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Modify Chunk Size in a Sharded Cluster

When the first mongos connects to a set of config servers, it initializes the sharded cluster with a default chunk size of 64 megabytes. This default chunk size works well for most deployments; however, if you notice that automatic migrations have more I/O than your hardware can handle, you may want to reduce the chunk size. For automatic splits and migrations, a small chunk size leads to more rapid and frequent migrations.

To modify the chunk size, use the following procedure:

  1. Connect to any mongos in the cluster using the mongo shell.

  2. Issue the following command to switch to the Config Database:

    use config
  3. Issue the following save() operation to store the global chunk size configuration value: { _id:"chunksize", value: <sizeInMB> } )


The chunkSize and --chunkSize options, passed at startup to the mongos, do not affect the chunk size after you have initialized the cluster.

To avoid confusion, always set the chunk size using the above procedure instead of the startup options.

Modifying the chunk size has several limitations:

  • Automatic splitting only occurs on insert or update.
  • If you lower the chunk size, it may take time for all chunks to split to the new size.
  • Splits cannot be undone.
  • If you increase the chunk size, existing chunks grow only through insertion or updates until they reach the new size.