I did this lab three times. I failed in the first time, because the sdf9 couldn't be used. Then I used 4 disks to do the lab, it succeeded. After I asked Chris, I found
the problem. The sdf9 was used by another raid(md_d8, it is inactive), so I stopped it. Finally, I finished the lab. I compared the result with Milton's and my second times. I find the disk speed of the raid 10 is slower than raid0(it should be) and the speed using 6 disks is faster than 4 disks.