# Mender A/B Updates with Separate Kernel and Rootfs Partitions on Yocto **URL:** \ **Category:** General Discussions\ **Tags:** yocto, mender-client, mender-server\ **Created:** [October 10, 2025, 9:40am UTC](https://hub.mender.io/t/mender-a-b-updates-with-separate-kernel-and-rootfs-partitions-on-yocto/8048 "2025-10-10T09:40:32Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![Stan](https://yyz2.discourse-cdn.com/flex036/user_avatar/hub.mender.io/stan/32/2571_2.png) [@Stan](https://hub.mender.io/u/Stan)\ **Post date:** [October 10, 2025, 9:40am UTC](https://hub.mender.io/t/mender-a-b-updates-with-separate-kernel-and-rootfs-partitions-on-yocto/8048/1 "2025-10-10T09:40:32Z")
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`** ```bash # 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`** ```bash # 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! ---
**Author:** ![TheYoctoJester](https://yyz2.discourse-cdn.com/flex036/user_avatar/hub.mender.io/theyoctojester/32/1444_2.png) [@TheYoctoJester](https://hub.mender.io/u/TheYoctoJester)\ **Post date:** [October 12, 2025, 12:59pm UTC](https://hub.mender.io/t/mender-a-b-updates-with-separate-kernel-and-rootfs-partitions-on-yocto/8048/2 "2025-10-12T12:59:33Z")
Hi @Stan, As far as I know, there is no public implementation of such a setup yet, unfortunately. So if you’re working on it, I’m happy to help with it and polish for inclusion. As a starting point, I would probably pick [meta-mender-community/kas/demos at wic · TheYoctoJester/meta-mender-community · GitHub](https://github.com/TheYoctoJester/meta-mender-community/tree/wic/kas/demos), where I have an example to use a fully custom `wks` file instead of having it automatically created as part of the `mender-image` approach. This way you get full control, which should enable you to create such a partition setup. Greetz, Josef ---
**Author:** ![Stan](https://yyz2.discourse-cdn.com/flex036/user_avatar/hub.mender.io/stan/32/2571_2.png) [@Stan](https://hub.mender.io/u/Stan)\ **Post date:** [October 14, 2025, 1:58pm UTC](https://hub.mender.io/t/mender-a-b-updates-with-separate-kernel-and-rootfs-partitions-on-yocto/8048/3 "2025-10-14T13:58:21Z")
Hi Josef, Thanks a lot for your answer and the pointer — I’ll definitely dig deeper in that direction! In the meantime, I wanted to check if there’s a temporary approach I could use: Would it be possible to update just the kernel (which currently resides under `/boot` inside the rootfs) using a _single-file update module_? I was thinking about deploying the new kernel that way and then triggering a reboot to apply it. If so, would it still be possible to retain rollback support in case the new kernel fails to boot? And if that’s achievable, should I handle this logic through a custom state script, or is there a recommended Mender mechanism for triggering such a controlled reboot with rollback capability? Thanks again for your help! Best, Stan