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Modify Chunk Size in a Sharded Cluster

The default chunk size for a sharded cluster is 128 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. The allowed range of the chunk size is between 1 and 1024 megabytes, inclusive.

To modify the chunk size, use the following procedure:

  1. Connect to any mongos in the cluster using mongosh

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

    use config
  3. Issue the following command to store the global chunk size configuration value:

    db.settings.updateOne(
    { _id: "chunksize" },
    { $set: { _id: "chunksize", value: <sizeInMB> } },
    { upsert: true }
    )

Modifying the chunk size has several limitations:

  • Starting in MongoDB 6.1, automatic chunk splitting is not performed. This is because of balancing policy improvements. Auto-splitting commands still exist, but do not perform an operation. For details, see Balancing Policy Changes.

    Starting in MongoDB 6.1, the following auto-splitting commands do not perform an operation:

  • In MongoDB versions earlier than 6.1, automatic splitting only occurs for insert or update operations.

  • 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.

  • The allowed range of the chunk size is between 1 and 1024 megabytes, inclusive.

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