When to use the different log levels?

Trace – Only when I would be “tracing” the code and trying to find one part of a function specifically.

Debug – Information that is diagnostically helpful to people more than just developers (IT, sysadmins, etc.).

Info – Generally useful information to log (service start/stop, configuration assumptions, etc). Info I want to always have available but usually don’t care about
under normal circumstances. This is my out-of-the-box config level.

Warn – Anything that can potentially cause application oddities, but for which I am automatically recovering. (Such as switching from a primary to backup server, retrying an operation, missing secondary data, etc.)

Error – Any error which is fatal to the operation, but not the service or application (can’t open a required file, missing data, etc.). These errors will force user (administrator, or direct user) intervention. These are usually reserved (in my apps) for incorrect connection strings, missing services, etc.

Fatal – Any error that is forcing a shutdown of the service or application to prevent data loss (or further data loss). I reserve these only for the most heinous errors and situations where there is guaranteed to have been data corruption or loss.