What mode should I keep the standby database ????

If you keep a standby database in Read Only mode then it will not apply archivals.

Quote:

While a standby database is open for read-only or read/write access, it does not apply redo data received from the primary database, thus it is not kept transactionally consistent with the primary database. When a physical standby database is open, redo data from the primary database is received by the standby database, but the log files are not applied. At some point, you need to resume Redo Apply on the standby database, and apply the archived redo log files to resynchronize the standby database with the primary database.Because of the additional time required to apply any accumulated archived redo log files, having a standby database open for read-only access can increase the time required to complete failovers or switchovers To open a standby database for read-only access when it is currently performing Redo Apply: 1. Cancel Redo Apply: SQL> ALTER DATABASE RECOVER MANAGED STANDBY DATABASE CANCEL; 2. Open the database for read-only access: SQL> ALTER DATABASE OPEN; You do not need to shut down the instance to open it for read-only access. To change the standby database from being open for read-only access to performing Redo Apply: 1. Terminate all active user sessions on the standby database if there. 2. Restart Redo Apply. To start Redo Apply, issue the following statement: SQL> ALTER DATABASE RECOVER MANAGED STANDBY DATABASE DISCONNECT FROM SESSION; To enable real-time apply, include the USING CURRENT LOGFILE clause: SQL> ALTER DATABASE RECOVER MANAGED STANDBY DATABASE USING CURRENT LOGFILE; You do not need to shut down the instance to start either of these apply modes .