Changing the size and number of the online redo log files is sometimes done to tune the database. You can add online redo log file groups or members to the primary database without affecting the standby database. Similarly, you can drop log file groups or members from the primary database without affecting your standby database. However, these changes do affect the performance of the standby database after switchover.
Whenever you add an online redo log file to the primary database, you must add a corresponding standby redo log file to the standby database.
For example, if the primary database has 10 online redo log files and the standby database has 2, and then you switch over to the standby database so that it functions as the new primary database, the new primary database is forced to archive more frequently than the original primary database.
Consequently, when you add or drop an online redo log file at the primary site, it is important that you synchronize the changes in the standby database by following these steps:
(1) If Redo Apply is running, you must cancel Redo Apply before you can change the log files.
(2) If the STANDBY_FILE_MANAGEMENT initialization parameter is set to AUTO, change the value to MANUAL.
(3) Add or drop an online redo log file:
(3-1) To add an online redo log file, use a SQL statement such as this:
SQL> ALTER DATABASE ADD LOGFILE
‘/disk1/oracle/oradata/payroll/prmy3.log’ SIZE 100M;
(3-2) To drop an online redo log file, use a SQL statement such as this:
SQL> ALTER DATABASE DROP LOGFILE
(4) Repeat the statement you used in Step 3 on each standby database.
(5) Restore the STANDBY_FILE_MANAGEMENT initialization parameter and the Redo Apply options to their original states.
Ref : http://download-west.oracle.com/docs/cd/B14117_01/server.101/b10823/manage_ps.htm#1010567