1 闪回区有足够空间,并且在 flashback retention target要求范围内,flashback log 将要被创建
2 一旦超过flashback retention target时间要求，flashback log将要被覆盖
3 闪回区已满或者磁盘空间不足时， flashback log将要被覆盖
4 闪回区已满，为了为其他文件腾出空间,归档日志会被删除,flashback log 也可能被删除http://www.cndba.cn/hbhe0316/article/108662

The following rules govern the flash recovery area’s creation, retention, overwriting and deletion of flashback logs:

A flashback log is created whenever necessary to satisfy the flashback retention target, as long as there is enough space in the flash recovery area.http://www.cndba.cn/hbhe0316/article/108662http://www.cndba.cn/hbhe0316/article/108662

A flashback log can be reused, once it is old enough that it is no longer needed to satisfy the flashback retention target.

If the database needs to create a new flashback log and the flash recovery area is full or there is no disk space, then the oldest flashback log is reused instead.

Note:http://www.cndba.cn/hbhe0316/article/108662http://www.cndba.cn/hbhe0316/article/108662

Re-using the oldest flashback log shortens the flashback database window. If enough flashback logs are reused due to a lack of disk space, the flashback retention target may not be satisfied.
If the flash recovery area is full, then an archived redo log may be automatically deleted by the flash recovery area to make space for other files. In such a case, any flashback logs that would require the use of that redo log file for the use of FLASHBACK DATABASE are also deleted.

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