RMAN> run { 
2> allocate channel ch00 type disk; 
3> backup format '/dbbackup/db_%T' database; 
4> release channel ch00; 
5> }

报出以下错误：

released channel: ch00 
RMAN-00571: =========================================================== 
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS =============== 
RMAN-00571: =========================================================== 
RMAN-03009: failure of backup command on ch00 channel at 08/11/2015 17:32:39 
ORA-19504: failed to create file "/dbbackup/db_20150811" 
ORA-27038: created file already exists Additional information: 1

RMAN>

原因是控制文件和参数文件需要单独生成一个备份集，不能和数据库共用一个备份集，而在给数据文件指定备份集时又只指定一个单一的文件名，所以报了个文件已经存在的错误。 解决方法是在format里加一个%U，这样就会自动生成不同又唯一的文件名了。

控制文件之所以不能和数据文件共存于一个备份集是因为二者的block size不一样，参看ref2有一段话。

Note: 
If the control file block size is not equal to the block size for data file 1, then the control file cannot be written into the same backup set as the data file. RMAN writes the control file into a backup set by itself if the block size is different. The V$CONTROLFILE.BLOCK_SIZE column indicates the control file block size, whereas the DB_BLOCK_SIZE initialization parameter indicates the block size of data file 1.

 
SQL> select name,block_size from v$controlfile;

NAME                                                         BLOCK_SIZE 
------------------------------------------------------------ ---------- 
/u01/app/oracle/oradata/dbt/control01.ctl                         16384 
/u01/app/oracle/oradata/dbt/control02.ctl                         16384

SQL> show parameter db_block_size

NAME                                 TYPE        VALUE 
------------------------------------ ----------- ------------------------------ 
db_block_size                        integer     8192 

SQL>