额，chenyu，你查的是一个归档的起始scn与结束scn。与增量检查点纪录rba有什么关系呢？与v$datafile中的checkpoint_change#更是没什么关系。因为一般来说，你查出来的值会远远小于当前v$datafile中的checkpoint_change#。退一步来说，就算不是归档模式，logfile 还是要switch的。

我只是要回答你第二个问题
“如果说switch logfile做检查点，那为何做完switch logfile之前之后,查出来的值都没变过？”只是想告诉你，switch logfile后scn的值是变化的。

Redo Byte Address (RBA)

Recent entries in the redo thread of an Oracle instance are addressed using a 3-part redo byte address, or RBA. An RBA is comprised of
the log file sequence number (4 bytes)
the log file block number (4 bytes)
the byte offset into the block at which the redo record starts (2 bytes)
RBAs are not necessarily unique within their thread, because the log file sequence number may be reset to 1 in all threads if a database is opened with the RESETLOGS option.
RBAs are used in the following important ways.

With respect to a dirty block in the buffer cache, the low RBA is the address of the redo for the first change that was applied to the block since it was last clean, and the high RBA is the address of the redo for the most recent change to have been applied to the block.
Dirty buffers are maintained on the buffer cache checkpoint queues in low RBA order. The checkpoint RBA is the point up to which DBWn has written buffers from the checkpoint queues if incremental checkpointing is enabled -- otherwise it is the RBA of last full thread checkpoint. The checkpoint RBA is copied into the checkpoint progress record of the controlfile by the checkpoint heartbeat once every 3 seconds. Instance recovery, when needed, begins from the checkpoint RBA recorded in the controlfile. The target RBA is the point up to which DBWn should seek to advance the checkpoint RBA to satisfy instance recovery objectives.

The on-disk RBA is the point up to which LGWR has flushed the redo thread to the online log files. DBWn may not write a block for which the high RBA is beyond the on-disk RBA. Otherwise transaction recovery (rollback) would not be possible, because the redo needed to undo a change is always in the same redo record as the redo for the change itself.

The term sync RBA is sometimes used to refer to the point up to which LGWR is required to sync the thread. However, this is not a full RBA -- only a redo block number is used at this point.

The low and high RBAs for dirty buffers can be seen in X$BH. (There is also a recovery RBA which is used to record the progress of partial block recovery by PMON.) The incremental checkpoint RBA, the target RBA and the on-disk RBA can all be seen in X$TARGETRBA. The incremental checkpoint RBA and the on-disk RBA can also be seen in X$KCCCP. The full thread checkpoint RBA can be seen in X$KCCRT.

on-disk RBA和low RBA都在控制文件中，Oracle不会去频繁写数据文件头部的。
否则容易出现性能问题。