Reshard to the Same Shard Key

Starting in MongoDB 8.0, you can reshard to the same shard key to move data with no downtime or impact on workload. This enables you to:

Use the Reshard to Shard technique to shard a collection and distribute its data across all relevant shards
Add new shards faster
Remove shards faster
Rewrite collections to reclaim disk space

Starting in MongoDB 8.0, resharding reads data using natural order scan. Resharding first clones all the data and then builds relevant indexes, resulting in orders of magnitude speed improvement of the resharding process.

Command Syntax

You can reshard to the same key using the reshardCollection command with forceRedistribution set to true.

The reshardCollection command has the following syntax:

db.adminCommand(
   {
     reshardCollection: "<database>.<collection>",
     key: { "<shardkey>" },
     unique: <boolean>,
     numInitialChunks: <integer>,
     collation: { locale: "simple" },
     zones: [
         {
             min: { "<document with same shape as shardkey>" },
             max: { "<document with same shape as shardkey>" },
             zone: <string> | null
         },
     ],
     forceRedistribution: <bool>
   }
)

For details, see reshardCollection.

Use Cases

Resharding is a strategy to move data with no downtime or impact on workload. Use the Reshard to Shard technique to shard a collection and distribute data across all shards.

Use resharding to distribute your collections across all relevant shards faster than chunk migrations. Resharding writes to all shards in parallel while each shard can only participate in one chunk migration at a time. Resharding drops the old collection at the end of the process. There are no orphan documents at the end of resharding.

Reshard to Shard

The Reshard to Shard technique lets you use resharding to shard a collection and distribute the data to all of the shards in a cluster.

Consider using Reshard to Shard when you are initially sharding a collection of any size across any number of shards. If your deployment meets the resource requirements, use Reshard to Shard no matter how large the collection you want to shard.

Behavior

Storage

Calculate the required storage space for the resharding operation by adding your collection size and index size, assuming a minimum oplog window of 24 hours by using this formula:

Available storage required on each shard = [(collection size + index size) *2 ] / number of shards the collection will be distributed across.

For example, a 2TB collection and 400GB of indexes distributed across 4 shards will need a minimum of 1.2TB of available storage per shard:

[ (2 TB + 400GB) * 2 ] / 4 shards = 1.2 TB / shard

You must confirm that you have the available storage space in your cluster.

If there is insufficient space or I/O headroom available, you must increase the storage size. If there is insufficient CPU headroom, you must scale up the cluster by selecting a higher instance size.

Tip

If your MongoDB cluster is hosted on Atlas, you can use the Atlas UI to review storage, CPU, and I/O headroom metrics.

Latency

You must ensure that your application can tolerate two seconds where the collection being resharded blocks writes. When writes are blocked, your application experiences an increase in latency. If your workload cannot tolerate this requirement, use chunk migrations to balance your cluster.

Additional Resource Requirements

Your cluster must meet these additional requirements:

A minimum oplog window of 24 hours.
I/O capacity below 50%.
CPU load below 80%.

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