mergeAllChunksOnShard (database command)

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

• 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:

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.

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

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