Creating swap file
A swap file can be created and mounted using the dd command. Say for example you need are planning to add 1 GB extra space for your swap memory.
# dd if=/dev/zero of=/usr/local/extraswap bs=1M count=1024
After that you need to use the mkswap to format the file to swap file sytem.
# mkswap /usr/local/extraswap
Then use the swapon command to enable the new swap file.
# swapon /usr/local/extraswap
Check the swap memory using free -m command and confirm it with
# swapon -s
To load the swap file on the next boot. Just edit the /etc/fstab and add the following lines.
/usr/local/extraswap none swap defaults 0 0
Save and exit, make sure you have made the backup of /etc/fstab before adding the lines.