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mergeAllChunksOnShard
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Definition
mergeAllChunksOnShardmergeAllChunksOnShardfinds and merges all mergeable chunks that a shard owns for a given collection.
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.
jumbochunks 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 |