对于Oracle数据库来说，db_name和instance_name可以不同。
我们来看一下Oracle文档中对于db_name的定义:
DB_NAME必须是一个不超过8个字符的文本串.在数据库创建过程中,db_name被记录在数据文件，日志文件和控制文件中。假如数据库实例启动过程中参数文件中的db_name和控制文件中的数据库名称不一致，则数据库不能启动。
一个实例可以mount并打开任何数据库，但是同一时间一个实例只能打开一个数据库。
一个数据库可以被一个或多个实例所mount并打开（在OPS/RAC环境下，一个数据库可以被多个实例所打开）.
我们看一下我的数据库:
[oracle@jumper dbs]$ grep name initeygle.ora
*.db_name='eygle'
*.instance_name='eygle'
当前参数设置下的数据库:
SQL> select name from v$datafile;
NAME
-----------------------------------------------------
/opt/oracle/oradata/eygle/system01.dbf
/opt/oracle/oradata/eygle/undotbs01.dbf
/opt/oracle/oradata/eygle/users01.dbf
/opt/oracle/oradata/eygle/eygle01.dbf
SQL> show parameter db_name
NAME TYPE VALUE
------------------------------------ ----------- -----------
db_name string eygle
SQL> show parameter instance_name
NAME TYPE VALUE
------------------------------------ ----------- -----------
instance_name string eygle
SQL> create pfile from spfile;
File created.
SQL> exit
Disconnected from Oracle9i Enterprise Edition Release 9.2.0.4.0 - ProdUCtion
With the Partitioning option
JServer Release 9.2.0.4.0 - Production
我们创建一个新的pfile为julia这个新的实例使用:
[oracle@jumper oracle]$ cd $ORACLE_HOME/dbs
[oracle@jumper dbs]$ cp initeygle.ora initjulia.ora
[oracle@jumper dbs]$ ll init*
-rw-r--r-- 1 oracle dba 982 Jul 25 14:03 initeygle.ora
-rw-r--r-- 1 oracle dba 982 Jul 25 14:04 initjulia.ora
-rw-r--r-- 1 oracle dba 8385 Mar 9 2002 init.ora
修改这个文件更改:
instance_name = julia
修改后的参数设置:
[oracle@jumper dbs]$ grep name initjulia.ora
*.db_name='eygle'
*.instance_name='julia'
然后我们启动实例名称为julia的instance:
[oracle@jumper dbs]$ eXPort ORACLE_SID=julia
[oracle@jumper dbs]$ sqlplus "/ as sysdba"
SQL*Plus: Release 9.2.0.4.0 - Production on Tue Jul 25 14:04:15 2006
Copyright (c) 1982, 2002, Oracle Corporation. All rights reserved.
Connected to an idle instance.
SQL> startup mount;
ORACLE instance started.
Total System Global Area 139531744 bytes
Fixed Size 452064 bytes
Variable Size 121634816 bytes
Database Buffers 16777216 bytes
Redo Buffers 667648 bytes
ORA-01102: cannot mount database in EXCLUSIVE mode
SQL> exit
Disconnected from Oracle9i Enterprise Edition Release 9.2.0.4.0 - Production
With the Partitioning option
JServer Release 9.2.0.4.0 - Production
注重，此时试图加载数据库时出现错误，因为当前数据库被另外一个实例(instance)加载。在非并行模式(Ops/RAC)下，一个数据库同时只能被一个实例加载。
此时已经启动了两个数据库实例，从后台进程可以看出：
[oracle@jumper dbs]$ ps -efgrep ora
oracle 27321 1 0 Jul14 ? 00:00:00 ora_pmon_eygle
oracle 27323 1 0 Jul14 ? 00:00:00 ora_dbw0_eygle
oracle 27325 1 0 Jul14 ? 00:00:00 ora_lgwr_eygle
oracle 27327 1 0 Jul14 ? 00:00:00 ora_ckpt_eygle
oracle 27329 1 0 Jul14 ? 00:00:32 ora_smon_eygle
oracle 27331 1 0 Jul14 ? 00:00:00 ora_reco_eygle
oracle 27333 1 0 Jul14 ? 00:00:00 ora_cjq0_eygle
root 15388 656 0 14:02 ? 00:00:00 sshd: oracle [priv]
oracle 15390 15388 0 14:02 ? 00:00:00 sshd: oracle@pts/2
oracle 15391 15390 0 14:02 pts/2 00:00:00 -bash
oracle 15445 1 0 14:04 ? 00:00:00 ora_pmon_julia
oracle 15447 1 0 14:04 ? 00:00:00 ora_dbw0_julia
oracle 15449 1 0 14:04 ? 00:00:00 ora_lgwr_julia
oracle 15451 1 0 14:04 ? 00:00:00 ora_ckpt_julia
oracle 15453 1 0 14:04 ? 00:00:00 ora_smon_julia
oracle 15455 1 0 14:04 ? 00:00:00 ora_reco_julia
oracle 15457 1 0 14:04 ? 00:00:00 ora_cjq0_julia
oracle 15459 15391 0 14:04 pts/2 00:00:00 ps -ef
oracle 15460 15391 0 14:04 pts/2 00:00:00 grep ora
我们关闭eygle这个数据库实例:
[oracle@jumper dbs]$ export ORACLE_SID=eygle
[oracle@jumper dbs]$ sqlplus "/ as sysdba"
SQL*Plus: Release 9.2.0.4.0 - Production on Tue Jul 25 14:04:39 2006
Copyright (c) 1982, 2002, Oracle Corporation. All rights reserved.
