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Create Chunks in a Sharded Cluster

Pre-splitting the chunk ranges in an empty sharded collection allows clients to insert data into an already partitioned collection. In most situations a sharded cluster will create and distribute chunks automatically without user intervention. However, in a limited number of cases, MongoDB cannot create enough chunks or distribute data fast enough to support required throughput. For example:

  • If you want to partition an existing data collection that resides on a single shard.
  • If you want to ingest a large volume of data into a cluster that isn’t balanced, or where the ingestion of data will lead to data imbalance. For example, monotonically increasing or decreasing shard keys insert all data into a single chunk.

These operations are resource intensive for several reasons:

  • Chunk migration requires copying all the data in the chunk from one shard to another.

  • No shard can participate in more than one migration at any given time. To migrate multiple chunks from a shard, the balancer migrates the chunks one at a time.

    Changed in version 3.4: Observing the restriction that a shard can participate in at most one migration at a time, for a sharded cluster with n shards, MongoDB can perform at most n/2 (rounded down) simultaneous chunk migrations.

  • MongoDB creates splits only after an insert operation.


Only pre-split an empty collection. If a collection already has data, MongoDB automatically splits the collection’s data when you enable sharding for the collection. Subsequent attempts to manually create splits can lead to unpredictable chunk ranges and sizes as well as inefficient or ineffective balancing behavior.

To create chunks manually, use the following procedure:

  1. Split empty chunks in your collection by manually performing the split command on chunks.


    To create chunks for documents in the myapp.users collection using the email field as the shard key, use the following operation in the mongo shell:

    for ( var x=97; x<97+26; x++ ){
      for( var y=97; y<97+26; y+=6 ) {
        var prefix = String.fromCharCode(x) + String.fromCharCode(y);
        db.adminCommand( { split : "myapp.users" , middle : { email : prefix } } );

    This assumes a collection size of 100 million documents.

    For information on the balancer and automatic distribution of chunks across shards, see Cluster Balancer and Chunk Migration. For information on manually migrating chunks, see Migrate Chunks in a Sharded Cluster.