Operating System: CentOS 6.5
Arch : x86_54
Symptom MySQL service failed to start after upgrading from MySQL 5.1 to 5.6 version.
2014-11-29 02:22:56 2175 [ERROR] InnoDB: auto-extending data file ./ibdata1 is of a different size 640 pages (rounded down to MB) than specified in the .cnf file: initial 768 pages, max 0 (relevant if non-zero) pages!
2014-11-29 02:22:56 2175 [ERROR] InnoDB: Could not open or create the system tablespace. If you tried to add new data files to the system tablespace, and it failed here, you should now edit innodb_data_file_path in my.cnf back to what it was, and remove the new ibdata files InnoDB created in this failed attempt. InnoDB only wrote those files full of zeros, but did not yet use them in any way. But be careful: do not remove old data files which contain your precious data!
To solve this problem add the below given line in /etc/my.cnf file inside
innodb_data_file_path = ibdata1:10M:autoextend
Now restart the mysql service.
service mysqld restart
I hope the issue is resolved for you also.