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Poc kernel dedup with symlink from overlay2 #3894

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Poc kernel dedup with symlink from overlay2 #3894

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95bf2a7
link hostapp's kernel in boot directory
ycardaillac Jun 16, 2026
c7534c4
symlink boot from overlay2 instead of using a hardlink
ycardaillac Jun 19, 2026
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45 changes: 39 additions & 6 deletions meta-balena-common/recipes-containers/hostapp-update/files/hostapp-update
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Original file line number Diff line number Diff line change
Expand Up @@ -23,6 +23,43 @@ ERROR() {
fi
}

resolve_boot_dir_from_image() {
hostapp_image="$1"
overlay_root="${SYSROOT}/balena/overlay2"
layerdb_root="${SYSROOT}/balena/image/overlay2/layerdb/sha256"

# We expect Hostapp images to be single-layer
# get the unique layer_id from the image
layer_id=$(balena inspect -f '{{index .RootFS.Layers 0}}' "$hostapp_image")
if [ -z "$layer_id" ]; then
ERROR "overlay2: image ${hostapp_image} has no RootFS layers"
exit 1
fi

# use cache-id to resolve boot dir
cache_id_path="${layerdb_root}/${layer_id#sha256:}/cache-id"
boot_dir="${overlay_root}/$(cat "$cache_id_path")/diff/boot"
if [ ! -d "$boot_dir" ]; then
ERROR "overlay2: failed to resolve boot dir for image ${hostapp_image}"
exit 1
fi

echo "$boot_dir"
}

# Symlink hostapps boot to overlay2 layer diff/boot (no copy).
symlink_boot_from_overlay_layers() {
hostapp_new="${SYSROOT}/hostapps/.new"

rm -rf "$hostapp_new"
mkdir -p "$hostapp_new"

boot_dir=$(resolve_boot_dir_from_image "$HOSTAPP_IMAGE")
cache_id=$(basename "$(dirname "$(dirname "$boot_dir")")")
ln -sfn "../../balena/overlay2/${cache_id}/diff/boot" "${hostapp_new}/boot"
INFO "Symlinked ${hostapp_new}/boot -> ../../balena/overlay2/${cache_id}/diff/boot"
}

run_current_hooks_and_recover () {
if [ "$hooks_rollback" = 1 ]; then
# Run the current ones to cleanup the system.
Expand Down Expand Up @@ -124,13 +161,9 @@ elif [ "$remote_image" != "" ]; then
_out="$(balena pull "$HOSTAPP_IMAGE" 2>&1)"
INFO "${_out}"
fi
CONTAINER_ID=$(balena create --runtime="bare" --volume=/boot "$HOSTAPP_IMAGE" /bin/sh)
BOOTSTRAP=$(balena inspect -f "{{range .Mounts}}{{.Destination}} {{.Source}}{{end}}" "$CONTAINER_ID" | awk '$1 == "/boot" { print $2 }' | head -n1)
CONTAINER_ID=$(balena create --runtime="bare" "$HOSTAPP_IMAGE" /bin/sh)

# Create boot entry
rm -rf "$SYSROOT/hostapps/.new"
mkdir -p "$SYSROOT/hostapps/.new"
ln -sr "$BOOTSTRAP" "$SYSROOT/hostapps/.new/boot"
symlink_boot_from_overlay_layers
sync -f "$SYSROOT"
mv -T "$SYSROOT/hostapps/.new" "$SYSROOT/hostapps/$CONTAINER_ID"
sync -f "$SYSROOT"
Expand Down
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