From shell in mongos with >1 shard added mongos>sh.enableSharding( "ismbtest" ) mongos>sh.shardCollection( "ismbtest.audit" ,{region:1,_id:1}) mongos>sh.splitAt( "ismbtest.audit" ,{region: "NA" ,_id: null }) mongos>sh.setBalancerState( false ) mongos>sh.status() — Sharding Status — sharding version: { "_id" : 1, "version" : 4, "minCompatibleVersion" : 4, "currentVersion" : 5, "clusterId" : ObjectId( "53be36b80698700e22a3bb6f" ) } shards: { "_id" : "shard0000" , "host" : "localhost:27502" } { "_id" : "shard0001" , "host" : "localhost:27503" } databases: { "_id" : "admin" , "partitioned" : false , "primary" : "config" } { "_id" : "test" , "partitioned" : false , "primary" : "shard0000" } { "_id" : "ismbtest" , "partitioned" : true , "primary" : "shard0000" } ismbtest.audit shard key: { "region" : 1, "_id" : 1 } chunks: shard0001 1 shard0000 1 { "region" : { "$minKey" : 1 }, "_id" : { "$minKey" : 1 } } -->> { "region" : "NA" , "_id" : { "$minKey" : 1 } } on : shard0001 Timestamp(2, 0) { "region" : "NA" , "_id" : { "$minKey" : 1 } } -->> { "region" : { "$maxKey" : 1 }, "_id" : { "$maxKey" : 1 } } on : shard0000 Timestamp(2, 1) { "_id" : "isbmtest" , "partitioned" : false , "primary" : "shard0001" } mongos> db.audit.insert({region: "EMEA" ,_id: "123-456-789-101112" ,text: "On Emea" }) WriteResult({ "nInserted" : 1 }) mongos> db.audit.insert({region: "NA" ,_id: "123-456-789-101113" ,text: "On NA" }) WriteResult({ "nInserted" : 1 }) mongos> db.audit.stats() { "sharded" : true , "systemFlags" : 1, "userFlags" : 1, "ns" : "ismbtest.audit" , "count" : 2, "numExtents" : 2, "size" : 224, "storageSize" : 16384, "totalIndexSize" : 32704, "indexSizes" : { "_id_" : 16352, "region_1__id_1" : 16352 }, "avgObjSize" : 112, "nindexes" : 2, "nchunks" : 2, "shards" : { "shard0000" : { "ns" : "ismbtest.audit" , "count" : 0, "size" : 0, "storageSize" : 8192, "numExtents" : 1, "nindexes" : 2, "lastExtentSize" : 8192, "paddingFactor" : 1, "systemFlags" : 1, "userFlags" : 1, "totalIndexSize" : 16352, "indexSizes" : { "_id_" : 8176, "region_1__id_1" : 8176 }, "ok" : 1 }, "shard0001" : { "ns" : "ismbtest.audit" , "count" : 2, "size" : 224, "avgObjSize" : 112, "storageSize" : 8192, "numExtents" : 1, "nindexes" : 2, "lastExtentSize" : 8192, "paddingFactor" : 1, "systemFlags" : 1, "userFlags" : 1, "totalIndexSize" : 16352, "indexSizes" : { "_id_" : 8176, "region_1__id_1" : 8176 }, "ok" : 1 } }, "ok" : 1 } Both went to same shard, it appears that a chunk range of { "region" : { "$minKey" : 1 }, "_id" : { "$minKey" : 1 } } -->> { "region" : "NA" , "_id" : { "$minKey" : 1 } } on : shard0001 Timestamp(2, 0) Somehow includes {region: "NA" , "_id:" 123-456-789-101113"} in sorting does minKey not come first?