Why getting error mongod dead but subsys locked and Insufficient free space for journal files on Linux?

You can add following to the config file provided when running mongod --config mongod.conf

For MongoDB 3.x (latest version)

storage:
   mmapv1:
      smallFiles: true

For version 2.6+

storage:
   smallFiles: true

For version 2.4 and less

smallfiles = true

Then just execute mongod to accept your config file (here it assumes that location of the config is /etc/mongodb.conf):

mongod -f /etc/mongodb.conf

Documentation for smallfiles parameter:

Set to true to modify MongoDB to use a smaller default data file size. 
Specifically, smallfiles reduces the initial size for data files and
limits them to 512 megabytes. The smallfiles setting also reduces the
size of each journal files from 1 gigabyte to 128 megabytes.