备份指定数据文件方法有两种，指定数据文件和指定表空间

指定备份数据文件
RMAN> backup datafile 4;

Starting backup at 2017-07-30 13:09:40
using target database control file instead of recovery catalog
allocated channel: ORA_DISK_1
channel ORA_DISK_1: SID=18 device type=DISK
channel ORA_DISK_1: starting full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00004 name=/u01/app/oracle/oradata/test/users01.dbf
channel ORA_DISK_1: starting piece 1 at 2017-07-30 13:09:40
channel ORA_DISK_1: finished piece 1 at 2017-07-30 13:09:43
piece handle=/home/oracle/1asal8ck_1_1 tag=TAG20170730T130940 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:03
Finished backup at 2017-07-30 13:09:43


备份表空间
备份表空间的时候其实就是在备份表空间下面的数据文件
RMAN> backup tablespace users;

Starting backup at 2017-07-30 13:10:30
using channel ORA_DISK_1
channel ORA_DISK_1: starting full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00004 name=/u01/app/oracle/oradata/test/users01.dbf
channel ORA_DISK_1: starting piece 1 at 2017-07-30 13:10:30
channel ORA_DISK_1: finished piece 1 at 2017-07-30 13:10:31
piece handle=/home/oracle/1bsal8e6_1_1 tag=TAG20170730T131030 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:01
Finished backup at 2017-07-30 13:10:31