Wrong sharding on mongoDB 3.0

Question

I have a mongos with 2 shard servers (and 3 config servers)

sh.status()
--- Sharding Status ---
  sharding version: {
        "_id" : 1,
        "minCompatibleVersion" : 5,
        "currentVersion" : 6,
        "clusterId" : ObjectId("55ae2ddab9e8641e489ce39b")
}
  shards:
        {  "_id" : "shard0000",  "host" : "192.168.212.182:27018" }
        {  "_id" : "shard0001",  "host" : "192.168.212.106:27018" }
  databases:
        {  "_id" : "admin",  "partitioned" : false,  "primary" : "config" }
        {  "_id" : "Vision_1282",  "partitioned" : true,  "primary" : "shard0001" }
                Vision_1282.Group
                        shard key: { "groepNr" : 1, "valId" : 1 }
                        chunks:
                                shard0001       1
                        { "groepNr" : { "$minKey" : 1 }, "valId" : { "$minKey" : 1 } } -->> { "groepNr" : { "$maxKey" : 1 }, "valId" : { "$maxKey" : 1 } } on : shard0001 Timestamp(1, 0)
        {  "_id" : "db",  "partitioned" : false,  "primary" : "shard0000" }

I made the shard key as follows:

db.runCommand({ shardcollection : "Vision_1282.Group", key : {groepNr : 1, valId : 1}})

I insert about 10.000 records, where the groepNr is 1, 2, 3 or 4 and a valId can be between 1 and 4001.

When I look at the shardDistribution I see that shard001 only has records where groepNr = 1 and valId = 1. All other records are in shard002.

I thought that the value behind the key value (here groepNr and valId) is 1 for ascending and -1 for descending.

PS. in PHP code, where I do the inserts I also ensure to use the index.
$collection->ensureIndex(array("groepNr" => 1 , "valId" => 1));

Did I do something wrong with creating the shardkey?

Answer 1

I have lowered the chunksize to 1 MB:

 { "_id" : "chunksize", "value" : 1 }

I inserted now about 10.000 documents.

If I look at the sh.status(), I see this:

Shard shard0000 at 192.168.212.182:27018
 data : 1.33Mb docs : 5761 chunks : 4
 estimated data per chunk : 343kb
 estimated docs per chunk : 1440

Shard shard0001 at 192.168.212.106:27018
 data : 1017kb docs : 4249 chunks : 3
 estimated data per chunk : 339kb
 estimated docs per chunk : 1416

Totals
 data : 2.33Mb docs : 10010 chunks : 7
 Shard shard0000 contains 57.41% data, 57.55% docs in cluster, avg obj size on shard : 243b
 Shard shard0001 contains 42.58% data, 42.44% docs in cluster, avg obj size on shard : 245b

Also I run the scripts AllChunkInfo("Vision_1282.Group"), with this result:

mongos> AllChunkInfo("Vision_1282.Group")
ChunkID,Shard,ChunkSize,ObjectsInChunk
Vision_1282.Group-groepNr_MinKeyvalId_MinKey,shard0001,83504,301
Vision_1282.Group-groepNr_"1"valId_"2000",shard0001,1920,8
Vision_1282.Group-groepNr_"1"valId_"2002",shard0001,974144,4012
Vision_1282.Group-groepNr_"2"valId_"1",shard0000,524384,2138
Vision_1282.Group-groepNr_"3"valId_"4",shard0000,498704,2031
Vision_1282.Group-groepNr_"4"valId_"4",shard0000,292800,1220
Vision_1282.Group-groepNr_"4"valId_"6",shard0000,113520,473
***********Summary Chunk Information***********
Total Chunks: 7
Average Chunk Size (bytes): 355568
Empty Chunks: 0
Average Chunk Size (non-empty): 355568

So you are right, the records were to small and the chunksize was to big.

Tx.

Answer 2

No need to do the ensureIndex in php. When sharding your collection, the index gets created automatically. Mostly I try to create the index before the shardCollection, just a habit.

First make sure you don't have Vision_1282 databases on your shards themselves:

shardA:PRIMARY> show dbs
local    6.075GB
shardA:PRIMARY> exit
bye

shardB:PRIMARY> show dbs
local    6.075GB
shardB:PRIMARY> exit
bye

Then I did the following steps:

(using a great script from Adam Comerford to check size/count/place of chunks : https://github.com/comerford/mongodb-scripts)

