There is a lot of conflicting information out there on how to create a swapfile correctly. Although these guides (should) all work perfeclty fine, I spent the time to research and find out what is actually the most correct.

In summary: make sure to use the dd command to create the swapfile, and create an fstab entry that looks similar to /swapfile none swap sw 0 0. In this article, I will go more in-depth into the correct steps to create a swapfile on Linux, and explain each step.

Tutorial

Step 1: Create the file

The first step is to allocate the file.

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# dd if=/dev/zero of=/swapfile bs=1M count=[size in MiB] status=progress

Replace [size in MiB] with the size of your swapfile in Mebibytes (MiB). You can use this online converter to convert from Gibibytes (GiB, often confused with Gigabytes, but that doesn’t matter right now) to MiB, which you can put in the command.

Or you can look at this table for common sizes:

If you aren’t sure how big your swapfile should be, take a look at the Ubuntu Swap FAQ. It has a table of common RAM sizes and how much swap you should use.

However, I would take it with a grain of salt; for the with hibernation column, you probably don’t need as much as it says. On my computer I have 32 GiB of RAM, but only a 16 GiB swap partition.¹

Remember that if that’s not enough, you can add always multiple swapfiles later.

Step 2: Change swapfile permissions

The swapfile should only be readable by the system (root user). Run this command to change it:

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# chmod 600 /swapfile

Step 3: Format the swapfile

Use the mkswap command to format the file to be used as swap (basically just add a header to identify it):

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# mkswap /swapfile
Setting up swapspace version 1, size = 4 GiB (4294963200 bytes)
no label, UUID=a0b87eca-b951-4344-be2d-020d77cdef48

Step 4: Create an entry in /etc/fstab

An entry needs to be added to /etc/fstab for the swapfile to be enabled during bootup. Open /etc/fstab in a text editor, and add this line to the end (add spaces to line up with other entries if you want, whitespace is ignored):

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/swapfile none swap sw 0 0

Step 5: Enable the swapfile

Adding the fstab entry won’t enable the swapfile until a reboot. To enable it now, use the swapon command.

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# swapon /swapfile

Finally: check the swap status

Use swapon and free to verify that your new swapfile has been added:

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$ swapon --show
NAME       TYPE SIZE USED PRIO
/swapfile  file   4G   0B   -2

$ free -h
               total        used        free      shared  buff/cache   available
Mem:            31Gi       3.6Gi        20Gi       233Mi       6.9Gi        26Gi
Swap:          4.0Gi          0B       4.0Gi