Updating files on readonly root with update module?

General Discussions
1

Hi,

We are using Podman and Quadlet files to start containers with SystemD. Inside those quadlet files are the image tag that the container runs. We are also using a readonly root.

What we’d like to be able to do is update this tag without redeploying a whole filesystem or requiring a reboot - is that possible with update modules?

We toyed with putting the Quadlet files on the /data partition but then they can’t be updated in a root update.

What is the best approach here? :slight_smile:

2

Hi @chriswiggins,

It’s definitely possible, but I don’t have a definite best practise here right now mainly due to my inexperience with containers.
An elaborate guess would be to use an overlayfs backed on /data, combined with a mechanism which wipes it if the root filesystem brings new content. State scripts or a specialised rootfs update module might be viable routes here. I’ll see if I can dig up some more details.

Greets,
Josef

3

Thanks @TheYoctoJester - lets ignore the fact that it has to do with containers.

Imagine I just want to update a file, lets call it /usr/share/yocto_is_cool

Can I use mender update modules to update /usr/share/yocto_is_cool if the root is RO?

4

Hi @chriswiggins,

I couldn’t reach the person whom I know to use overlayfs, sorry. So my explanation will have to suffice at the moment. :frowning:
When the root filesystem is read only, you can’t modify the files there, also not through an Update Module. Usually you want the filesystem to be unmodified, because this is a requirement for delta updates.
What you can do is to use overlayfs (see https://elinux.org/images/6/6c/OverlayFS_in_Yocto._Vyacheslav_Yurkov.pdf), which allows modifications to the root filesystem and stores them at an additional persistent place, like /data. If such a setup is in place, you can modify /usr/share/... through an Update Module. The only additional thing to take into consideration is how and when to reset the overlay storage if a root filesystem upgrade happens.

Hope this helps,
Josef

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5

Thanks Josef - good to know that is the approach to take :slight_smile:

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6

Just to comment, this is the approach we have taken and it is working well for us. We have overlays for both /etc and /var directories. The overlays are set up by initramfs (/sbin/preinint) which is set as an argument on the kernel command line (init=/sbin/preinit).

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