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 其中一个宕机，这集群的原始数据只能变成只读。可以读写 shards分片，但是不能再检查移动分片信息。如果全部的config 服务器挂掉，集群将不可使用。