The instance mounts a database to associate the database with that instance. To mount
the database, the instance finds the database control files and opens them. Control files
are specified in the CONTROL_FILES initialization parameter in the parameter file
used to start the instance. Oracle then reads the control files to get the names of the
database’s datafiles and redo log files.

At this point, the database is still closed and is accessible only to the database
administrator. The database administrator can keep the database closed while
completing specific maintenance operations. However, the database is not yet
available for normal operations.

装载数据库
1. 在挂载数据库时,实例首先需要找到并打开数据库控制文件(CONTROL_FILES),
接着 Oracle 将读取控制文件获取数据库的数据文件名及重做日志文件名。
2. 这个时候数据库还没有打开, 只有DBA才能访问数据库