Not the best practices to add a file as swap space, but it comes handy when you have an urgent requirement of adding swap space. The post below describes steps to add swap file.
Adding swap file
Determine the size of the new swap file in MB and multiple by 1024 to determine the block size. For example, the block size of a 64 MB swap file is 65536 (64*1024).
1. At a shell prompt as root, type the following command with count being equal to the desired block size:
# dd if=/dev/zero of=/swapfile bs=1024 count=65536
2. Change the permissions to readable and writeable only by root:
# chmod 600 /swapfile
3. Setup the swap file with the command:
# mkswap /swapfile
4. To enable the swap file immediately but not automatically at boot time:
# swapon /swapfile
5. To enable it at boot time, edit /etc/fstab to include:
/swapfile swap swap defaults 0 0
New swap file will be enabled after next system boot.
6. After adding the new swap file and enabling it, verify it is enabled by viewing the output of the command cat /proc/swaps or free.
# cat /proc/swaps Filename Type Size Used Priority /dev/sda3 partition 65535996 372404 -1 /swapfile file 65532 0 -2