Mariadb - Variable 'innodb_log_file_size' is a read only variable
You might have to consider two things before doing this.


Ensure innodb_fast_shutdown SHOULDN'T BE 2. If it is? change it
to 1 globally. 
Verify  step 1, query: show variables like 'innodb_fast_shutdown';
Shutdown Mysql.
Ensure you copy the iblogfile0&amp;1 into a safer place and then
remove from data dir.
Make changes to my.cnf for innodb_log_file_size as per your
desired option.
Start Mysql


Rollback: If it doesn't start for some reason put back the iblogfiles and start.

Please see link https://dev.mysql.com/doc/refman/5.5/en/innodb-data-log-reconfiguration.html for detailed explanation


  UPDATE: In our chat we narrowed down that my.cnf was pointed to a
  different file or picked default ones. We forced mysql to pick the
  mentioned my.cnf via mysqld_safe --defaults-file. And it worked.