While trying to set up a new MySQL slave on Centos 5, i copied over the /var/lib/mysql directory and made necessary updates but Mysql would not start, in /var/log/mysql.log there was this
120910 13:40:00 mysqld_safe mysqld from pid file /var/lib/mysql/omni1.omniweb.com.pid ended
120910 13:42:08 mysqld_safe Starting mysqld daemon with databases from /var/lib/mysql
120910 13:42:08 [ERROR] /usr/sbin/mysqld: Can't find file: './mysql/plugin.frm' (errno: 13)
120910 13:42:08 [ERROR] Can't open the mysql.plugin table. Please run mysql_upgrade to create it.
120910 13:42:08 [ERROR] Falcon: can't open file "/var/lib/mysql/falcon_master.fts": Permission denied (13)
120910 13:42:08 [ERROR] Plugin 'Falcon' init function returned error.
120910 13:42:08 [ERROR] Plugin 'Falcon' registration as a STORAGE ENGINE failed.
120910 13:42:08 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.
This particular error /usr/sbin/mysqld: Can't find file: './mysql/plugin.frm' (errno: 13) led me to a lot of posts about apparmor, i shut off iptables but the winner was SELINUX.
setenforce 0
then mysql started right up!
shut it off for good,