If the config server replica set loses its primary and cannot elect a primary, the cluster’s metadata becomes read only. You can still read and write data from the shards, but no chunk migration or chunk splits will occur until the replica set can elect a primary. If all config databases become unavailable, the cluster can become inoperable.
大致意思：如果config server 其中一個宕機，這集羣的原始數據只能變成只讀。可以讀寫 shards分片，但是不能再檢查移動分片信息。如果全部的config 服務器掛掉，集羣將不可使用。