How to clean logs or find largest files and reduce the space? also i do not understand 1.7T is not even used in this critical moment.

Mysqld is not running because 99% is used.

# df -h Filesystem Size Used Avail Use% Mounted on rootfs 20G 19G 279M 99% / /dev/root 20G 19G 279M 99% / devtmpfs 32G 268K 32G 1% /dev /dev/md3 1.8T 196M 1.7T 1% /home tmpfs 32G 0 32G 0% /dev/shm /dev/root 20G 19G 279M 99% /var/named/chroot/etc/named /dev/root 20G 19G 279M 99% /var/named/chroot/var/named /dev/root 20G 19G 279M 99% /var/named/chroot/etc/named.conf /dev/root 20G 19G 279M 99% /var/named/chroot/etc/named.rfc1912.zones /dev/root 20G 19G 279M 99% /var/named/chroot/etc/rndc.key /dev/root 20G 19G 279M 99% /var/named/chroot/usr/lib64/bind /dev/root 20G 19G 279M 99% /var/named/chroot/etc/named.iscdlv.key /dev/root 20G 19G 279M 99% /var/named/chroot/etc/named.root.key