RMAN备份时报“ORA-19504: failed to create file”和“ORA-27038: created file already exists”

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

报出以下错误：

released channel: ch00
RMAN-: ===========================================================
RMAN-: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-: ===========================================================
RMAN-: failure of backup command on ch00 channel at // ::
ORA-: failed to create file "/dbbackup/db_20150811"
ORA-: created file already exists Additional information: 

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 , 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 .

SQL> select name,block_size from v$controlfile;

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

SQL> show parameter db_block_size

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

SQL>