linux 디스크 입출력 속도 확인 캐시 드랍

dd if=something of=somewhere bs=4K/1M/2M/4M count=testsize oflag=dsync

To free pagecache:
```
# echo 1 > /proc/sys/vm/drop_caches
```
To free dentries and inodes:
```
# echo 2 > /proc/sys/vm/drop_caches
```
To free pagecache, dentries and inodes:
```
# echo 3 > /proc/sys/vm/drop_caches
```

http://unix.stackexchange.com/questions/87908/how-do-you-empty-the-buffers-and-cache-on-a-linux-system

https://romanrm.net/dd-benchmark

https://www.thomas-krenn.com/en/wiki/Linux_I/O_Performance_Tests_using_dd