MongoDB Documentation  Release 3.0.4 Process Overview Indications When you are aware of a mongod instance running without journaling that stops unexpectedly and you’re not running with replication  you should always run the repair operation before starting MongoDB again. If you’re using replication  then restore from a backup and allow replication to perform an initial sync (page 593) to restore data. If the mongod.lock file in the data directory specified by dbPath   /data/db by default  is not a zero-byte file  then mongod will refuse to start  and you will find a message that contains the following line in your MongoDB log our output: This indicates that you need to run mongod with the --repair option. If you run repair when the mongodb.lock file exists in your dbPath   or the optional --repairpath   you will see a message that contains the following line: old lock file: /data/db/mongod.lock. probably means unclean shutdown Unclean shutdown detected. If you see this message  as a last resort you may remove the lockfile and run the repair operation before starting the database normally  as in the following procedure: Warning: Recovering a member of a replica set. Do not use this procedure to recover a member of a replica set . Instead you should either restore from a backup (page 184) or perform an initial sync using data from an intact member of the set  as described in Resync a Member of a Replica Set (page 633). There are two processes to repair data files that result from an unexpected shutdown: • Use the --repair option in conjunction with the --repairpath option. mongod will read the existing data files  and write the existing data to new data files. You do not need to remove the mongod.lock file before using this procedure. • Use the --repair option. mongod will read the existing data files  write the existing data to new files and replace the existing  possibly corrupt  files with new files. You must remove the mongod.lock file before using this procedure. Note: --repair functionality is also available in the shell with the db.repairDatabase() helper for the repairDatabase command. Procedures Important: Always Run mongod as the same user to avoid changing the permissions of the MongoDB data files. Repair Data Files and Preserve Original Files To repair your data files using the --repairpath option to preserve the original data files unmodified. Repair Data Files without Preserving Original Files To repair your data files without preserving the original files  do not use the --repairpath option  as in the following procedure: Warning: After you remove the mongod.lock file you must run the --repair process before using your database. Chapter 5. Administration 264