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Reshard a Collection

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  • About this Task
  • Before you Begin
  • Steps
  • Start the resharding operation.
  • Monitor the resharding operation.
  • Finish the resharding operation.
  • Block writes early to force resharding to complete
  • Abort resharding operation
  • Behavior
  • Minimum Duration of a Resharding Operation
  • Retryable Writes
  • Error Case
  • Duplicate _id Values

New in version 5.0.

The ideal shard key allows MongoDB to distribute documents evenly throughout the cluster while facilitating common query patterns. A suboptimal shard key can lead to performance or scaling issues due to uneven data distribution.

Starting in MongoDB 5.0, you can change the shard key for a collection to change the distribution of your data across a cluster.

Starting in MongoDB 7.2, you can reshard a collection on the same shard key, allowing you to redistribute data to include new shards or to different zones without changing your shard key.

Note

Before resharding your collection, read Troubleshoot Shard Keys for information on common performance and scaling issues and advice on how to fix them.

Before you reshard 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.

    If your workload cannot tolerate this requirement, consider refining your shard key instead.

  • Your database meets these resource requirements:

    • Ensure that the available storage space on each shard the collection will be distributed across is at least twice the size of the collection that you want to reshard and its total index size, divided by the number of shards.

      storage_req = ( ( collection_storage_size + index_size ) * 2 ) / shard_count

      For example, consider a collection that contains 2 TB of data and has a 400 GB index distributed across four shards. To perform a resharding operation on this collection, each shard would require 1.2 TB of available storage.

      1.2 TB storage = ( ( 2 TB collection + 0.4 TB index ) * 2 ) / 4 shards

      To meet storage requirements, you may need to upgrade to the next tier of storage during the resharding operation. You can scale down once the operation completes.

    • 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.

  • You must rewrite your application's queries to use both the current shard key and the new shard key.

    Tip

    If your application can tolerate downtime, you can perform these steps to avoid rewriting your application's queries to use both the current and new shard keys:

    1. Stop your application.

    2. Rewrite your application to use the new shard key.

    3. Wait until resharding completes. To monitor the resharding process, use the $currentOp pipeline stage.

    4. Deploy your rewritten application.

    Before resharding completes, the following queries return an error if the query filter does not include either the current shard key or a unique field (like _id):

    For optimal performance, we recommend that you also rewrite other queries to include the new shard key.

    Once the resharding operation completes, you can remove the old shard key from the queries.

  • No index builds are in progress. Use db.currentOp() to check for any running index builds:

    db.adminCommand(
    {
    currentOp: true,
    $or: [
    { op: "command", "command.createIndexes": { $exists: true } },
    { op: "none", "msg" : /^Index Build/ }
    ]
    }
    )

    In the result document, if the inprog field value is an empty array, there are no index builds in progress:

    {
    inprog: [],
    ok: 1,
    '$clusterTime': { ... },
    operationTime: <timestamp>
    }

Note

Resharding is a write-intensive process which can generate increased rates of oplog. You may wish to:

  • set a fixed oplog size to prevent unbounded oplog growth.

  • increase the oplog size to minimize the chance that one or more secondary nodes becomes stale.

See the Replica Set Oplog documentation for more details.

Important

We strongly recommend that you check the About this Task and read the Steps section in full before resharding your collection.

In a collection resharding operation, a shard can be a:

  • donor, which currently stores chunks for the sharded collection.

  • recipient, which stores new chunks for the sharded collection based on the shard keys and zones.

A shard can be donor and a recipient at the same time. The set of donor shards is identical to the recipient shards, unless you use zones.

The config server primary is always the resharding coordinator and starts each phase of the resharding operation.

1

While connected to the mongos, issue a reshardCollection command that specifies the collection to be resharded and the new shard key:

db.adminCommand({
reshardCollection: "<database>.<collection>",
key: <shardkey>
})

MongoDB sets the max number of seconds to block writes to two seconds and begins the resharding operation.

2

To monitor the resharding operation, you can use the $currentOp pipeline stage:

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

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 resharding operation. It is returned as -1 when a new resharding operation starts.

    Starting in:

    • MongoDB 5.0, but before MongoDB 6.1, remainingOperationTimeEstimatedSecs is only available on a recipient shard during a resharding operation.

    • MongoDB 6.1, remainingOperationTimeEstimatedSecs is also available on the coordinator during a resharding operation.

    The resharding operation performs these phases in order:

    1. The clone phase duplicates the current collection data.

    2. The catch-up phase applies any pending write operations to the resharded collection.

    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.

[
{
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>,
...
},
...
]
3

Throughout the resharding process, the estimated time to complete the resharding operation (remainingOperationTimeEstimatedSecs) decreases. When the estimated time is below two seconds, MongoDB blocks writes and completes the resharding operation. Until the estimated time to complete the resharing operation is below two seconds, the resharding operation does not block writes by default. During the time period where writes are blocked your application experiences an increase in latency.

Once the resharding process has completed, the resharding command returns ok: 1.

{
ok: 1,
'$clusterTime': {
clusterTime: <timestamp>,
signature: {
hash: Binary(Buffer.from("0000000000000000000000000000000000000000", "hex"), 0),
keyId: <number>
}
},
operationTime: <timestamp>
}

To see whether the resharding operation completed successfully, check the output of the sh.status() method:

sh.status()

The sh.status() method output contains a subsection for the databases. If resharding has completed successfully, the output lists the new shard key for the collection:

databases
[
{
database: {
_id: '<database>',
primary: '<shard>',
version: {
uuid: <uuid>,
timestamp: <timestamp>,
lastMod: <number>
}
},
collections: {
'<database>.<collection>': {
shardKey: <shardkey>,
unique: <boolean>,
balancing: <boolean>,
chunks: [],
tags: []
}
}
}
...
]

Note

If the resharded collection uses Atlas Search, the search index will become unavailable when the resharding operation completes. You need to manually rebuild the search index once the resharding operation completes.

You can manually force the resharding operation to complete by issuing the commitReshardCollection command. This is useful if the current time estimate to complete the resharding operation is an acceptable duration for your collection to block writes. The commitReshardCollection command blocks writes for the duration of the critical section and forces the resharding operation to complete. The command has the following syntax:

db.adminCommand({
commitReshardCollection: "<database>.<collection>"
})

You can abort the resharding operation during any stage of the resharding operation, even after running the commitReshardCollection, until shards have fully caught up.

For example, if remainingOperationTimeEstimatedSecs does not decrease, you can abort the resharding operation with the abortReshardCollection command:

db.adminCommand({
abortReshardCollection: "<database>.<collection>"
})

After canceling the operation, you can retry the resharding operation during a time window with lower write volume. If this is not possible, add more shards before retrying.

The minimum duration of a resharding operation is always 5 minutes.

Retryable writes initiated before or during resharding can be retried during and after the collection has been resharded for up to 5 minutes. After 5 minutes you may be unable to find the definitive result of the write and subsequent attempts to retry the write fail with an IncompleteTransactionHistory error.

The resharding operation fails if _id values are not globally unique to avoid corrupting collection data. Duplicate _id values can also prevent successful chunk migration. If you have documents with duplicate _id values, copy the data from each into a new document, and then delete the duplicate documents.

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