这里的配置碎片是什么意思？

Question

我创建了一个 MongoDB 分片集群，其中副本集为 rsa、rsb 和 rsc。配置服务器副本集为 configRS。最初，分片状态仅列出这些分片作为分片数据库的主分片。但现在我可以看到列出了另一个分片“config”。这个分片来自哪里？这是什么意思？

 [root@akscentnewtest 1]#  mongo --port 27777 --host 172.16.66.74 -u cv_user -p'xxxxx' --authenticationDatabase 'admin' --eval "db.adminCommand( { listDatabases: 1 } )"
    MongoDB shell version v3.6.6
    connecting to: mongodb://172.16.66.74:27777/
    MongoDB server version: 3.6.6
    {
        "databases" : [
            {
                    "name" : "admin",
                    "sizeOnDisk" : 147456,
                    "empty" : false,
                    "shards" : {
                            "config" : 147456
                    }
            },
            {
                    "name" : "config",
                    "sizeOnDisk" : 1228800,
                    "empty" : false,
                    "shards" : {
                            "rsa" : 200704,
                            "rsb" : 102400,
                            "rsc" : 118784,
                            "config" : 806912
                    }
            },
            {
                    "name" : "test",
                    "sizeOnDisk" : 32768,
                    "empty" : false,
                    "shards" : {
                            "rsb" : 32768
                    }
            }
        ],
        "totalSize" : 1409024,
        "totalSizeMb" : 1,
        "ok" : 1,
        "$clusterTime" : {
            "clusterTime" : Timestamp(1565775023, 1),
            "signature" : {
                    "hash" : BinData(0,"vIxGUfXD3ZzE5gkr5wSHDBwJuSg="),
                    "keyId" : NumberLong("6721659792886071315")
            }
        },
        "operationTime" : Timestamp(1565775023, 1)
    }

分片状态

mongos> sh.status()
--- Sharding Status ---
  sharding version: {
    "_id" : 1,
    "minCompatibleVersion" : 5,
    "currentVersion" : 6,
    "clusterId" : ObjectId("5d4821f8ffb408d9b4ea389a")
  }
  shards:
        {  "_id" : "rsa",  "host" : "rsa/172.16.63.8:27000",  "state" : 1 }
        {  "_id" : "rsb",  "host" : "rsb/172.16.66.74:27000",  "state" : 1 }
        {  "_id" : "rsc",  "host" : "rsc/172.16.73.51:27001",  "state" : 1 }
  active mongoses:
        "3.6.6" : 1
  autosplit:
        Currently enabled: yes
  balancer:
        Currently enabled:  yes
        Currently running:  yes
        Failed balancer rounds in last 5 attempts:  5
        Last reported error:  Could not find host matching read preference { mode: "primary" } for set rsa
        Time of Reported error:  Wed Aug 14 2019 15:14:01 GMT+0530 (IST)
        Migration Results for the last 24 hours:
                No recent migrations
  databases:
        {  "_id" : "config",  "primary" : "config",  "partitioned" : true }
                config.system.sessions
                        shard key: { "_id" : 1 }
                        unique: false
                        balancing: true
                        chunks:
                                rsa     1
                        { "_id" : { "$minKey" : 1 } } -->> { "_id" : { "$maxKey" : 1 } } on : rsa Timestamp(1, 0)
        {  "_id" : "test",  "primary" : "rsb",  "partitioned" : false }