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mergeAllChunksOnShard

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  • Definition
  • Syntax
  • Command Fields
  • Behavior
  • Example
mergeAllChunksOnShard

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

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 */
}
)

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.

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:

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.

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
1
db.adminCommand( { mergeAllChunksOnShard: "db.coll", shard: "Shard0" } )

This command merges the contiguous sequences of chunks:

  • A-B-C-D

  • G-H

2
db.adminCommand( { mergeAllChunksOnShard: "db.coll", shard: "Shard1" } )

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

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
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