理解redo（5）深入学习RBA
RBA就是redo entries在重做日志文件中所对应的地址。定义：块第一次变脏时所对应的redo entries在redo log file里的位置CKPT-Q按
1 定义
RBA就是redo entries在重做日志文件中所对应的地址
2 组成
RBA由三部分组成：
序列号（4个字节)
块号(4个字节）
起始字节（2个字节）
以上都是16进制存储
3 类型
3.1 LRBA
定义：块第一次变脏时所对应的redo entries在redo log file里的位置
注释：CKPT-Q按LRBA的顺序维护脏块，执行检查点事件时，DBWn从CKPT-Q按LRBA的顺序写出。LRBA也就是checkpoint position。从检查点位置（LRBA)处，，应用重做记录到On disk rba处，这个过程就是前滚。
3.2 HRBA
定义：脏块在buffer cache中最近一次被更新时产生的redo entries在redo log file里的位置
注释：如果HRBA > on-disk RABA，那么HRBA所对应的相关redo entries还在redo log buffer中，如果实例crash，则HRBA是不可能被恢复的。
3.3 checkpoint RBA
定义：在CKPT-Q上第一个脏数据块的LRBA,这个RBA之前的脏数据已经被全部写入磁盘。
注释：当一个checkpoint事件发生的时候，CKPT进程会记录下当时所写的重做日志记录的地址（即RBA),此时记录的RBA被称之为checkpoint RBA,从上一个checkpoint RBA到当前checkpoint RBA之间的日志所保护的buffer cache中的脏块接下来将会被写进数据文件。
3.4 on-disk RBA
定义：LGWR 写日志文件的最末位置的地址，磁盘中重做日志文件的最后一条重做记录。
注释：on disk rba是Oracle前滚操作的终点。on disk 顾名思义 就是'在磁盘上'的意思.比这个更高的rba,都在log buffer中,还没有来的急被写进磁盘中的日志文件.所以是不能被用于恢复的。
3.5 target RBA
定义：检查点事件结束时的checkpoint RBA
注释：为了缩短实例恢复的时间，DBWn会尽力把checkpoint RBA的目标值定的长远点。
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