执行测试命令之前，一定先清除缓存：
# echo 3> /proc/sys/vm/drop_caches
SD卡读取的速度
# echo 3> /proc/sys/vm/drop_caches
# time dd if=/dev/mmcblk0 of=/dev/null bs=1M count=500 conv=sync
500+0 records in
500+0 records out
524288000 bytes (524 MB) copied, 0.20453 s, 2.6 GB/s
real 0m0.206s
user 0m0.000s
sys 0m0.204s
SD卡写的能力：
# echo 3> /proc/sys/vm/drop_caches
# time dd bs=1M count=500 if=/dev/zero of=/mnt/aa
500+0 records in
500+0 records out
524288000 bytes (524 MB) copied, 11.2617 s, 46.6 MB/s
real 0m11.378s
user 0m0.000s
sys 0m1.092s
参考文献：https://blog.csdn.net/benkaoya/article/details/70112094