Oracle启动四状态一：SHUTDOWN
第一状态没什么好解释的，oracle的所有文件都静静的躺在磁盘里，一切都还未开始，属于关机状态
Oracle启动四状态二：NOMOUNT
Starting the instance (nomount)
* Reading the initialization file from $ORACLE_HOME/dbs in the following order:
-first spfileSID.ora
-if not found then, spfile.ora
-if not found then, initSID.ora
Specifying the PFILE parameter with STARTUP overrides the default behavior.
* Allocating the SGA
* Starting the background processes
* Opening the alertSID.log file and the trace files
The database must be named with the DB_NAME parameter either in the initialization
Parameter file or in the STARTUP command.
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* 读取环境变量下dbs目录下的参数文件（spfile/pfile）
[ora10@localhost dbs]$ pwd
/ora10/product/10.2.0/db_1/dbs
[ora10@localhost dbs]$ ll
-rw-r----- 1 ora10 dba 3584 07-19 22:07 spfilechongshi.ora
-rw-r----- 1 ora10 dba 3584 07-23 22:00 spfile.ora
-rw-r--r-- 1 ora10 dba 1106 07-19 21:47 initchongshi.ora
查找参数文件的顺序如上面列表的，读取优先级
spfilechongshi.ora > spfile.ora>initchongshi.ora
如果三个文件都找不到的话，那么将启动失败。
* 启动算法，分配内存
* 启动后台进程
* 开放alertSID.log文件和跟踪文件
[ora10@localhost dbs]$ sqlplus
SQL*Plus: Release 10.2.0.1.0 - Production on Tue Jul 24 22:33:54 2012
Copyright (c) 1982, 2005, Oracle. All rights reserved.
Enter user-name: /as sysdba
Connected to an idle instance.
SQL> startup nomount;
ORACLE instance started.
Total System Global Area 205520896 bytes
Fixed Size 1218532 bytes
Variable Size 79693852 bytes
Database Buffers 121634816 bytes
Redo Buffers 2973696 bytes
现在就处在一个nomount状态。
Oracle启动四状态三：mount
Mounting a database includes the following tasks:
* Associating a database with a previously started instance
* Locating and opening the control files specified in the parameter file
* Reading the control files to obtain the names and status of the data files and online redo log files.However,no checks are performed to verify the existence of the data files and online redo log files at this time.
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* 把一个数据库和启动的实例关联起来
* 在参数文件（spfile/pfile）中找到控制文件进行读取
查看参数文件：
[ora10@localhost dbs]$ strings spfileora10.ora | more
......
*.control_files='/ora10/product/oradata/ora10/control01.ctl','/ora10/product/ora
data/ora10/control02.ctl','/ora10/product/oradata/ora10/control03.ctl'
.......
控制文件：
[ora10@localhost ~]$ cd /ora10/product/oradata/ora10/
[ora10@localhost ora10]$ ll
总计 954196
-rw-r----- 1 ora10 dba 7061504 07-23 23:01 control01.ctl
-rw-r----- 1 ora10 dba 7061504 07-23 23:01 control02.ctl
-rw-r----- 1 ora10 dba 7061504 07-23 23:01 control03.ctl
* 读取控制文件，获得的数据文件
dba培训和联机重做日志文件，然而，在这个时候没有进行检查以验证存在的数据文件和联机重做日志文件
[ora10@localhost dbs]$ sqlplus
SQL*Plus: Release 10.2.0.1.0 - Production on Tue Jul 24 23:02:28 2012
Copyright (c) 1982, 2005, Oracle. All rights reserved.
Enter user-name: /as sysdba
Connected to an idle instance.
SQL> startup mount;
ORACLE instance started.
Total System Global Area 205520896 bytes
Fixed Size 1218532 bytes
Variable Size 79693852 bytes
Database Buffers 121634816 bytes
Redo Buffers 2973696 bytes
Database mounted.
现在进入了数据库的mount状态，我们通过mount启动的时候，下面会多一句提示“Database mounted.” 数据库准备就绪。
Oracle启动四状态四：open
opening the database includes the following tasks
opening the online data log files
opening the onling redo log files
If any of the datafiles or noline redo log files are not present when you attempt to open the database ,the oracle server returns an error.
During this final stage,the oracle server verfies that all the data files and online redo log files can be opened and checks the consistency of the database . If necessary , the SMON background process initiates instance recovery.
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打开数据库包括下列任务:
打开在线数据日志文件
打开联机重做日志文件
如果任何数据文件或非线性重做日志文件不存在，当您试图打开的数据库，服务器返回错误。
在这最后阶段，该服务器验证所有数据文件和联机重做日志文件可以打开并检查数据库的一致性。如果需要，该系统监控进程开始实例恢复。
