AlwaysOn Availability Groups allow you to increase fault tolerance between active and hot-standby databases without involving shared physical disks, which is quite important for virtualization of Microsoft SQL Servers. Veeam Backup & Replication supports AlwaysOn Availability Groups for virtualized Microsoft SQL Server 2012 and later.
Image-level Backup of Microsoft SQL Server VMs
During image-level backup of a Microsoft SQL Server VM, Veeam Backup & Replication requests and analyzes information about databases that are included in the AlwaysOn Availability Groups. Depending on the retrieved information, Veeam Backup & Replication creates a VSS snapshot with or without COPY_ONLY flag. The VSS_BS_COPY flag for VSS snapshot is triggered if the VM represents a secondary node for at least one AlwaysOn Availability Group.
Veeam Backup & Replication also detects to what cluster the database belongs. If the backup job does not include all VMs from the cluster, a warning message will be issued.
Retrieved information is saved for further log identification.
Transaction Log Backup
Transaction log backup can be performed only for those databases that were successfully backed up, either on the primary or on the secondary node of AlwaysOn Availability Group.
The transaction logs processing interval may be the same or may differ through VMs included in AlwaysOn Availability Group. If the interval is different, Veeam Backup & Replication will use minimal value (by default, 15 minutes).
At each log processing interval, Veeam Backup & Replication chooses the AlwaysOn Availability Group node for which transaction logs will be backed up.
Logs are backed up from one node of the AlwaysOn Availability Group. To become a subject for log backup, the node must meet the following criteria:
- Required Veeam Backup & Replication components can be installed on this node (the VM must be running).
- If there are any logs remaining in the temporary folder on the node of AlwaysOn Availability Group, this means these logs were not backed up to the backup repository during the previous session of the transaction log backup job, so this AlwaysOn Availability Group node must be processed first.
- Databases in the AlwaysOn Availability Group(s) for this node were successfully backed up for the last two processing intervals.
- Veeam Backup & Replication can establish a network connection to the node or VIX connection, if a connection over the network cannot be established
- The VM is in the list of preferred nodes for backup retrieved from the Microsoft SQL Server. If there are no preferred nodes, any node can be chosen.
If you are using Basic Availability Groups on Microsoft SQL Server 2016 Standard Edition, consider that the secondary replica node cannot be backed up because the secondary node is not readable. For more information, see https://blogs.msdn.microsoft.com/alwaysonpro/2016/05/02/sql-server-2016-alwayson-availability-group-enhancements-basic-availability-group-on-standard-edition/.