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scripts/mkimg.arm.sh: build Pi img as resizable FAT16 partition

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Construct a partitioned disk, resizable with libparted-based tools (gparted and al.).
libparted has limitations to cross cluster size boundaries in resize operations [1].
Use the minimum image file size to ensure FAT16 partition crosses the 2K/4K cluster size boundary (128MB+)[2].
In MB alignment, this is a full image 130MB = 129MB + 1MB offset for partition table (sector 2048 =1MB).

[1] https://www.gnu.org/software/parted/manual/html_node/parted_58.html
[2] https://www.pctechguide.com/hard-disks/file-systems-fat-fat8-fat16-fat32-and-ntfs-explained
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macmpi 2025-10-12 17:34:29 +02:00 committed by macmpi
parent 6afe4d3803
commit 657b468231
1 changed files with 6 additions and 4 deletions
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10
scripts/mkimg.arm.sh
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@ -52,12 +52,14 @@ profile_rpi() {
}
create_image_imggz() {
MIN_IMG_SIZE=130 # exceed 128MB minimum FAT16 partition size in MB to cross 4k cluster size boundary
sync "$DESTDIR"
local image_size=$(du -L -k -s "$DESTDIR" | awk '{print $1 + 8192}' )
local imgfile="${OUTDIR}/${output_filename%.gz}"
dd if=/dev/zero of="$imgfile" bs=1M count=$(( image_size / 1024 ))
mformat -i "$imgfile" -N 0 ::
mcopy -s -i "$imgfile" "$DESTDIR"/* "$DESTDIR"/.alpine-release ::
local image_size=$(du -L -k -s "$DESTDIR" | awk '{print int(($1 + 8192) / 1024)}' )
dd if=/dev/zero of="$imgfile" bs=1M count=$(( image_size > $MIN_IMG_SIZE ? image_size : $MIN_IMG_SIZE ))
echo 'start=2048, type=6, bootable' | sfdisk "$imgfile" # create partition table with FAT16 at standard 2048 sector (1MB)
mkfs.vfat -n PIBOOT -F 16 --offset 2048 "$imgfile"
mcopy -s -i "$imgfile"@@2048s "$DESTDIR"/* "$DESTDIR"/.alpine-release ::
echo "Compressing $imgfile..."
pigz -v -f -9 "$imgfile" || gzip -f -9 "$imgfile"
}