i follow http://savanna.readthedocs.org/en/latest/userdoc/installation.guide.html (http://savanna.readthedocs.org/en/lat...), but do not know whether i can set "sahara-db-manage --config-file /etc/sahara/sahara.conf upgrade head" many times if config is wrong or need to edit again, i do not understand how sahara use MySQL

i have never used hadoop before, how to write a hello world program to use sahara? and how client use it through horizon web and how to set horizon for client to use?

Packstack will run for you "sahara-db-manage --config-file /etc/sahara/sahara.conf upgrade head"
Generate standard answer file via packstack --gen-answer-file answerAIO.txt. Then update answerAIO.txt as was advised , then run packstack --answer-file=./answerAIO.txt.

how to input question to packstack for it to generate answer? how do packstack know?

First run:- packstack --gen-answer-file answerAIO.txt
Second edit:- answerAIO.txt as advised just 3 lines
Third run :- packstack --answer-file=./answerAIO.txt