1) redo和undo的“藕断丝连”
        前滚redo
        回滚undo
        undo受到redo的保护，以便历经前滚后在buffer cache中得到undo，并将这些undo运用到buffer cache里的数据块和索引块上，使那些数据文件能“赶上”数据库的其余部分，从而保持数据库的一致性。
        二者是DBA和开发人员的桥梁
        回滚过程绝不涉及重做日志，只有恢复和归档时会读取重做日志。因为，重做日志是用来写的，而不是用来读！

    2) 删除联机重做日志文件组的注意事项

         ①日志组为active和current状态时不可以删除
         ②日志组在数据库级别删除后操作系统上的文件不会被级链删除
         ③对于一个Oracle数据库实例，至少要包含两个联机重做日志组

    3) redo buffer由谁来写入？

        > redo buffer的写入 也是 lgwr吗？

            No. Server (shadow) processes write to log buffer. LGWR writes redo records from the log buffer to the log file.

        > 而undo buffer因为就是在db cache buffer,所以也是DBWR读入的吗？

            Not quite sure what you mean. "Undo buffer" is just a buffer in buffer cache that corresponds to a block in an undo tablespace.
            Treat it like any other data block. It's written into buffer cache by a server process and written out to datafiles by DBWn.

        > 涉及IO（特别是异步），就需要一个专门的进程来完成

            A server process can always read datafiles. But normally only DBWn can write to them. Also remember a server process can read and write a tempfile