Ext3: Why data=journal is better than data=ordered when data needs to be read from and written to disk at the same time

Hi,

I see many literatures mentioned this, but I have never seen any one
explains it in detail.(Although this link exposed the original story:
http://lkml.indiana.edu/hypermail//linux/kernel/0107.1/0364.html)

"Journal mode: This mode is the slowest except when data needs to be
read from and written to disk at the same time where it outperform all
others mode."

Since this is pretty counter-intuitive, I believe many people are not
aware about the root cause, thus I won't be the last one to ask this
same question. Can any one kindly explain it so as to make it more
clear? Thank you!

Regards
Jidong