#!/bin/bash

# Usage: ./script.sh arg1
#
# Arg1: the number of the loopback device to create
#       (must be a number that does not match the entries under
#       /dev/loop<N>)
#
# e.g.: if `ls /dev` shows /dev/loop{0,1,2}, here you can
#       use something like `10`.
#
# ps.: it depends on `btrfs-tools` and must be run as root.

set -o errexit
set -o xtrace

readonly MOUNT_POINT_PREFIX="/mnt/btrfs"

main() {
	local btrfs_instance="$1"

	if [[ -z $1 ]]; then
		echo "Usage: $0 <btrfs_instance_no>"
		exit 1
	fi

	local btrfs_instance=$1
	local volume_number=1

	setup_btrfs $btrfs_instance
	setup_base_volume $btrfs_instance
	for i in $(seq 3001 10000); do
		setup_volume $btrfs_instance $i
	done
}

setup_base_volume() {
	local mount_point=$MOUNT_POINT_PREFIX-$1
	local base_volume=$mount_point/base_volume

	echo "Setting up base volume
MOUNT_POINT: $mount_point
BASE_VOLUME: $base_volume
"

	btrfs subvolume create $base_volume
	dd if=/dev/zero of=$base_volume/file1 bs=1024 count=1024
}

setup_volume() {
	local mount_point=$MOUNT_POINT_PREFIX-$1
	local base_volume=$mount_point/base_volume
	local new_volume=$mount_point/vol$2

	echo "Snapshotting volume (base=$base_volume, new=$new_volume)"

	btrfs subvolume snapshot $base_volume $new_volume
}

setup_btrfs() {
	local loopback_device="/dev/loop$1"
	local backing_image="/img-$1"
	local mount_point="$MOUNT_POINT_PREFIX-$1"

	echo "INFO: Setting up BTRFS
LOOPBACK_DEVICE $loopback_device
BACKING_IMAGE   $backing_image
MOUNT_POINT     $mount_point
"

	mknod -m 0660 $loopback_device b 7 $1

	fallocate -l 256M $backing_image
	losetup $loopback_device $backing_image

	mkfs.btrfs $loopback_device


	mkdir -p $mount_point
	mount -t btrfs $loopback_device $mount_point
}

main "$@"