理论上他就是个Leaner 本身不会投票也就没半数以上的说法，我感觉是连接处理不够好，当某个tiflash返回一些报错后导致SQL报错，所以才有fallback_to_tikv这参数
Problem Summary:
When we get a backoff error of TiFlash, the SQL will always fail while the TiKV nodes are actually alive. We hope to fallback to TiKV after TiFlash is down.