#!/bin/bash

# Run multiple dd processes in parallel to copy a single file/blockdevice. Each
# dd process will operate on a dedicated part of the file at a time (1GB chunks
# for big files).

set -e

# Source: https://stackoverflow.com/a/25268449
min() {
    printf "%s\n" "${@:2}" | sort "$1" | head -n1
}
max() {
    # using sort's -r (reverse) option - using tail instead of head is also possible
    min "${1}r" "${@:2}"
}

inputfile=$1
if [[ -b "$inputfile" ]]; then
	inputsize=$(blockdev --getsize64 "$inputfile")
else
	inputsize=$(stat --format %s "$inputfile")
fi
outputfile=$2

stepsize=1000
parallel=6
# 1GB blocks or smaller blocks if we cannot achieve the required parallelization with 1GB
blocksize=$(min -g 1000000 "$((inputsize / stepsize / parallel))" )

seq 0 $stepsize $(($inputsize / $blocksize )) | xargs -P$parallel -I{} dd if="$inputfile" bs=$blocksize skip={} conv=sparse seek={} count=$stepsize status=none of="$outputfile"