1.先来看下外国网站上的资料怎么说

• Nomount – The database instance has been started (processes and memory structures have been allocated, but control file is not yet accessed).  数据库实例已经开始（进程和内存已经分配，但是控制文件还没有启动）

• Mount – Instance has accessed the control file, but has not yet validated its entry or accessed the datafiles.   实例已经开启控制文件，但是还没有开启数据文件

• Open – Instance has validated the entries in the control files and is accessing the datafiles – it is now open for business.  开启数据文件

2.nomount阶段

• read the spfileSID.ora initSID.ora 
•  Allocating the SGA
• Starting the background processes
• Opening the alertSID.log file and the trace files

3.mount阶段

• 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.

4.open状态

• opening the online datafiles
• opening the online redologs

oracle之nomount、mount、open三种状态