This project can be used to build an initramfs image for a Raspberry Pi that can be used to create a full disk backup of the rootfs before booting the actual system.

The following steps are required in order to build the ram disk:

1. Compile or download a statically linked version of BusyBox.
2. Copy the BusyBox executable to rootfs/bin.
3. Adjust the configuration file in rootfs/etc/config.
4. Create a cpio image of the rootfs directory.

The build script contains functions to automate all steps:

$ ./build getbusybox arm
$ cp ./busybox ./rootfs/bin
$ ./build package

Copy the built ram disk from the working directory to /boot/initramfs.cpio on your Raspberry Pi.

In order to load and execute the ram disk during boot, add the following directive to your /boot/config.txt:

initramfs initramfs.cpio followkernel

To trigger the creation of a backup, create a .renew flag file on the backup target device and reboot the system.

For example, consider the following configuration setup in rootfs/etc/config:

BACKUP_SOURCE=DEVICE=/dev/mmcblk0                       # mounted to / (sdcard)
BACKUP_TARGET=UUID=bb57cb7a-94a8-45d2-821a-9daa2f716d9e # mounted to /mnt (UUID of external hdd)
BACKUP_FILE=backups/rootfs.img

Then the existence of /mnt/backups/rootfs.img.renew would trigger a backup on the next system (re)boot.

Once a backup image has been created use losetup to create a loop device for each partition within the image. Then use mount on each partition you want to access.

$ losetup --partscan --find --show rootfs.img # prints loop device name (e.g. /dev/loop0)
$ mount /dev/loop0p2 /mnt # mounts partition 2 of rootfs.img

To unmount the backup image do:

$ umount /mnt
$ losetup -d /dev/loop0 # cleans up loop device