$ mongo --shell allChunkInfo.js
MongoDB shell version: 3.0.3
connecting to: test
type "help" for help
mongos> show dbs
admin    (empty)
config   0.016GB
mongos> sh.enableSharding("Vision_1282")
{ "ok" : 1 }
mongos> use Vision_1282
switched to db Vision_1282
mongos> db.Group.createIndex({groepNr : 1, valId : 1})
{
    "raw" : {
        "shardB/mongo2-2-1:27018,mongo2-2-2:27018" : {
            "createdCollectionAutomatically" : true,
            "numIndexesBefore" : 1,
            "numIndexesAfter" : 2,
            "ok" : 1,
            "$gleStats" : {
                "lastOpTime" : Timestamp(1437540334, 5),
                "electionId" : ObjectId("55a72087bc8dee49cc03fce5")
            }
        }
    },
    "ok" : 1
}
mongos> use admin
switched to db admin
mongos> db.runCommand({ shardcollection : "Vision_1282.Group", key : {groepNr : 1, valId : 1}})
{ "collectionsharded" : "Vision_1282.Group", "ok" : 1 }
mongos> use Vision_1282
switched to db Vision_1282
mongos> db.Group.insert({groepNr:1, valId:3})
WriteResult({ "nInserted" : 1 })
mongos> db.Group.find()
{ "_id" : ObjectId("55af205a0eb443db0213e359"), "groepNr" : 1, "valId" : 3 }
mongos> AllChunkInfo("Vision_1282.Group")
ChunkID,Shard,ChunkSize,ObjectsInChunk
Vision_1282.Group-groepNr_MinKeyvalId_MinKey,shardB,112,1
***********Summary Chunk Information***********
Total Chunks: 1
Average Chunk Size (bytes): 112
Empty Chunks: 0
Average Chunk Size (non-empty): 112
mongos> db.Group.remove({})
WriteResult({ "nRemoved" : 1 })
mongos> db.Group.count()
0

Using a python script to import 10.000 records, then the result was indeed 9999 records on 1 shard and 1 document in the other.

mongos> AllChunkInfo("Vision_1282.Group")
ChunkID,Shard,ChunkSize,ObjectsInChunk
Vision_1282.Group-groepNr_MinKeyvalId_MinKey,shardA,48,1
Vision_1282.Group-groepNr_1valId_2,shardB,360128,7502
Vision_1282.Group-groepNr_4valId_5,shardB,119872,2497
***********Summary Chunk Information***********
Total Chunks: 3
Average Chunk Size (bytes): 160016
Empty Chunks: 0
Average Chunk Size (non-empty): 160016

But... after inserting bigger records with more data... and adding a lot more records ... and patience ... finally you can see the sharding in action:

mongos> AllChunkInfo("Vision_1282.Group")
ChunkID,Shard,ChunkSize,ObjectsInChunk
Vision_1282.Group-groepNr_MinKeyvalId_MinKey,shardA,576,8
Vision_1282.Group-groepNr_1valId_2,shardA,10490992,106806
Vision_1282.Group-groepNr_2valId_4455,shardB,11521744,110015
Vision_1282.Group-groepNr_3valId_14462,shardB,4757472,42486
Vision_1282.Group-groepNr_4valId_5,shardB,8998288,87476
***********Summary Chunk Information***********
Total Chunks: 5
Average Chunk Size (bytes): 7153814.4
Empty Chunks: 0
Average Chunk Size (non-empty): 7153814.4

mongos> sh.status()
--- Sharding Status --- 
  sharding version: {
    "_id" : 1,
    "minCompatibleVersion" : 5,
    "currentVersion" : 6,
    "clusterId" : ObjectId("55a754a9ac01a84a90b137cf")
}
  shards:
    {  "_id" : "shardA",  "host" : "shardA/mongo1:27018,mongo2:27018" }
    {  "_id" : "shardB",  "host" : "shardB/mongo3:27018,mongo4:27018" }
  balancer:
    Currently enabled:  yes
    Currently running:  yes
        Balancer lock taken at Wed Jul 22 2015 05:15:40 GMT+0000 (UTC) by ac2-1:27017:1437029544:1804289383:Balancer:1681692777
    Failed balancer rounds in last 5 attempts:  0
  databases:
    {  "_id" : "admin",  "partitioned" : false,  "primary" : "config" }

    {  "_id" : "Vision_1282",  "partitioned" : true,  "primary" : "shardB" }
        Vision_1282.Group
            shard key: { "groepNr" : 1, "valId" : 1 }
            chunks:
                shardA  2
                shardB  3
            { "groepNr" : { "$minKey" : 1 }, "valId" : { "$minKey" : 1 } } -->> { "groepNr" : 1, "valId" : 2 } on : shardA Timestamp(2, 0) 
            { "groepNr" : 1, "valId" : 2 } -->> { "groepNr" : 2, "valId" : 4455 } on : shardA Timestamp(3, 0) 
            { "groepNr" : 2, "valId" : 4455 } -->> { "groepNr" : 3, "valId" : 14462 } on : shardB Timestamp(3, 1) 
            { "groepNr" : 3, "valId" : 14462 } -->> { "groepNr" : 4, "valId" : 5 } on : shardB Timestamp(2, 4) 
            { "groepNr" : 4, "valId" : 5 } -->> { "groepNr" : { "$maxKey" : 1 }, "valId" : { "$maxKey" : 1 } } on : shardB Timestamp(1, 3) 

Conclusion: because the records were too small, and the chunk was not big enough to split yet.

To see the configuration of the size of the chunks:

mongos> use config
switched to db config
mongos> db.settings.find()
{ "_id" : "chunksize", "value" : 64 }

• http://docs.mongodb.org/manual/tutorial/modify-chunk-size-in-sharded-cluster/
• http://docs.mongodb.org/manual/core/sharding-chunk-splitting/