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:

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