RMAN Backup of Oracle Database

[oracle@ora1 ~]$ sqlplus / as sysdba

SQL> archive log list;

SQL> show parameter recovery_file_dest;

SQL> alter system set db_recovery_file_dest_size=75g scope=both;

SQL> archive log list;

SQL> shutdown immediate;

SQL> startup mount;

SQL> alter database archivelog;

SQL> alter database open;

SQL> archive log list;

SQL> exit;

[oracle@ora1 ~]$ rman target /;

RMAN> backup database plus archivelog;