To take backup manually or when cloning a database using the following SQL one can place the database in backup mode.

SQL> ALTER DATABASE BEGIN BACKUP;
Database altered.

To find if database or any tablespace is in backup mode, the status in V$BACKUP is ACTIVE
SQL> select * from v$backup;

FILE# STATUS CHANGE# TIME
———- —————— ———- ———
1 ACTIVE 11453595 31-OCT-10
2 ACTIVE 11453595 31-OCT-10
3 ACTIVE 11453595 31-OCT-10
4 ACTIVE 11453595 31-OCT-10

… perform database manual or cloning
SQL> ALTER DATABASE END BACKUP;
Database altered.

To check if database is in backup mode
SQL> select * from v$backup;

FILE# STATUS CHANGE# TIME
———- —————— ———- ———
1 NOT ACTIVE 11453595 31-OCT-10
2 NOT ACTIVE 11453595 31-OCT-10
3 NOT ACTIVE 11453595 31-OCT-10
4 NOT ACTIVE 11453595 31-OCT-10

If one tries to perform shutdown of database when database is in BACKUP mode, you will receive the following error “ORA-01149”
SQL> shutdown immediate;
ORA-01149: cannot shutdown – file 1 has online backup set
ORA-01110: data file 1: ‘/u01/oradata/TEST/system_01.dbf’

If one tries to set the database that is already in backup mode, one will receive the following error “ORA-01146”
SQL> alter database begin backup;
alter database begin backup
*
ERROR at line 1:
ORA-01146: cannot start online backup – file 1 is already in backup
ORA-01110: data file 1: ‘/u01/oradata/TEST/system_01.dbf’

In 9i to set database is backup mode, “ALTER DATABASE BEGIN/END BACKUP” doesn’t exists so it has to be done at individual table space level by executing “ALTER TABLESPACE <tablespace> BEGIN/END BACKUP”.

If one tries to perform incremental backup using RMAN it will skip backing up datafiles as datafiles are in backup
RMAN> backup incremental level 1 database;

Starting backup at 31-OCT-10
using channel ORA_DISK_1
RMAN-06554: WARNING: file 1 is in backup mode
RMAN-06554: WARNING: file 2 is in backup mode
RMAN-06554: WARNING: file 3 is in backup mode
RMAN-06554: WARNING: file 4 is in backup mode
channel ORA_DISK_1: starting incremental level 1 datafile backupset
channel ORA_DISK_1: specifying datafile(s) in backupset
input datafile fno=00003 name=/u01/oradata/TEST/sysaux_01.dbf
skipping datafile 00003 because it has not changed
input datafile fno=00001 name=/u01/oradata/TEST/system_01.dbf
skipping datafile 00001 because it has not changed
input datafile fno=00002 name=/u01/oradata/TEST/undo_01.dbf
skipping datafile 00002 because it has not changed
input datafile fno=00004 name=/u01/oradata/TEST/test_01.dbf
skipping datafile 00004 because it has not changed
channel ORA_DISK_1: backup cancelled because all files were skipped