How to Create a Swap File

On servers with low amounts of memory, it can be a good idea to create a swap file to prevent out of memory crashes. A swap file is a a designated space on your disk that can be used in addition to regular memory to give your system more, albeit much slower, memory.

A swap file should be the same size as your available physical memory. Having a very large swap file is not a good idea and if you are experiencing frequent crashing, you should expand your physical memory.

Creating the swap file

To create the swap file, log into the console through SSH.

Once in, you will want to create the file that you would like to use with the following command:

Change the count value to the size that you need. You can use the formula (<count>/1024) to determine how many megabytes you will be setting aside for swap. In this example, we are crafting a 64MB swap. 

dd if=/dev/zero of=/swapfile bs=1024 count=65535

Once you've created the file, you need to change it's permissions to keep it secure. 

chmod 600 /swapfile

Now that your swap file is secure, you can turn it into a usable swap file: 

mkswap /swapfile

Now it's ready to be used, so let's turn it on: 

swapon /swapfile

That's all there is to it!

You can test to make sure that it was added by running the free -m command: 

# free -m
              total        used        free      shared  buff/cache   available
Mem:            983         343          12          51         627         561
Swap:            63           0          63

If you would like to have your swapfile always mounted on boot, you will need to add it inside of /etc/fstab. You can add the following line to mount your swap file on boot. 

/swapfile swap swap defaults 0 0