PHASE1:nomount阶段. 打开了初始参数文件和backupground_dump_dest下的   alert_sid.log和background processes 的trace files.

　　PHASE2:mount阶段是打开了control file.

　　control file中存放的东东如下:

Database name and identifier
Time stamp of database creation
Tablespace names
Names and locations of data files and redo log files
Current redo log file sequence number
Checkpoint information
Begin and end of undo segments
Redo log archive information
Backup information

　　所以偶们在PHASE0中所做操作得在PHASE3真正open这些文件之前，告诉Oracle(因为信息记录在control file中，所以又得在PHASE2中，control file被open后做)，偶们已改了file location.

　　于是可用alter database的data file clause或log file clause的rename 命令来更新control file，于是Oracle会在PHASE3时，到新file location去找相应的文件.

　　PHASE3:open阶段打开所有非offline的data files和redo log files.

　　因为文件已打开了.所以此时，只能对已经offline或还可以offline的文件作rename操作.原理也是通过更新control file中的内容来告诉Oracle:File location has been changed.