redo log的大小可以影响 DBWR 和 checkpoint ；larger redo log files provide better performance. Undersized logfiles increase checkpoint activity and reduce performance.适当大小的log file可以提供更好的性能；redo logfile 设置过小会增加checkpoint 和降低性能；A rough guide is to switch log files at most once every 20 minutes.（推荐日志切换的时间不要超多20分钟）.