I don't know what is the reason to vote-down this question, can you all help me find out this possible vote-down reason?
It's not a bad question. It's clear enough for people who know well haproxy and they could surely answer it. The concept is technically valid. The problem with the question: it's very hard to parse for people who aren't versed in this particular area of haproxy config (i.e. the
Why are these people's understanding relevant? Well, here we care mainly about them - this site's main goal isn't to help you (brutally speaking). It's 1% to help you and 99% to help all other people who will google at some point for the same class of problems.
For the sake of them, try not to focus on
backup here is also a part of solution, not a part of the 'business' problem. Surely, after you re-visit the question (after some time) you'll find a way to express the problem in a way that is understandable to people who have many backends, use haproxy, have a limited throughput, but never used
backup keyword yet.