How to create swap partition/file on a Yocto distribution

I'm trying to create a swap partition/file on my board where a core-image-minimal has been installed.

The fdisk -l command doesn't show any partition thus I'm not able to figure out which block device I need to use to create a new partition.

Secondly, launchig a swapon command on a swapfile correctly initialized using mkswap will raise an invalid argument error saying that the file contains holes even though I created it using dd.

At this point I'm not sure if I can do something like this since the free output looks like:

              total        used        free      shared  buff/cache   available
Mem:         503304       32108      101108         216      370088      465180
Swap:             0           0           0

Solution

To add any partition to your image, you need to modify the wks file that is used for your build.

To get the current wks file run :

bitbake -e | grep ^WKS_FILE=

Then, look for that file in your layers sources.

In that file you can add (example 1GB swap):

part swap --ondisk mmcblk0 --size 44 --label swap --fstype=swap --size=1024M --overhead-factor 1

For a real example, you can see the raspberry-pi machine swap support commit here.

You can use a custom wks file and set it to your custom machine conf file:

WKS_FILE ?= "custom-image.wks"

For detailed info, check the Yocto reference about wks.