Create writable device files from readonly images on Linux using device mapper

add_cow.sh

copied from https://git.scc.kit.edu/-/snippets/563, found on search for "device mapper volatile writeable readonly"

#!/bin/bash

set -e

# test if $1 exists
test -f "$1"

BASEDIR=$(dirname "$1")
original_image=$(basename "$1")
BASENAME=`basename "${original_image}" .raw`
cow_file=".${BASENAME}.cow"
cow_device_name="${BASENAME}.cow"

# cow-size in bytes
cow_file_size=$((800*1024*1024))

# create new loopdev for original image
losetup --read-only --verbose --find "${BASEDIR}/${original_image}"

# find existing loopdev
loopdev_image=$(losetup --associated "${BASEDIR}/${original_image}" --output NAME --noheadings | cut -d: -f1)

# create new COW file
fallocate -l ${cow_file_size} "${cow_file}"

# create new loopdev for COW file
losetup --verbose --find "${cow_file}"

# find existing loopdev
loopdev_cow=$(losetup --associated "${cow_file}" --output NAME --noheadings | cut -d: -f1)

# construct cow-file
dmsetup create "${cow_device_name}" --table "0 $(blockdev --getsz ${loopdev_image}) snapshot ${loopdev_image} ${loopdev_cow} p 64"

# add partitions
kpartx -v -a "/dev/mapper/${cow_device_name}"