1、环境描述：
Oracle 10g RAC ,在线修改db_files参数

2、操作如下

SQL> show parameter db_files

NAME                     TYPE    VALUE
------------------------------------ ----------- ------------------------------
db_files                 integer     200

SQL> alter system set db_files=1000 scope=spfile sid='*';

System altered.

SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup
ORACLE instance started.

Total System Global Area  536870912 bytes
Fixed Size          1268508 bytes
Variable Size         150996196 bytes
Database Buffers      381681664 bytes
Redo Buffers            2924544 bytes
ORA-01105: mount is incompatible with mounts by other instances
ORA-01174: DB_FILES is 1000 buts needs to be 200 to be compatible

--报错，提示数据库Mount与另一个实例mount不一致，修改后的参数为1000与另一个节点的参数200不一致。

3、解决方法
关闭第二个节点的实例，这样就是同时关闭了两个节点的实例，或者说需要关闭所有节点的实例。必然影响业务了。有的参数可以在线更改，有的参数可以滚动修改，这些都不影响业务。但是db_files 这个参数必然影响业务了，只能找一个对业务影响最小的时间点重启所有实例即可。

SQL> conn / as sysdba
Connected.
SQL> startup
ORA-01081: cannot start already-running ORACLE - shut it down first
SQL> shutdown abort;
ORACLE instance shut down.
SQL> startup
ORACLE instance started.

Total System Global Area  536870912 bytes
Fixed Size          1268508 bytes
Variable Size         150996196 bytes
Database Buffers      381681664 bytes
Redo Buffers            2924544 bytes
Database mounted.
Database opened.
SQL>