Offload supported backups to secondary replicas of an availability group
Applies to: 
SQL Server
The Always On availability groups active secondary capabilities include support for taking backups on secondary replicas. Backup operations can put significant strain on I/O and CPU (with backup compression). Offloading backups to a synchronized or synchronizing secondary replica allows you to use the resources on server instance that hosts the primary replica for your tier-1 workloads.
Note
RESTORE statements are not allowed on either the primary or secondary databases of an availability group.
Backup Types Supported on Secondary Replicas
BACKUP DATABASE supports only copy-only full backups of databases, files, or filegroups when it's executed on secondary replicas. Copy-only backups don't impact the log chain or clear the differential bitmap.
Differential backups aren't supported on secondary replicas.
Concurrent backups, such as executing a transaction log backup on the primary replica while a full database backup is executing on the secondary replica, is currently not supported.
BACKUP LOG supports only regular log backups (the COPY_ONLY option is not supported for log backups on secondary replicas).
A consistent log chain is ensured across log backups taken on any of the replicas (primary or secondary), irrespective of their availability mode (synchronous-commit or asynchronous-commit).
To back up a secondary database, a secondary replica must be able to communicate with the primary replica and must be SYNCHRONIZED or SYNCHRONIZING.
In a distributed availability group, backups can be performed on secondary replicas in the same availability group as the active primary replica, or on the primary replica of any secondary availability groups. Backups cannot be performed on a secondary replica in a secondary availability group because secondary replicas only communicate with the primary replica in their own availability group. Only replicas that communicate directly with the global primary replica can perform backup operations.
Configuring Where Backup Jobs Run
Performing backups on a secondary replica to offload the backup workload from the primary production server is a great benefit. However, performing backups on secondary replicas introduce significant complexity to the process of determining where backup jobs should run. To address this, configure where backup jobs run as follows:
Configure the availability group to specify which availability replicas where you would prefer backups to be performed. For more information, see AUTOMATED_BACKUP_PREFERENCE and BACKUP_PRIORITY parameters in CREATE AVAILABILITY GROUP (Transact-SQL) or ALTER AVAILABILITY GROUP (Transact-SQL).
Create scripted backup jobs for every availability database on every server instance that hosts an availability replica that is a candidate for performing backups. For more information, see the "Follow Up: After Configuring Backup on Secondary Replicas" section of Configure Backup on Availability Replicas (SQL Server).
Related Tasks
To configure backup on secondary replicas
To determine whether the current replica is the preferred backup replica
To create a backup job
See Also
Overview of Always On Availability Groups (SQL Server)
Copy-Only Backups (SQL Server)
CREATE AVAILABILITY GROUP (Transact-SQL)
ALTER AVAILABILITY GROUP (Transact-SQL)