Login as root:
sudo su
Create a swap file with 4 blocks of 1GB:
dd if=/dev/zero of=/mnt/swap bs=1G count=4
Enable the file for swap usage:
chmod 600 /mnt/swap mkswap /mnt/swap swapon /mnt/swap
Persist the swap file:
- add to /etc/fstab (bottom):
/mnt/swap none swap sw 0 0
- add to /etc/sysctl.conf (bottom):
vm.swappiness=30
Detailed procedure here