今天在用RMAN备份数据库的时候遇到ORA-15021错误：

RMAN> connect target *
2> run{
3> allocate channel d1 type disk;
4> allocate channel d2 type disk;
5> allocate channel d3 type disk;
6> allocate channel d4 type disk;
7> allocate channel d5 type disk;
8> allocate channel d6 type disk;
9> allocate channel d7 type disk;
10> allocate channel d8 type disk;
11> backup as compressed backupset full database format '/backups_20140501/20140506/fulldb_%u%p%s.rmn'
12> include current controlfile plus archivelog format '/backups_20140501/20140506/arch_ctl_%u%p%s.rmn';
13> release channel d1;
14> release channel d2;
15> release channel d3;
16> release channel d4;
17> release channel d5;
18> release channel d6;
19> release channel d7;
20> release channel d8;
21> }
22>
RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-12001: could not open channel default
RMAN-10008: could not create channel context
RMAN-10002: ORACLE error: ORA-15021: parameter "remote_dependencies_mode" is not valid in asm instance
RMAN-10006: error running SQL statement: alter session set remote_dependencies_mode = signature

Recovery Manager complete.

这个错误提示的很明显，remote_dependencies_mode参数指定的不正确，要改成signature才可以，这我就纳闷了，为什么提示的是ASM实例，后来想到，之前使用ASMCMD命令登录了ASM，查看了磁盘组信息，忘了把ORACLE_SID改回来，现在的ORACLE_SID还是+ASM呢。

-bash-3.2$ echo $ORACLE_SID
+ASM
-bash-3.2$ export ORACLE_SID=headdb2

将ORACLE_SID改为数据库实例名后，再次使用RMAN备份成功，问题解决。
后来经过测试，按照RMAN的提示，将remote_dependencies_mode参数的值设置为signature也是可以的。
但是将alter语句直接写在RMAN脚本里也是不行的，因为这是数据库参数，ASM实例无法修改，所以如果不修改ORACLE_SID环境变量的情况下，只能在数据库实例里修改这个参数，而且还要全局修改才可以。

-bash-3.2$ export ORACLE_SID=headdb2
-bash-3.2$ sqlplus / as sysdba

SQL*Plus: Release 10.2.0.4.0 - Production on Tue May 6 17:07:23 2014

Copyright (c) 1982, 2007, Oracle.  All Rights Reserved.


Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

SQL> show parameter remote_dependencies_mode

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
remote_dependencies_mode             string      TIMESTAMP


SQL> alter system set remote_dependencies_mode=signature;

System altered.

这样，就可以在ORACLE_SID即使指定的是ASM实例，也可以通过RMAN备份数据库了。
remote_dependencies_mode参数影响的是远程调用PL/SQL或DBLINK，修改后一般不会对数据库造成严重影响。这是动态参数，可以根据需求随时调整。
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本文固定链接: https://www.dbdream.com.cn/2014/05/ora-15021-parameter-remote_dependencies_mode-is-not-valid-in-asm-instance/ | 信春哥，系统稳，闭眼上线不回滚！