xtrabackup 备份为什么要--prepare，增量合并的时候为什么需要--apply-log-only 呢？

--prepare 主要是做什么动作？我只找到这么一句话
"because they were copied at different times as the program ran, and they might have been changed while this was happening. If you try to start InnoDB with these data files, it will detect corruption and crash itself to prevent you from running on damaged data."

意思是说，全量备份完成时，整个备份集是不可用的，为什么不可用呢？

--apply-log-only 应该是为了保留未提交的事务，因为有可能在下一次增量的时候，进行提交，按这种逻辑，应该最后一次增量也需要做这个动作，为什么文档建议最后一次增量合并不加这个参数呢