Separate boot partition and different root partitions → simple man’s A+B update possible?

[LanMarc77]

Getting into buildroot and yocto seems a bit too much for me to get an A+B style update possibility. I would like to stick with the quality images provided by Armbian.

The following applies to a NanoPi-NEO3 others SBC might have different partition layouts.

 

From what I understood is that uboot looks for /boot in /dev/mmcblk0p1 reads the boot.cmd/scr and armbianEnv.txt and then boots the kernel accordingly.

The armbianEnv.txt seems to point the kernel to the root partition using the rootdev directive.

 

If /dev/mmcblk0p1 would now only hold /boot would it be possible to point to a different partition for the rootfs using armbianEnv.txt?

If so I could imagine changing the partition layout to the following:

 

/dev/mmcblk0:

/dev/mmcblk0p1: boot partition

/dev/mmcblk0p2: rootfs partitionA

/dev/mmcblk0p3: rootfs partitionB

/dev/mmcblk0pX: multiple more partitions that survive updates

 

Too now allow consistency with the rootfs partitions also the boot partition directory structure would slightly change. It would contain 2 directories: /bootA, /bootB AND a hardlink /boot which either points to /bootA or /bootB

In the running rootfs the fitting /boot is mounted via a bind mount hoping normal apt-get updates can deal with this. This would allow creating a rootfs by updating and verifying everything works locally and then creating an image from it.

 

Now to OTA a relatively simple script running from rootfsA can download a new rootfs image/file structure and place it in the partition of rootfsB. Same with the new boot partition just into /bootB. The values of the armbianEnv.txt from the new boot image always point to the corresponding rootfs partition either because the image was already built for this or the script dynamically adjusts them. Same goes for fstab, machine-id, ssh identity and other rootfs specifics. The script will then verify that the write worked to avoid sd card issues.

The (almost) atomic operation for switching the systems would be changing the hardlink of /boot to the /bootB directory and reboot.

 

This idea will not detect any boot issues and revert automatically back as uboot is not involved. Also this only works if uboot is still compatible with the new boot files provided. But an additional new uboot image could also be provided during the update.

Reverting can be done manually (relatively fast) by changing the hardlink back and rewriting a previously created backup copy of a might be updated uboot image.

 

Independent of the fact that buildroot, yocto, rauc, mender… systems have a better feature set, do you think this could work?

[eselarm]

19 minutes ago, LanMarc77 said:

If /dev/mmcblk0p1 would now only hold /boot would it be possible to point to a different partition for the rootfs using armbianEnv.txt?

That is basically what I have been doing. I reconstructed image/partitions to have bootfs and rootfs separated. Typically format bootfs as simple FAT where you can fix things even on Windows computers maybe when SD-cards. For new fast SBCs I just have many partitions on NVMe where various Linux distros rootfs are and just copy or rename armbianEnv.txt which is then essentially only a UUID of a rootfs.

 

When I know the SBC well enough, it is more the kernel only that matters, so i made a extlinux.conf generator so I can select a kernel via U-Boot serial console.

 

I don't see what A+B brings me as I use Btrfs for rootfs and use snapper to make 'last-known-good' snapshots and also transfer those to NAS or so. If you want totally unattended, then A+B is an option, look at Android as well I would say.

But more towards PC like systems, one can put efi bootloader (efi-grub) on the bootFAT, that allows you to select last-known-good more or less if you deploy snapshots. I usually do update in-place, so more like rolling release, but you can also do updates on a new read-write snapshot and fallback if it would fail to boot, see https://kubic.opensuse.org/documentation/man-pages/transactional-update.8.html  for example how that works.

 

U-Boot also has options I think to get to last-known-good automatically, but I don't know how that would work. For my stuff at home, it is good enough that I can quickly go back to older snapshot and that works for more than a decade on my PCs, so I do the same on SBCs/embedded, as long as they have a serial console cable (or HDMI,keyboard). For the NanoPi-NEO I also had Btrfs capable U-Boot, so then U-Boot can directly boot a certain rootfs partition, but it is not really default, so I keep an extra bootFAT.