## 3.4 Full Sync Option for SAP HANA System Replication
To reach a true RPO=0 for synchronous system replication, the full sync option can be enabled for SYNC replication mode.
With the activated full sync option, the transaction processing on the primary system blocks when the secondary is currently not connected and newly created log buffers cannot be shipped to the secondary
system. This behavior ensures that no transaction can be committed on the primary without shipping the log buffers to the secondary system.
The full sync option can be switched on and off using the command:
hdbnsutil -sr\_fullsync --enable|--disable
This changes the setting of the parameter enable\_full\_sync in the [system\_replication] section of the global.ini file accordingly.
##  Recommendation
When configuring the full sync option, proceed as follows:
1. Configure your system replication with the SYNC replication mode. This replication mode is the prerequisite for enabling the full sync option.
2. Check that the system replication status is active and in sync for all services.
3. Enable the full sync option with hdbnsutil -sr\_fullsync --enable
In the M\_SERVICE\_REPLICATION system view, the setting of the full sync option can be viewed in the column FULL\_SYNC . It can have the following values:
- DISABLED: Full sync is not configured at all.
The parameter enable\_full\_sync = false in the system\_replication section of the global.ini file.
- ENABLED: Full sync is configured, but it is not yet active. Transactions do not block in this state. To become active, the secondary has to connect and the replication status has to be ACTIVE.
- ACTIVE: Full sync mode is configured and active.
If the network connection to a connected secondary is closed, the transactions on the primary system will block in this state.
If full sync is enabled when an active secondary is currently connected, the FULL\_SYNC column will be immediately set to ACTIVE.
If the secondary is stopped, disable the full sync option. Otherwise the primary blocks and it is not possible to stop it.
##  Note
Use the hdbnsutil command to resolve a blocking situation of the primary system caused by the enabled full sync option. This is important because a configuration changing command could also block in this state. You must do this also when you want to shut down the currently blocking primary system. Otherwise it is not possible to stop it.
The Cluster Manager that is used to operate SAP HANA system replication landscapes could provide a timeout after which the blocking situation is resolved automatically using the hdbnsutil command and deactivating the full sync option. However, after the reason for the blocking situation disappears, you must activate the full sync option again (manually or automatically with the help of the cluster manager tool).
In a multitarget system replication setup, configure the full sync option on the primary system. Enter the site name used for the secondary system when registering it:
```
global.ini [system_replication] enable_full_sync[<secondary_site_name>] = true
```
##  Note
In a multitarget system replication setup, you can use hdbnsutil -sr\_fullsync to turn off the full sync option.
## Related Information
SAP HANA System Replication Command Line Reference [page 71]