## 2.3 Operation Modes for SAP HANA System Replication
While registering the secondary system, you need to decide in which operation mode to run SAP HANA system replication.
System replication can be run in three operation modes: delta\_datashipping , logreplay or logreplay\_readaccess . Depending on the configured operation mode, the database sends different types of data packages to the secondary system. For more information, see Data Transferred to the Secondary System .
| Operation Mode       | Description                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|----------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| delta_datashipping   | This mode establishes a system replication where occasionally (per default every 10 minutes) a delta data shipping takes place in addition to the continuous log shipping. The secondary system persists the received log entries but it does not replay them until it has to take over. To shorten the log replay time, data snapshots are transmitted from time to time from the primary to the secondary system. The data snapshots are transferred asynchronously as dif­ ferential backups (data backup deltas) triggered by the secondary system, which asks for a data backup delta with changes since the last one. During takeover the redo log needs to be replayed up to the last arrived delta data shipment.                                                       |
| logreplay            | In this operation mode, a redo log shipping is done after system replication was initially config­ ured with one full data shipping.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
| logreplay_readaccess | This mode is required for replication to an Active/Active (read enabled) secondary system. Using this operation mode while configuring your system replication, read access becomes possi­ ble on the secondary system by establishing a direct connection to the secondary system or by providing a SELECT statement from the primary system with a HINT. For more information , see also Client Support for Active/Active (Read Enabled) and SAP HANASQLReference Guide. This operation mode is similar to the logreplay operation mode regarding the continuous log shipping, the redo log replay on the secondary system, as well as the required initial full data ship­ ping and the takeover. As with the logreplay operation mode, the redo log is replicated to the sec­ |
##  Note
In a multitier or multitarget system replication it is not possible to combine the logreplay and delta\_datashipping operation modes.
## Limitations
Before you begin preparing a replication strategy for an SAP HANA system, consider the following important aspects regarding the operation modes logreplay and logreplay\_readaccess :
- Registering a secondary with operation mode logreplay against a primary running on an SAP HANA revision less than or equal to SPS10 will not work, because the primary does not yet support this feature. Furthermore, for operation mode logreplay\_readaccess the primary must be running on a revision SAP HANA 2 SPS00 or higher.
- Only the operation mode delta\_datashipping will work when registering the original primary (failback) after upgrade of the secondary during a near zero downtime upgrade from an SAP HANA revision less than or equal to SPS 10 to SPS 11, because the former primary's version does not yet support logreplay .
- If the connection to the secondary is not available, the primary system will keep writing the redo log segments in the online log area to be prepared for the delta log shipping after the connection is reestablished. These log segments are marked as RetainedFree until the secondary is in sync again. In this case there is a risk that the log volume may run full. To prevent this:
- If a secondary is not used anymore, it must be unregistered (sr\_unregister).
- If a takeover to the secondary was done, the former primary should be disabled (sr\_disable).
For more information, see How to Avoid Log Full Situations in LogReplay: Managing the Size of the Log File .
- The logreplay operation modes do not support history tables.
##  Note
If you plan to add SAP HANA dynamic tiering to your landscape in the future, please check supported operation modes in SAP Note 2447994 before you enable SAP HANA system replication.
##  Note
When selecting an operation mode, keep in mind that the selected operation mode impacts the network throughput requirements of the communication channel used in SAP HANA system replication. For more information about this, see Network Recommendations .
For information about how to change the operation mode, see Changing the Operation Mode .
## Related Information
Data Transferred to the Secondary System [page 16]
Active/Active (Read Enabled) SAP HANA System Replication Command Line Reference [page 71] LogReplay: Managing the Size of the Log File [page 227]