Hello everyone,
I’m currently working on a Yocto project on the Scarthgap branch, setting up OTA updates using Mender. By default, Mender proposes the following partitioning scheme:
| Partition | Usage | Size Variable | Typical Contents |
|---|---|---|---|
| 1 (boot) | Bootloader | MENDER_BOOT_PART_SIZE_MB | U-Boot, EFI |
| 2 (A) | Rootfs A | auto (depends on total available) | Kernel, Rootfs, DTB |
| 3 (B) | Rootfs B | auto (depends on total available) | Kernel, Rootfs, DTB (rollback) |
| 4 (data) | Data | MENDER_DATA_PART_SIZE_MB | Persistent data |
However, I would like to have a partitioning scheme that isolates the kernel from the rootfs, while still keeping the A/B layout for atomic updates and rollback for both kernel and rootfs:
| Partition | Usage | Size Variable | Typical Contents |
|---|---|---|---|
| 1 (boot) | Bootloader | MENDER_BOOT_PART_SIZE_MB | U-Boot, EFI |
| 2 (A) | Kernel A | auto (depends on total available) | Kernel, DTB |
| 3 (B) | Kernel B | auto (depends on total available) | Kernel, DTB (rollback) |
| 4 (A) | Rootfs A | auto (depends on total available) | Rootfs |
| 5 (B) | Rootfs B | auto (depends on total available) | Rootfs (rollback) |
| 6 (data) | Data | MENDER_DATA_PART_SIZE_MB | Persistent data |
The problem is, I don’t know how to proceed. Does Mender support this setup? What would be the recommended approach? Has anyone done this before? I haven’t been able to find references on this.
I was thinking about using a custom .wic image and then integrating it with Mender and the bootloader. I made a first attempt with the following files:
.bbappend
# meta-my-layer/meta-wic/recipes-core/images/core-image-minimal.bbappend
IMAGE_FSTYPES += "wic wic.bz2 sdimg"
# Check the wks file name
WKS_FILE = "custom-qemuarm64.wks.in"
WKS_FILE_DEPENDS += "virtual/kernel"
# These variables must match the exact labels in your .wks.in
IMAGE_BOOT_FILES_label-KERNEL_A = "Image qemuarm64.dtb"
IMAGE_BOOT_FILES_label-KERNEL_B = "Image qemuarm64.dtb"
# Generic EFI bootloader for QEMU
IMAGE_BOOT_FILES = "grub-efi-bootaa64.efi"
custom-qemuarm64.wks.in
# core-image-minimal-qemuarm64.wks.in
# GPT partition table for QEMU ARM64 with A/B kernel and rootfs
bootloader --ptable gpt --append="rootwait rootfstype=ext4"
# Boot partition (U-Boot)
part /boot --source bootimg-partition --ondisk vda --fstype=vfat --label boot --active --size 128
# Kernel A
part /bootA --source bootimg-partition --fstype=vfat --label kernelA --align 1024 --size 128
# Kernel B
part /bootB --source bootimg-partition --fstype=vfat --label kernelB --align 1024 --size 128
# Rootfs A
part / --source rootfs --fstype=ext4 --label rootA --align 1024 --size 2048
# Rootfs B
part / --source rootfs --fstype=ext4 --label rootB --align 1024 --size 2048
# Persistent data
part /data --fstype=ext4 --label data --align 1024 --size 256
The problem with this attempt is that the bootloader ends up in my kernel partitions instead of just the Image and DTB.
If anyone has pointers or examples on how to set up a separated A/B kernel + rootfs layout with Mender, that would be greatly appreciated!