Connected to:
Oracle9i Enterprise Edition Release 9.2.0.4.0 - Production
With the Partitioning option
JServer Release 9.2.0.4.0 - Production
SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> exit
Disconnected from Oracle9i Enterprise Edition Release 9.2.0.4.0 - Production
With the Partitioning option
JServer Release 9.2.0.4.0 - Production
此时就可以通过实例julia加载并打开db_name=eygle的数据库了:
[oracle@jumper dbs]$ export ORACLE_SID=julia
[oracle@jumper dbs]$ sqlplus "/ as sysdba"
SQL*Plus: Release 9.2.0.4.0 - Production on Tue Jul 25 14:05:06 2006
Copyright (c) 1982, 2002, Oracle Corporation. All rights reserved.
Connected to:
Oracle9i Enterprise Edition Release 9.2.0.4.0 - Production
With the Partitioning option
JServer Release 9.2.0.4.0 - Production
SQL> alter database mount;
alter database mount
*
ERROR at line 1:
ORA-01990: error opening passWord file '/opt/oracle/product/9.2.0/dbs/orapw'
ORA-27037: unable to oBTain file status
Linux Error: 2: No such file or Directory
Additional information: 3
SQL> alter database open;
Database altered.
SQL> select name from v$datafile;
NAME
----------------------------------------------------------------------------
/opt/oracle/oradata/eygle/system01.dbf
/opt/oracle/oradata/eygle/undotbs01.dbf
/opt/oracle/oradata/eygle/users01.dbf
/opt/oracle/oradata/eygle/eygle01.dbf
SQL> ! ps -efgrep ora
root 15388 656 0 14:02 ? 00:00:00 sshd: oracle [priv]
oracle 15390 15388 0 14:02 ? 00:00:00 sshd: oracle@pts/2
oracle 15391 15390 0 14:02 pts/2 00:00:00 -bash
oracle 15445 1 0 14:04 ? 00:00:00 ora_pmon_julia
oracle 15447 1 0 14:04 ? 00:00:00 ora_dbw0_julia
oracle 15449 1 0 14:04 ? 00:00:00 ora_lgwr_julia
oracle 15451 1 0 14:04 ? 00:00:00 ora_ckpt_julia
oracle 15453 1 0 14:04 ? 00:00:00 ora_smon_julia
oracle 15455 1 0 14:04 ? 00:00:00 ora_reco_julia
oracle 15457 1 0 14:04 ? 00:00:00 ora_cjq0_julia
oracle 15513 15391 0 14:05 pts/2 00:00:00 sqlplus
oracle 15514 15513 3 14:05 ? 00:00:01 oraclejulia (DESCRIPTION=(LOCAL=YES)(ADDRESS=(PROTOCOL=beq)))
oracle 15515 15513 0 14:05 pts/2 00:00:00 /bin/bash -c ps -efgrep ora
oracle 15516 15515 0 14:05 pts/2 00:00:00 ps -ef
SQL> show parameter instance_name
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
instance_name string julia
SQL> show parameter db_name
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
db_name string eygle
我们再看看假如参数文件中的db_name和控制文件中的db_name不一致会出现什么错误.
修改参数db_name:
[oracle@jumper dbs]$ grep name initjulia.ora
*.db_name='julia'
*.instance_name='julia'
在启动过程中，我们看到，在mount阶段，数据库会对参数文件和控制文件进行比较，假如两者记录的db_name不一致，则数据库无法启动:
SQL> startup nomount;
ORACLE instance started.
Total System Global Area 139531744 bytes
Fixed Size 452064 bytes
Variable Size 121634816 bytes
Database Buffers 16777216 bytes
Redo Buffers 667648 bytes
SQL> alter database mount;
alter database mount
*
ERROR at line 1:
ORA-01103: database name 'EYGLE' in controlfile is not 'JULIA'
另外的一个区别是，db_name通常限制在8个字符之内；instance_name最多应该支持21个字符。通常我们都是设置db_name和instance_name一致。需要注重的是假如db_name设置过长，则会被Oracle截断为8个字符，而instance_name仍然会保留在21个字符之内，假如你的环境变量设置的instance_name=db_name，则启动时就会出现问题，你需要重建pfile/spfile并且修改环境变量才能启动实例，加载数据库。
这个问题可以参考Itpub曾经出现的案例:
http://www.itpub.net/showthread.PHP?threadid=604507
本文通过实例来介绍instance_name和db_name的区别，希望大家能对这两个参数以及instance和database有进一步的熟悉。