Hello all,
ext4 is discussing the concept of 'allocate-on-flush'. I
know its concept, advantages but, want to know more about it. If any
one of you can help me I reuest you to guide me.
Thank you in advance.

These are simple results from Google - "allocate-on-flush":

http://en.wikipedia.org/wiki/Delayed_allocation
https://ols2006.108.redhat.com/2007/Reprints/sato-Reprint.pdf
http://en.wikipedia.org/wiki/File_system_fragmentation - a good
discussion available.
http://uwsg.iu.edu/hypermail/linux/kernel/0008.0/0354.html