mergeAllChunksOnShard

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Definition

Compatibility
Syntax
Command Fields
Behavior
Example

Definition

mergeAllChunksOnShard: mergeAllChunksOnShard finds and merges all mergeable chunks that a shard owns for a given collection.

Compatibility

This command is available in deployments hosted in the following environments:

MongoDB Atlas: The fully managed service for MongoDB deployments in the cloud

Important

This command is not supported in serverless instances. For more information, see Unsupported Commands.

MongoDB Enterprise: The subscription-based, self-managed version of MongoDB
MongoDB Community: The source-available, free-to-use, and self-managed version of MongoDB

Syntax

The command has the following syntax:

db.adminCommand(
  {
    mergeAllChunksOnShard: <name of the collection>,
    shard: <name of the shard>,
    maxNumberOfChunksToMerge: <maximum number of chunks to merge> /* optional */
  }
)

Command Fields

The command takes the following fields:

Field	Type	Necessity	Description
`mergeAllChunksOnShard`	string	Required	Name of the collection.
`shard`	string	Required	Name of the shard.
`maxNumberOfChunksToMerge`	integer	Optional	Maximum number of chunks to merge.

Behavior

mergeAllChunksOnShard finds and merges all mergeable chunks for a collection on the same shard. Two or more contiguous chunks in the same collection are mergeable when they meet all of these conditions:

They are owned by the same shard.
They are not jumbo chunks. jumbo chunks are not mergeable because they cannot participate in migrations.
Their history can be purged safely, without breaking transactions and snapshot reads:
- The last migration involving the chunk happened at least as many seconds ago as the value of minSnapshotHistoryWindowInSeconds.
- The last migration involving the chunk happened at least as many seconds ago as the value of transactionLifetimeLimitSeconds.

Example

This example assumes that history is empty for all chunks and all chunks are non-jumbo. Since both conditions are true, all contiguous intervals on the same shard are mergeable.

Setup

These chunks belong to a collection named coll with shard key x. There are nine chunks in total.

Chunk ID	Min	Max	Shard
A	`x: 0`	`x: 10`	Shard0
B	`x: 10`	`x: 20`	Shard0
C	`x: 20`	`x: 30`	Shard0
D	`x: 30`	`x: 40`	Shard0
E	`x: 40`	`x: 50`	Shard1
F	`x: 50`	`x: 60`	Shard1
G	`x: 60`	`x: 70`	Shard0
H	`x: 70`	`x: 80`	Shard0
I	`x: 80`	`x: 90`	Shard1

Steps

Merge All Mergeable Chunks on Shard0

db.adminCommand( { mergeAllChunksOnShard: "db.coll", shard: "Shard0" } )

This command merges the contiguous sequences of chunks:

A-B-C-D
G-H

Merge All Mergeable Chunks on Shard1

db.adminCommand( { mergeAllChunksOnShard: "db.coll", shard: "Shard1" } )

This command merges the contiguous sequences of chunks E-F.

Result

After these commands have completed, the contiguous chunks have been merged. There are four total chunks instead of the original nine.

Chunk ID	Min	Max	Shard
A-B-C-D	`x: 0`	`x: 40`	Shard0
E-F	`x: 40`	`x: 60`	Shard1
G-H	`x: 60`	`x: 80`	Shard0
I	`x: 80`	`x: 90`	Shard1

Back

listShards

moveChunk