What will happen if the primary shard for a mongodb database is down? Well as I understand each database has it's primary shard, which is mostly the first shard added to mongodb cluster. And we know that balancer will not split chunks until the chunksize is bigger than the half of the max chunk size, so the balancer will not move any data to the specified shards, for example when we use tag aware sharding. So I wonder what will happen when the primary shard is down?

Thanks a lot.
I guess you mean primary shard is down, because the term primary database is not applicable. The behavior will be the same as losing any other shard. The difference is that both sharded and unsharded collections will get affected.