centos 7

vmlinuz initrd=initrd.img linux dd quiet

vmlinuz initrd=initrd.img inst.stage2=hd:/dev/sdb4 quiet

vi /etc/default/grub
#line6 add
ipv6.disable=1
grub2-mkconfig -o /boot/grub2/grub.cfg
reboot

echo "ulimit -SHn 102400" >> /etc/rc.local
cat >> /etc/security/limits.conf << EOF
 *           soft   nofile       102400
 *           hard   nofile       102400
 *           soft   nproc        102400
 *           hard   nproc        102400
EOF

tzselect
yum install -y ntpdate
ntpdate -d time.nist.gov
hwclock -w

rpm -qa postfix
rpm -e --nodeps postfix