I'm reading the debugging section of NGINX and it says to turn on debugging, you have to compile or start nginx a certain way and then change a config option. I don't understand why this is a two step process and I'm inferring that it means, "you don't want to run nginx in debug mode for long, even if you're not logging debug messages because it's bad".
Since the config option (error_log) already sets the logging level, couldn't I just always compile/run in debug mode and change the config when I want to see the debug level logs? What are the downsides to this? Will nginx run slower if I compile/start it in debug mode even if I'm not logging debug messages?
First off, to run nginx in debug you need to run the nginx-debug binary, not the normal nginx, as described in the nginx docs. If you don't do that, it won't mater if you set the error_log to debug, as it won't work. If you want to find out WHY it is a 2 step process, I can't tell you why exactly the decision was made to do so.
Debug spits out a lot of logs, fd info and so much more, so yes it can slow down your system for example as it has to write all that logs. On a dev server, that is fine, on a production server with hundreds or thousands of requests, you can see how the disk I/O generated by that log can cause the server to slow down and other services to get stuck waiting on some free disk I/O. Also, disk space can run out quickly too.
Also, what would be the reason to run always in debug mode ? Is there anything special you are looking for in those logs ? I guess i'm trying to figure out why would you want it.
And it's maybe worth mentioning that if you do want to run debug in production, at least use the debug_connection directive and log only certain IPs.