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Moveable Collections

Move a Collection

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  • About this Task
  • Access Control
  • Before you Begin
  • Steps
  • Learn More

Starting in MongoDB 8.0, you can move an unsharded collection to a different shard using the moveCollection command.

About this Task

Access Control

If your deployment has access control enabled, the enableSharding role grants you access to run the moveCollection command.

Before you Begin

Before you move your collection, ensure that you meet the following requirements:

  • Your application can tolerate a period of two seconds where the affected collection blocks writes. During the time period where writes are blocked, your application experiences an increase in latency.

  • Your database meets these resource requirements:

    • Ensure the shard you are moving the collection to has enough storage space for the collection and its indexes. The destination shard requires at least ( Collection storage size + Index Size ) * 2 bytes available.

    • Ensure that your I/O capacity is below 50%.

    • Ensure that your CPU load is below 80%.

Important

These requirements are not enforced by the database. A failure to allocate enough resources can result in:

  • the database running out of space and shutting down

  • decreased performance

  • the operation taking longer than expected

If your application has time periods with less traffic, perform this operation on the collection during that time if possible.

Steps

1

Move the collection.

To move an unsharded collection named inventory on the app database to the shard02 shard, run moveCollection:

db.adminCommand(
  {
    moveCollection: "app.inventory",
    toShard: "shard02"
  }
)

To get a list of the available shard IDs, run sh.status(). For details, see sh.status() Output.

2

Monitor the progress of the moveCollection operation.

  1. Monitor the time remaining.

    To monitor the time remaining for the moveCollection operation, use the $currentOp pipeline stage.

    This example shows how to check the progress of moveCollection on the app.inventory collection:

    db.getSiblingDB("admin").aggregate( [
       { $currentOp: { allUsers: true, localOps: false } },
       {
          $match: {
          type: "op",
          "originatingCommand.reshardCollection": "app.inventory"
          }
       }
    ] )

    Note

    To see updated values, you need to continuously run the preceeding pipeline.

    The $currentOp pipeline outputs:

    • totalOperationTimeElapsedSecs: elapsed operation time in seconds

    • remainingOperationTimeEstimatedSecs: estimated time remaining in seconds for the current moveCollection operation. It is returned as -1 when a new moveCollection operation starts.

    Note

    remainingOperationTimeEstimatedSecs is set to a pessimistic time estimate:

    • The catch-up phase time estimate is set to the clone phase time, which
      is a relatively long time.
    • In practice, if there are only a few pending write operations, the
      actual catch-up phase time is relatively short.

    This pipeline stage has output similar to the following:

    [
       {
          shard: '<shard>',
          type: 'op',
          desc: 'ReshardingRecipientService | ReshardingDonorService | ReshardingCoordinatorService <reshardingUUID>',
          op: 'command',
          ns: '<database>.<collection>',
          originatingCommand: {
          reshardCollection: '<database>.<collection>',
          key: <shardkey>,
          unique: <boolean>,
          collation: { locale: 'simple' }
          },
          totalOperationTimeElapsedSecs: <number>,
          remainingOperationTimeEstimatedSecs: <number>,
          ...
       },
       ...
    ]

  2. Monitor the number of bytes transferred.

    To monitor the number of bytes transferred, use shardingStatistics.resharding.active.bytesCopied and compare against the number of bytes in the collection.

3

Confirm the collection has been moved.

To confirm the collection has been moved to the expected shard, use the $collStats pipeline stage.

This example shows how to confirm that the app.inventory collection exists on the expected shard:

db.inventory.aggregate( [
  { $collStats: {} },
  { $project: { "shard": 1 } }
] )

This pipeline stage has output similar to the following:

[ { shard: 'shard02' } ]

Learn More

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