start mongod by service mongod start, attempt to create a lock file on a read-only directory /mgodb I changed /etc/mongod.conf's dbpath to /mgodb and then I execute the follow command:


mkdir /mgodb
chown mongod:mongod /mgodb
chmod 0755 /mgodb  


Then I run service mongod start, and get errors like:


  initAndListen: 20 Attempted to create a lock file on a read-only directory: /mgodb


I tried to change /usr/lib/systemd/system/mongod.service's User and Group to root, it also get the same error.

But I cat run mongod --fork --config /etc/mongod.conf through command line.
If you look that /etc/mongod.conf, what are values of processManagement.pidFilePath and storage.dbPath? It sounds like that system tries to create files to some subdirectory, what don't exists or is not owned by user what is used to start mongod process.

You should never start your mongod with root user. And if you have done that, f.ex. when starting mongod to maintenance mode, always remember to chown -R user: &lt;dir&gt; data files back to the right owner.