So you wanted to change the value of “innodb_log_file_size” on your mysql server.
As it happens, you can’t just change it the “my.cnf” file, restart and make it work. If you do, Mysql will refuse to start and throw this error instead. You can the details of the startup error in your mysql error log file.
To safely change this setting follow these steps
Shutdown your mysql server, make sure to check the error log that the shutdown had no issues
Move the “ib_logfile0″ and “ib_logfile1″ files from your mysql data directory.
Change “innodb_log_file_size” to your desired setting.
Restart mysql. Check the error log to make sure everything started successfully, mysql may complain that the log files don’t exist, it will make new ones and start.
Make sure if you are changing things on a production server, you take due care of the mysql downtime due to the changes and restart, and have backups to recover quickly in case something goes wrong.