[systemd-devel] /etc overlay

[Barbier, Renaud]

Hello,
We would like to have a ROFS volume on which the whole fs is and writeable config volume on which with have the same /etc.
Current kernel is 5.11
I have done an /etc/ overlay using the service var-volatile-etc for the RO FS. And then added another service to either mount the config /etc on top of the /etc  overlay when the config volume is rw or mount the config volume to a temp directory and then overlay it to the /etc/ overlay when the volume is RO.

It seems to be working as I am able to disable our  services (bit, sensor) on config volume and they don't start.
Also the OOB address is assigned from the configuration file.

Then I found https://lists.freedesktop.org/archives/systemd-devel/2019-February/042151.html further below in quote:

So is it my understanding that as long as the mount or overlay happen early enough which is around the service for var-volatile-etc then there is a rescan and all config from the /etc in the volume will be used then?

Following the thread, I also saw that it is advised to use initramfs. I am not clear why that makes a difference. This thread being 2 year old there may thing I am not aware of.
Cheers.


"> I have an embedded system with a readonly rootfs.
>
> I've setup a very early service that mounts a writable aufs overlay over /etc/.
>
>
> The problem I'm having is systemd is not starting any services that I have enabled in that overlay that were not initially enabled in the readonly rootfs.
>
> ?I've tried placing a calls to systemctl daemon-reload or systemctl daemon-reexec just after mounting the overlay but the services are not started, after the fact systemctl will report that the services are enabled but inactive.
>

If you are doing it as part of normal startup, then systemd has already
computed initial "transaction". While daemon-reload makes it rescan for
and see additional unit definitions, it does not change currently
running "transaction"."

Cheers,
Renaud


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