Launch dd against block device with multiple jobs

mdd.sh

#!/bin/bash
[[ -z "$1" ]] && echo "No given device" && exit 1
[[ -z "$2" ]] && echo "No given mode (read|write)" && exit 1
device=$1
mode=$2
cpu_num=$(cat /proc/cpuinfo | grep proc | wc -l)
bytes_to_write=$(fdisk -l $device | grep -E 'Disk .* bytes$' | awk '{print $(NF-1)}')

process_num=$((cpu_num*2))
count_by_process=$((bytes_to_write/1024/1024/process_num))

declare -a pids

# Launch dds
for p in $(seq 1 $process_num) ; do
    if [[ "$mode" -eq 'read' ]] ; then
        dd of=/dev/null if=$device bs=1M count=$count_by_process skip=$((count_by_process*p)) \
            iflag=skip_bytes &
    elif [[ "$mode" -eq 'write' ]] ; then
        dd if=/dev/zero of=$device bs=1M count=$count_by_process seek=$((count_by_process*p)) \
            oflag=seek_bytes oflag=direct &
    else
        echo "Bad acces mode '$mode' (read|write)" && exit
    fi
    pid=$!
    pids[$pid]=$pid
    echo "Launch dd ($pid)"
done

# Track dd tasks
while [[ $(echo ${pids[@]} | wc -w) -ne 0 ]] ; do
    sleep 3
    enumerate=0
    for pid in "${pids[@]}"; do
        sleep 1
        echo "===== $pid ====="
        kill -USR1 $pid 2> /dev/null
        if [[ $? -ne 0 ]] ; then
            unset pids[$pid]
            echo "End dd ($pid)"
        fi
    done
done

sync

echo "Done"

Tests with RunAbove ra.intel.ha.l 50GB High-Speed and Ubuntu 14.04

Read:
dd : 150 seconds
mdd : 40 seconds

Write:
mdd : 160 seconds
dd : 190 seconds