Control MDADM rebuild/reshape speed

edit etc/sysctl.conf
        add:
                dev.raid.speed_limit_min = 400000
                dev.raid.speed_limit_max = 400000

# sysctl -p

or temporarily change by:
echo 400000 > /proc/sys/dev/raid/speed_limit_min
echo 400000 > /proc/sys/dev/raid/speed_limit_max

Where 400000 is the desired speed. My system doesn't have a heavy load on it so I like to max out the reshape speed using 400000 (400MB/s) max.