Feature #447

Need upgrade path

Added by Hammel over 3 years ago. Updated 8 months ago.

Status:In ProgressStart date:15 Jun 2015
Priority:ImmediateDue date:
Assignee:Hammel% Done:


Target version:0.12.0
Severity:03 - Medium


There is no way to upgrade a system. I need one.

Plan A

Create 4 partitions:
  1. p0: FAT/boot
  2. p1: EXT4/Minimal update
  3. p2: EXT4/Runtime
  4. p3: EXT4/User Data w/update files area

An update causes a reboot into p1 which does the update to p2 and then reboots into P2. This takes the system out of action during the update but probably not for long as long as the update image has already been downloaded.

This also assumes that the update is a complete rewrite of the boot partition. That may not be desirable.

Plan B

Always use opkg's, even for files already on the rootfs.

Create 3 partitions:
  1. p0: FAT/boot
  2. p1: EXT4/Runtime
  3. p2: EXT4/User Data w/update files area

For example, a kernel update would be downloaded as an opkg and installed over the existing kernel (essentially). This requires postinst processing to switch to the new kernel (copy to p0).

The question is how will the system behave when not all files are initially package controlled. Replacing system files with opkg files may present problems.

Overview of the image and updates

The image build (re: rootfs) sits on a two partitions. The first is a vfat partition required by the bootloader. The second is the rootfs. Both are images copied byte-for-byte to their respective partitions in order to create the boot SD card. An update process can either update specific packages (of which there are none for the base platform) or it can replace the partition image. The latter is often required in embedded systems and might be more appropriate for PiBox unless PiBox switches to a fully package managed system.

An update to the rootfs can be done in place. A secondary rootfs partition can be installed and the boot process pointed at it so the next boot uses it. This requires a mechanism to fallback to the original partition if something fails in the new partition. A key sequence or other extenal impetus at boot time would be required to force the fallback.

The boot partition can't use an alternate. So files have to be where we expect them. The boot partition also doesn't use symlinks so we can't place alternates in directories and just update the symlinks. This implies that we have two directories, current and new, that contain the data to boot. Within that data is a kernel with an initramfs capable of replacing the booting kernel with the older version if the boot process fails.

The rootfs update is not overly difficult. It requires an extra boot script that validates the running system and resets if anything goes wrong. This verification tests that all components are online correctly, specifically wireless and USB support.

The boot partition update is more complex since it requires an error checking mechanism built into the initramfs. This error checking would be to verify that all drivers are available and can load without error.


#1 Updated by Hammel about 2 years ago

  • Description updated (diff)
  • Category set to Support
  • Assignee set to Hammel
  • Priority changed from Normal to Immediate
  • Target version set to 0.12.0

#3 Updated by Hammel 8 months ago

  • Description updated (diff)
  • Status changed from New to In Progress
  • % Done changed from 0 to 10

There are a couple of related solutions to consider.

Bank Switching

This is like Plan A except we use full images in each bank. Bank A is the initial install. Bank B is the first update. We download the update into Bank B, while Bank A is running, and validate it. When ready, we swap to Bank B. Swapping could be done with a reboot but it would be faster (and less error prone?) to do a kexec to a new kernel (which might be in the second bank). Note that since the new kernel is being loaded without booting from the VFAT partition (as Pi's demand) we'd need to copy in required files to the VFAT partition after the swap, but only after additional/final validation says this will be okay.

Overlay Filesystems

This would be layers of data, with the lowest being the core image and higher layers holding, in order from bottom to top:
  • Graveyard - deleted files
  • RW - updates and modifications to the core image, including end user customizations
  • presentation - what we see at run time

This also requires a kexec and has the same issues with the VFAT partition as the Bank Switching solution.

Additional Comments

  • Validation requires checksuming downloaded data to start, but goes as far as verifying operational status after kexec'ing and restarting the platform via the init processing.
  • Rollback capability is required in case the update fails.
  • There may need to be a small kernel/initramfs specifically designed to handle the update process only. This allows us to have a constant tool for upgrades irrespective of operational kernel updates. The update kernel/initramfs would, after update, kexec into the latest kernel image.

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