When the database is running in NOARCHIVELOG mode then you can take only offline full database backups. i.e. you cannot take online backups.
To take full backup using RMAN when the database is in NOARCHIVELOG mode, the database should not be opened but it should be in the mounted state
To take offline full database backup using RMAN do the following
Assuming the database is not started. Set the ORACLE_SID environment variable to your database SID and start RMAN by typing the following command
$rman target system/manager nocatalog
When you type the above command, RMAN will start with the following messages and you will get the RMAN prompt
Start the instance and mount the database by typing the following command at RMAN prompt
RMAN> startup mount;
To take full database backup type the following command
RMAN> backup full database tag 'myfullbackup';
The above the command will backup the full database and it will create the backupsets in the directory which is set in the DB_RECOVERY_FILE_DEST parameter. In our case the parameter is set to the following path "/u02/oracle/testdb/flash"
If you want to create the backupsets in other directory path then, you have to include the FORMAT option in the above command. For example the following command will create backupsets in the "/u02/oracle/mybackup" directory
RMAN> backup full database format '/u02/oracle/mybackup/%U' tag 'myfullbackup'
The %U means RMAN should generate unique filenames for backupsets
That's it the backup is over. We can start the database by typing the following
RMAN> alter database open;
If you want you can view the backupsets by going into the following directory using O/s commands