Huge logs are generated in mongo db shard logs
If you are using Linux or Unix as your platform, you could setup a shell script that monitors the size of the log files, and send the SIGUSR1 signal to mongod which forces it to cycle the log files once the files reach a certain size.

This Mongo documentation shows how to send the signal here

kill -SIGUSR1 &lt;mongod process id&gt;


You can get the &lt;mongod process id&gt; using:

ps -e | grep -i "mongod"