In order to recover Linux software raid, and increase speed of RAID rebuilds, I found a nice howto at

Warning !!! … these options are good for tweaking rebuilt process and may increase overall system load, high cpu and memory usage.

To increase speed, enter:

echo value > /proc/sys/dev/raid/speed_limit_min

sysctl -w

In this example, set it to 50000 K/Sec, enter:

# echo 120000 > /proc/sys/dev/raid/speed_limit_min