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Populating a Partition

Use Yocto and WIC to include a dedicated ext4 data partition in your disk image.

Yocto Published 02/05/2026 3 min read
YoctoWICPartitions
Populating a Partition

A common embedded use case is a disk image with a separate data partition that is not part of the root filesystem.

This article shows how to:

  • build an ext4 partition image from a recipe,
  • reference that file from a wks layout,
  • wire the dependency so wic includes it during image generation.

Build the data partition image

Create a recipe (for example data-rootfs.bb) that generates the filesystem image in do_compile() and deploys it in do_deploy().

SUMMARY = "Data partition filesystem image"
LICENSE = "MIT"
LIC_FILES_CHKSUM = "file://${COMMON_LICENSE_DIR}/MIT;md5=0835ade698e0bcf8506ecda2f7b4f302"

inherit deploy

S = "${WORKDIR}"

DATA_ROOTFS_DIR = "${WORKDIR}/data-rootfs"
DATA_IMAGE_SIZE_MB = "1024"

do_compile() {
    rm -rf ${DATA_ROOTFS_DIR}
    install -d ${DATA_ROOTFS_DIR}/empty

    rm -f ${WORKDIR}/data-rootfs.ext4
    mkfs.ext4 -F \
        -L data \
        -d ${DATA_ROOTFS_DIR} \
        ${WORKDIR}/data-rootfs.ext4 \
        ${DATA_IMAGE_SIZE_MB}M
}

do_deploy() {
    install -d ${DEPLOYDIR}
    install -m 0644 ${WORKDIR}/data-rootfs.ext4 ${DEPLOYDIR}/data-rootfs.ext4
}

addtask deploy after do_compile before do_build

DEPENDS += "e2fsprogs-native"

Notes:

  • inherit deploy makes it easy to copy the generated image to ${DEPLOYDIR}.
  • mkfs.ext4 ... -d ${DATA_ROOTFS_DIR} creates a filesystem image populated from that directory.
  • You can add files into ${DATA_ROOTFS_DIR} before creating the image if the partition requires content.

Define the partition in a WKS file

Put your wks layout in your BSP layer, for example meta-my-bsp/wic/mydisk.wks.in.

part /boot --source bootimg-partition --ondisk mmcblk --fstype=vfat --label boot --active --align 4 --size 64
part rootfsA --source rootfs --ondisk mmcblk --fstype=ext4 --label rootfsA --align 4 --size 2048
part rootfsB --source rootfs --ondisk mmcblk --fstype=ext4 --label rootfsB --align 4 --size 2048
part data --source rawcopy --sourceparams="file=data-rootfs.ext4" --ondisk mmcblk --fstype=ext4 --label data --align 4096 --size 8192

Key points:

  • rawcopy copies the prebuilt image file directly into the partition.
  • --sourceparams="file=data-rootfs.ext4" names the deployed artifact from the recipe.
  • --fstype=ext4 is used to describe the partition layout and label.

Make the WIC image depend on the partition image

In your image recipe, ensure the WIC image generation waits for the partition image to be ready.

IMAGE_FSTYPES += "wic"
WKS_FILE = "mydisk.wks.in"

# Ensure the partition image is built before do_image_wic runs.
do_image_wic[depends] += "data-rootfs:do_deploy"
WKS_FILE_DEPENDS:append = " data-rootfs"

That setup makes data-rootfs available when wic reads the wks file and builds the final disk image.

Practical tips

  • Keep the wks file focused on disk layout and let the recipe build the partition image.
  • Use explicit labels so the image can be inspected and mounted reliably.
  • Test the generated image with fdisk, parted, or losetup before flashing hardware.

A separate data partition is often the cleanest way to preserve user files, logs, or application state while keeping the root filesystem isolated.

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