Laptop:tmp user$ mongo MongoDB shell version: 3.2.0 connecting to: test mongos> use test switched to db test mongos> db.mycol.ensureIndex( { id : 1 }, { unique : true }) { "raw" : { "shard01/Laptop.local:27018,Laptop.local:27019,Laptop.local:27020" : { "createdCollectionAutomatically" : true , "numIndexesBefore" : 1, "numIndexesAfter" : 2, "ok" : 1, "$gleStats" : { "lastOpTime" : Timestamp(1452269862, 2), "electionId" : ObjectId( "568fe0d80000000000000001" ) } } }, "ok" : 1 } mongos> sh.enableSharding( 'test' ); { "ok" : 1 } mongos> sh.shardCollection( 'test.mycol' , {id: 1}); { "collectionsharded" : "test.mycol" , "ok" : 1 } mongos> sh.startBalancer() mongos> mongos> sh.getBalancerState() true mongos> mongos> sh.isBalancerRunning() false mongos> mongos> sh.status() --- Sharding Status --- sharding version: { "_id" : 1, "minCompatibleVersion" : 5, "currentVersion" : 6, "clusterId" : ObjectId( "568fe0cf098724379302dda5" ) } shards: { "_id" : "shard01" , "host" : "shard01/Laptop.local:27018,Laptop.local:27019,Laptop.local:27020" } { "_id" : "shard02" , "host" : "shard02/Laptop.local:27021,Laptop.local:27022,Laptop.local:27023" } { "_id" : "shard03" , "host" : "shard03/Laptop.local:27024,Laptop.local:27025,Laptop.local:27026" } active mongoses: "3.2.0" : 1 balancer: Currently enabled: yes Currently running: no Failed balancer rounds in last 5 attempts: 0 Migration Results for the last 24 hours: No recent migrations databases: { "_id" : "test" , "primary" : "shard01" , "partitioned" : true } test.mycol shard key: { "id" : 1 } unique: false balancing: true chunks: shard01 1 { "id" : { "$minKey" : 1 } } -->> { "id" : { "$maxKey" : 1 } } on : shard01 Timestamp(1, 0) mongos> sh.addShardTag( 'shard01' , 'TAG1' ); mongos> sh.addShardTag( 'shard02' , 'TAG2' ); mongos> sh.addShardTag( 'shard03' , 'TAG2' ); mongos> mongos> sh.addTagRange( 'shard01' , {id: 1}, {id: 200}, 'TAG1' ); mongos> sh.addTagRange( 'shard01' , {id: 100}, {id: 150}, 'TAG2' ); mongos> sh.addTagRange( 'shard01' , {id: 0}, {id: 300}, 'TAG2' ); mongos> mongos> use config switched to db config mongos> db.tags.find() { "_id" : { "ns" : "shard01" , "min" : { "id" : 1 } }, "ns" : "shard01" , "min" : { "id" : 1 }, "max" : { "id" : 200 }, "tag" : "TAG1" } { "_id" : { "ns" : "shard01" , "min" : { "id" : 100 } }, "ns" : "shard01" , "min" : { "id" : 100 }, "max" : { "id" : 150 }, "tag" : "TAG2" } { "_id" : { "ns" : "shard01" , "min" : { "id" : 0 } }, "ns" : "shard01" , "min" : { "id" : 0 }, "max" : { "id" : 300 }, "tag" : "TAG2" } mongos> We now have three overlapping ranges with two different TAGs. No errors were raised. The intended location of a chunk with "id" values from 110 => 125 appears to be ambiguous, and the expected behaviour of the balancer undefined.