[yocto] Read-only rootfs update

[Anders Darander]

Hi,

* Ruben Schwarz <r.schwarz at sotec.eu> [151124 10:58]:
> We are using yocto to build a custom linux image for our BeagleBoneBlack
> based hardware. We are using a read-only root file system to prevent data
> lost if power supply is lost.
> Now we wan't to be able to update our device. If it is possiple we wan't to
> use apt/dpkg to do this update. We set up an update server and everything
> works so far.

Ok, so you're only mounting a standard file system read-only.

> Following steps are done to do an update:

> mount -o remount,rw /
> apt-get update
> apt-get upgrade

> Everything works fine, new package versions are installed. After rebooting
> the device and executing "dpkg -l" the old package version is listed.
> Output of mount shows that there is tmpfs on /var/lib which hides the
> /var/lib/dpkg folder.

I guess that if you want this to work, you'd need to move the original
/var/lib/dpkg folder out to some other in your file system. Then you can
have that new folder mounted on top of your /var/lib tmpfs.

That's probably the easiest way to solve your issue.

> Is there an standard way how to update devices with read-only root file
> systems? Anybody here who did something similar?

Well, it depends.

For consistency, I'm in the camp that prefers to do image-based
upgrades. I.e. rewrite the whole rootfs; though that normallly requires
that you're either switching between two alternative rootfs', or that
you have a special maintenance/upgrade rootfs.

Cheers,
Anders

-- 
Anders Darander, Senior System Architect
ChargeStorm AB / eStorm AB