I am studying for my second test in Data Administration and I have a question. I am reading about redo log files and Archiving. Why do you have both. It seems to me that they are the same except for the fact that the archive files don't get over written. When the redo log file is written to the online files and a checkpoint is called it seems to me that the changes are in the datafiles and if the database crashes the info is already in the datafiles. Why do you need the archive logs??
REDOLOGS are online at any instant which keeps logging changes thats happening in database.
ARCHIVEDLOGS are saved copies of REDOLOGS for recovery purpose.
Say you don't have changes you made. How are you gonna recover at so and so point of time. say wednesday till 3:34 pm. you have TUES day night backup and you apply redolgs till 3:34 pm after applying tuesday night backup.
You need archiving for media recovery (in the case one of your disks crashes).
Lets say at 15:00 hours one of your database disks crashes. Say from 8:00 that morning users are doing transactions and already five log switches have taken place. When your database only has three online redologs the first two are already used twice that day.
When you now try to recover with the lastnight backup you have a problem because two archived redologs are missing. (These were overwritten but not archived).