oracle 启动三步骤
oracle启动会经过三个过程，分别是nomount、mount、open
一、nomount 阶段
nomount 阶段，可以看到实例已经启动。oracle进程会根据参数文件开创共享内存池。
SQL> startup nomount;
ORACLE instance started.
Total System Global Area 1653518336 bytes
Fixed Size 2213896 bytes
Variable Size 956303352 bytes
Database Buffers 687865856 bytes
Redo Buffers 7135232 bytes
SQL>可以看到共享内存已经开辟
[root@localhost dbs]# ipcs -m
------ Shared Memory Segments --------
key shmid owner perms bytes nattch status
0x00000000 98304 oracle 600 393216 2 dest
0x00000000 131073 oracle 600 393216 2 dest
0x00000000 163842 oracle 600 393216 2 dest
0x00000000 196611 oracle 600 393216 2 dest
0x00000000 229380 oracle 600 393216 2 dest
0x00000000 262149 oracle 600 393216 2 dest
0x00000000 294918 oracle 600 393216 2 dest
0x00000000 327687 oracle 600 393216 2 dest
0x00000000 360456 oracle 600 393216 2 dest
0x33554094 1048585 oracle 660 4096 0
0x00000000 425994 oracle 600 393216 2 dest
0x00000000 458763 oracle 600 393216 2 dest
0x00000000 491532 oracle 600 393216 2 dest
0x00000000 524301 oracle 600 393216 2 dest
0x00000000 557070 oracle 600 393216 2 dest
0x00000000 688143 oracle 600 393216 2 dest
0x00000000 720912 oracle 600 393216 2 dest 进程已经开启
oracle 2965 1 0 04:44 ? 00:00:00 ora_pmon_test
oracle 2967 1 0 04:44 ? 00:00:00 ora_vktm_test
oracle 2971 1 0 04:44 ? 00:00:00 ora_gen0_test
oracle 2973 1 0 04:44 ? 00:00:00 ora_diag_test
oracle 2975 1 0 04:44 ? 00:00:00 ora_dbrm_test
oracle 2977 1 0 04:44 ? 00:00:00 ora_psp0_test
oracle 2979 1 0 04:44 ? 00:00:00 ora_dia0_test
oracle 2981 1 0 04:44 ? 00:00:01 ora_mman_test
oracle 2983 1 0 04:44 ? 00:00:00 ora_dbw0_test
oracle 2985 1 0 04:44 ? 00:00:00 ora_lgwr_test
oracle 2987 1 0 04:44 ? 00:00:00 ora_ckpt_test
oracle 2989 1 0 04:44 ? 00:00:00 ora_smon_test
oracle 2991 1 0 04:44 ? 00:00:00 ora_reco_test
oracle 2993 1 0 04:44 ? 00:00:00 ora_mmon_test
oracle 2995 1 0 04:44 ? 00:00:00 ora_mmnl_test
oracle 2997 1 0 04:44 ? 00:00:00 ora_d000_test
oracle 2999 1 0 04:44 ? 00:00:00 ora_s000_test查看参数配置文件位置
SQL> show parameter spfile
NAME TYPE VALUE
------------------------------------ --------------------------------- ------------------------------
spfile string /u01/app/oracle/product/11.2.4
/db_1/dbs/spfiletest.ora
SQL> 移除配置文件后startup nomount，报错如下：
SQL> startup nomount;
ORA-01078: failure in processing system parameters
LRM-00109: could not open parameter file '/u01/app/oracle/product/11.2.4/db_1/dbs/inittest.ora'
SQL>二、mount 阶段
mount阶段，oracle会根据nomount阶段的参数文件来寻找控制文件的名称和位置，一旦查找到立即锁定该控制文件，控制文件里记录了数据库中的数据文件、日志文件、检查点信息等非常重要的信息。启动mount时，会先自动启动nomount
startup mount
mount阶段可以看到，比nomount阶段多了一个database mounted的提示。
SQL> startup mount;
ORACLE instance started.
Total System Global Area 1653518336 bytes
Fixed Size 2213896 bytes
Variable Size 956303352 bytes
Database Buffers 687865856 bytes
Redo Buffers 7135232 bytes
Database mounted.
SQL>
查看控制文件位置
SQL> show parameter control
NAME TYPE VALUE
------------------------------------ --------------------------------- ------------------------------
control_file_record_keep_time integer 7
control_files string /u01/app/oracle/oradata/test/c
ontrol01.ctl, /u01/app/oracle/
flash_recovery_area/test/contr
ol02.ctl
control_management_pack_access string DIAGNOSTIC+TUNING
SQL>
将配置文件移除，然后在启动一次。
SQL> startup mount;
ORACLE instance started.
Total System Global Area 1653518336 bytes
Fixed Size 2213896 bytes
Variable Size 956303352 bytes
Database Buffers 687865856 bytes
Redo Buffers 7135232 bytes
ORA-00205: error in identifying control file, check alert log for more info
SQL> 三、open阶段
open阶段会根据控制文件记录的信息，定位到数据库文件、日志文件等，正式开启实例和数据库之间的桥梁。如果数据文件或者日志文件缺少，那么open失败。open之后，如果有使用归档日志功能，便可看到归档日志的进程。
SQL> alter database open;
Database altered.
SQL> 查看日志文件和日志文件的位置
SQL> select file_name from dba_data_files;
FILE_NAME
------------------------------------------------
/u01/app/oracle/oradata/test/users01.dbf
/u01/app/oracle/oradata/test/undotbs01.dbf
/u01/app/oracle/oradata/test/sysaux01.dbf
/u01/app/oracle/oradata/test/system01.dbf
SQL>
SQL> select group#,member from v$logfile;
GROUP# MEMBER
-------------------------------------
3 /u01/app/oracle/oradata/test/redo03.log
2 /u01/app/oracle/oradata/test/redo02.log
1 /u01/app/oracle/oradata/test/redo01.log
移动一个数据文件后启动，报错如下
SQL> startup;
ORACLE instance started.
Total System Global Area 1653518336 bytes
Fixed Size 2213896 bytes
Variable Size 956303352 bytes
Database Buffers 687865856 bytes
Redo Buffers 7135232 bytes
Database mounted.
ORA-01157: cannot identify/lock data file 4 - see DBWR trace file
ORA-01110: data file 4: '/u01/app/oracle/oradata/test/users01.dbf'
SQL> 如果我随便复制一个文件进去可不可以呢？启动如下：
SQL> alter database open;
alter database open
*
ERROR at line 1:
ORA-01122: database file 4 failed verification check
ORA-01110: data file 4: '/u01/app/oracle/oradata/test/users01.dbf'
ORA-01210: data file header is media corrupt总结
我们可以使用statup来启动oracle数据库，也可以用shutdwon来关闭。如果使用startup启动，其实已经经过了三个过程。
总的来说，没有参数文件，实例无法创建，数据库无法nomount成功，没有配置文件，数据库无法mount；没有数据文件，数据库无法打开使用。