ORA-01123
Tried to make the whole database enter backup mode, but it failed with ORA-01123.
SQL> alter database begin backup;
alter database begin backup
*
ERROR at line 1:
ORA-01123: cannot start online backup; media recovery not enabled
ORA-01123 means that the archivelog mode is disabled, there's no media recovery to support online backup mode of the database.
Let's check the current archivelog mode of the database.
SQL> select log_mode from v$database;
LOG_MODE
------------
NOARCHIVELOG
As we can see, the archivelog mode is not enabled.
Solution
The solution is simple, you have to enable archivelog before entering backup mode.
Restart DB to Mount
SQL> shutdown immediate;
SQL> startup mount;
Enable Media Recovery
Which means to enable archivelog mode.
SQL> alter database archivelog;
Database altered.
Check Log Mode
SQL> select log_mode from v$database;
LOG_MODE
------------
ARCHIVELOG
Open DB
SQL> alter database open;
Database altered.
Begin Backup
SQL> alter database begin backup;
Database altered.
We're done.