How to design test case to differentiate `conv` options in `dd`?

[Jin Li]

Hi,

I am trying to test disk speed using `dd` program, and I have
questions about what are the differences between `conv` options, i.e.
`fdatasync`, `fsync`, `sync`. To illustrate the difference, I have the
following example.

~~~
~$ dd if=/dev/zero of="$tmpfile" bs=1MB count=500 conv=fsync
500+0 records in
500+0 records out
500000000 bytes (500 MB) copied, 1.58808 s, 315 MB/s

~$ dd if=/dev/zero of="$tmpfile" bs=1MB count=500 conv=sync
500+0 records in
500+0 records out
500000000 bytes (500 MB) copied, 1.5292 s, 327 MB/s

~$ dd if=/dev/zero of="$tmpfile" bs=1MB count=500 conv=fdatasync
500+0 records in
500+0 records out
500000000 bytes (500 MB) copied, 1.48944 s, 336 MB/s
~~~

No big difference can be found in these three options. Please find my
machine and `dd` version.
~~~
~$ sw_vers
ProductName: Mac OS X
ProductVersion: 10.11.2
BuildVersion: 15C50

~$ dd --version
dd (coreutils) 8.24
Copyright (C) 2015 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Written by Paul Rubin, David MacKenzie, and Stuart Kemp.
~~~

Could you show me how to design test case to differentiate the three
options? Thanks.

Best regards,
Jin