For backup of VMs on local storage and CSV, Veeam Backup & Replication leverages the Microsoft VSS framework and Microsoft Hyper-V VSS components. Veeam Backup & Replication acts as a Microsoft VSS requestor and communicates with the Microsoft VSS framework. It obtains information about available Microsoft VSS components from Microsoft VSS, prescribes what components Microsoft VSS must use, identifies volumes where VM files are located, and triggers the Microsoft VSS coordinator to create volume snapshots.
Before a snapshot of a volume is created, VMs on the volume must be quiesced: there must remain no incomplete transactions, open files and so on. Veeam Backup & Replication uses three methods to prepare Microsoft Hyper-V VMs for backup:
- Online backup (also known as Child VM Snapshot method) — native Microsoft Hyper-V mechanism that allows you to create an application-consistent image of running VMs without any downtime.
- Offline backup (also known as Saved State method) — native Microsoft Hyper-V mechanism that requires some VM downtime. To quiesce guest OS operations for a VM, Microsoft Hyper-V brings the VM to the save state (hibernates it).
Offline backup is not applicable to VMs registered on Microsoft Hyper-V Server 2016.
- Crash-consistent backup — Veeam’s proprietary mechanism that allows you to create a crash-consistent VM backup. During crash-consistent backup, VMs are not brought to the saved state.
Whenever possible, Veeam Backup & Replication uses online backup to quiesce VMs. If online backup cannot be performed, Veeam Backup & Replication employs one of the other two methods to prepare a VM for backup.
- If online backup is cannot be performed, Veeam Backup & Replication fails over to the crash-consistent backup.
- If you do not want to produce a crash-consistent backup, you can instruct Veeam Backup & Replication to use the offline backup method.
Veeam Backup & Replication does not fail over to the crash-consistent backup mode if you enable application-aware processing for a job and enable the Require successful processing option. In such situation, if application-aware processing fails, Veeam Backup & Replication will terminate the job with the Error status.