Mongo shard collection not distributed if shard key is a small int

Question

We did mock test on the sharded mongo environment, but if the shard key value is small int, the collection didn't distribute, but if the shard key is big int, its working fine. Please read on...

Code used to insert records from mongos shell.

var shId = 15;
for (var i = 0; i < 100; i++) {
  if(i%50 == 0){
      shId = shId + 1;
  }
  db.Foo.insert( { shKeyId : shId , text:"this is a test" } );
}

With shId = 15, Foo collection split to two shards didn't work.

Environment : Two shards, each shard with Primary1 and two secondary mongod instances. Mongo config is running on one of the shard.

Sharding enabled on 'Foo' collection by shKeyId as hashed shard key. db.runCommand({ shardcollection : "test.Foo", key : {shKeyId : "hashed"}});

sh.status() output

mongos> sh.status();
--- Sharding Status --- 
  sharding version: {
    "_id" : 1,
    "version" : 3,
    "minCompatibleVersion" : 3,
    "currentVersion" : 4,
    "clusterId" : ObjectId("516ea48e979736fd306973c9")
}
  shards:
    {  "_id" : "mongo-perf-shrd1",  "host" : "mongo-perf-shrd1/sh1-prim-ip:27017,sh1-sec1-ip:27017,sh1-sec2-ip:27017" }
    {  "_id" : "mongo-perf-shrd2",  "host" : "mongo-perf-shrd2/sh2-prim-ip:27017,sh2-sec1-ip:27017,sh2-sec2-ip:27017" }
  databases:
    {  "_id" : "admin",  "partitioned" : false,  "primary" : "config" }

    {  "_id" : "test",  "partitioned" : true,  "primary" : "mongo-perf-shrd2" }
        test.Foo
            shard key: { "shKeyId" : "hashed" }
            chunks:
                mongo-perf-shrd2    2
                mongo-perf-shrd1    2
            { "shKeyId" : { "$minKey" : 1 } } -->> { "shKeyId" : NumberLong("-4611686018427387902") } on : mongo-perf-shrd2 { "t" : 2, "i" : 2 } 
            { "shKeyId" : NumberLong("-4611686018427387902") } -->> { "shKeyId" : NumberLong(0) } on : mongo-perf-shrd2 { "t" : 2, "i" : 3 } 
            { "shKeyId" : NumberLong(0) } -->> { "shKeyId" : NumberLong("4611686018427387902") } on : mongo-perf-shrd1 { "t" : 2, "i" : 4 } 
            { "shKeyId" : NumberLong("4611686018427387902") } -->> { "shKeyId" : { "$maxKey" : 1 } } on : mongo-perf-shrd1 { "t" : 2, "i" : 5 }

Shard Distribution output

   mongos> db.Foo.getShardDistribution();

Shard mongo-perf-shrd1 at mongo-perf-shrd1/ip1:27017,ip2,ip3:27017
 data : 6KiB docs : 100 chunks : 2
 estimated data per chunk : 3KiB
 estimated docs per chunk : 50

Shard mongo-perf-shrd2 at mongo-perf-shrd2/ip4:27017,ip5:27017,ip6:27017
 data : 0B docs : 0 chunks : 2
 estimated data per chunk : 0B
 estimated docs per chunk : 0

Totals
 data : 6KiB docs : 100 chunks : 4
 Shard mongo-perf-shrd1 contains 100% data, 100% docs in cluster, avg obj size on shard : 64B
 Shard mongo-perf-shrd2 contains 0% data, 0% docs in cluster, avg obj size on shard : NaNGiB

Can you describe what you mean by "the sharding didn't work"? — shelman
I was expecting, 50 records go into shard1, and other 50 records go into shard2. But all 100 records saved into shard1 — Samba
Could you post the output of sh.status() run on the mongos? — shelman
Thanks shelman. I have added sh.status output into the question — Samba
Why do you think all 100 records are saved in shard1? It looks like, given the output, that the chunks are evenly spread amongst the two shards (2 chunks in each shard) — shelman

Vladimir Muzhilov Vladimir Muzhilov · Accepted Answer · 2013-04-17T10:13:47

you have just monotonically shard key, this is not good solution: http://docs.mongodb.org/manual/core/sharded-cluster-internals/

for just mock tested, enough to select shard key by default _id field

Mongo shard collection not distributed if shard key is a small int

1 Answers