Today I have updated a CentOS 6.8 system that has MariaDB-server installed from
http://yum.mariadb.org/5.5/centos6-amd64 and found that I had an SELinux issue when I tried to restart the service.

Using the information at https://wiki.centos.org/HowTos/SELinux I have created a local policy that seems to fix things

module marialocal 1.0;

require {
        type mysqld_safe_t;
        class capability { setuid setgid };
}

#============= mysqld_safe_t ==============

allow mysqld_safe_t self:capability setgid;
allow mysqld_safe_t self:capability setuid;