You might come across with following error while starting mysql.

-----------------------------------
071027 7:43:13 InnoDB: Operating system error number 13 in a file operation.

InnoDB: The error means mysqld does not have the access rights to
InnoDB: the directory.
InnoDB: File name ./ibdata1
InnoDB: File operation call: 'open'.
InnoDB: Cannot continue operation.
071027 07:43:13 mysqld ended
-----------------------------------

Resolution: This error may mean mysql does not have privilege to access database directory. (/var/lib/mysql)

Giving the required privilege will fix this error.

-----------------

chown -R mysql.mysql /var/lib/mysql

-----------------