Check which directories are using disk space
sudo du -h --max-depth=1
Delete the Mongo Log
sudo rm -rf /var/log/mongodb/mongod.log
Check which processes have not released the file lock
sudo lsof | egrep "deleted|COMMAND"
Restart Mongo DB to release the file lock
sudo service mongod restart
Or run the log rotate command on mongo
db.adminCommand({logRotate:1})