Oracle的db_name和instance_name

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。如果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

注意，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 -ef|grep 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 -ef|grep 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 -ef|grep 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

 我们再看看如果参数文件中的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，如果两者记录的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。需要注意的是如果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，如果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。