
- In the offhost backup mode, the Microsoft Hyper-V host VSS provider takes a snapshot of a volume on which VM disks are located.
The volume snapshot is mounted to the offhost backup proxy and presented to Veeam Backup & Replication. Veeam Backup & Replication reads VM data from the volume snapshot. After the backup job completes, the volume snapshot and checkpoint are deleted.