We had a problem with an application accessing our MongoDB cluster, and the log files didn’t provide much useful information. I could see the client connect and disconnect … but nothing in between. I discovered that the default logging level is very low. Good for disk utilization and I/O, but not great for troubleshooting.

db.runCommand({getParameter: 1, logLevel: 1}) # Get the current logging level
db.setLogLevel(3) # Fairly robust logging
db.setLogLevel(5) # don't try this is prod huge stream of text super logging
db.setLogLevel(0) # and set logging back to a low level once you are done troubleshooting

You can also increase the log level for individual components of MongoDB to minimize logging I/O:

db.setLogLevel(2, "accessControl" )