oracle启动过程的分析：
oracle的启动过程一般分为shutdown、nomount、mount、open，几个状态，
shutdown到nomount状态：oracle根据环境变量中的sid到默认目录加载初始化文件，初始化文件有两种，pfile和spfile，
格式分别是initsid.ora和spfilesid.ora,oracle在启动的时候会先加载spfilesid.ora，如果找不到就会加载initsid.ora,同时根据初始化参数文件分配相应的内存空间，创建必要的进程，此时数据库就到了nomount状态。
nomount到mount状态：根据初始化参数中读取到的控制文件的路径打开控制文件，then reads the control files to get the name of the database datafiles and redo controls,At this point,the databases is still closed and is accessible only to the database administrator.The database admin can keep the database closed while completing speclific maintenance operations,However the database is not available for nomal operations.
mount到open状态：when open the database, oracle database opens the online datafiles and redo log files.