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Replication Modes for SAP HANA System

There are three different modes of replication for SAP HANA system 

Synchronous in-memory (default): 
Synchronous in memory (mode=syncmem) means the log write is considered as successful, when the log entry has been written to the log volume of the primary and sending the log has been acknowledged by the secondary instance after copying to memory.
When the connection to the secondary system is lost, the primary system continues transaction processing and writes the changes only to the local disk.
Data loss can occur when primary and secondary fail at the same time as long as the secondary system is connected or when a takeover is executed, while the secondary system is disconnected. This option provides better performance because it is not necessary to wait for disk I/O on the secondary instance, but is more vulnerable to data loss.


Synchronous

Synchronous (mode=sync) means the log write is considered as successful when the log entry has been written to the log volume of the primary and the secondary instance.
When the connection to the secondary system is lost, the primary system continues transaction processing and writes the changes only to the local disk.
No data loss occurs in this scenario as long as the secondary system is connected. Data loss can occur, when a takeover is executed while the secondary system is disconnected.
Additionally, this replication mode can run with a full sync option. This means that log write is successful when the log buffer has been written to the log file of the primary and the secondary instance. In addition, when the secondary system is disconnected (for example, because of network failure) the primary systems suspends transaction processing until the connection to the secondary system is re-established. No data loss occurs in this scenario. You can set the full sync option for system replication only with the parameter [system_replication]/enable_full_sync). For more information on how to enable the full sync option, see Enable Full Sync Option for System Replication.


Asynchronous

Asynchronous (mode=async) means the primary system sends redo log buffers to the secondary system asynchronously. The primary system commits a transaction when it has been written to the log file of the primary system and sent to the secondary system through the network. It does not wait for confirmation from the secondary system.
This option provides better performance because it is not necessary to wait for log I/O on the secondary system. Database consistency across all services on the secondary system is guaranteed. However, it is more vulnerable to data loss. Data changes may be lost on takeover.

The replication mode can be changed without going through a full data shipping from the primary system to the secondary system afterwards.



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