Redo Log Files
Used to record changes made to data.
Server processes started on a user’s behalf will make changes on data in memory.
A record of those changes are immediately recorded in the redo log files by LGWR.
Should database crash before DBWR flushes changed data from memory to disk, on startup SMON will simply replay the redo log file to bring the database to a consistent state.
Due to critical nature of redo log files, should be multiplexed and, optionally, OS mirrored.