Redo log files can be lost due to several causes. The most recent which I had was because someone archived them :D Seriously, someone just decided to GZIP the archivelogs because I guess 500 MBs where to big for 25 GBs file system :D. Therefore, I had to fix last of 2 log files. But in general, loss of a redo log file can be a BIG BIG BIG problem (irreversible loss of data). So how to know if we have a problem. Well depends in what state the redo log group WAS when it was deleted, and it can be in one of three states:

• INACTIVE - Archived and the database doesn't need it for anything during normal operation
• ACTIVE - Archived, but the database still MIGHT need it for crash recovery.
• CURRENT - Currently written into.

So what happens if we delete a redo log file. Well let's see according to the group:

• INACTIVE - Nothing, the database will continue to work, as long as there is another member of the group, if not, it will skip
• ACTIVE - Database might abort in case of recovery and you MIGHT have loss of data.
• CURRENT - Database WILL ABORT and you WILL have lost of data.

So let's see the recoveries from each of the three scenarios:

1. Lost Inactive Redolog file
2. Lost Active Redolog file
3. Lost Current Redolog file