moveChunk¶

Definition定义¶

moveChunk¶

Internal administrative command. Moves chunks between shards. Issue the moveChunk command via a mongos instance while using the admin database. Use the following forms:

db.adminCommand( { moveChunk : <namespace> ,
                 find : <query> ,
                 to : <string>,
                 forceJumbo: <boolean>, // Starting in MongoDB 4.4
                 _secondaryThrottle : <boolean>,
                 writeConcern: <document>,
                 _waitForDelete : <boolean> } )

Alternatively:

db.adminCommand( { moveChunk : <namespace> ,
                 bounds : <array> ,
                 to : <string>,
                 forceJumbo: <boolean>, // Starting in MongoDB 4.4
                 _secondaryThrottle : <boolean>,
                 writeConcern: <document>,
                 _waitForDelete : <boolean> } )

The moveChunk command has the following fields:

Field字段	Type类型	Description描述
moveChunk	string	The namespace of the collection where the chunk exists. Specify the collection’s full namespace, including the database name.
find	document	An equality match on the shard key that specifies the shard-key value of the chunk to move. Specify either the bounds field or the find field but not both. Do not use the find field to select chunks in collections that use a hashed shard key.
bounds	array	The bounds of a specific chunk to move. The array must consist of two documents that specify the lower and upper shard key values of a chunk to move. Specify either the bounds field or the find field but not both. Use bounds to select chunks in collections that use a hashed shard key.
to	string	The name of the destination shard for the chunk.
forceJumbo	boolean	Optional. A flag that determines if the command can move a chunk that is too large to migrate. The chunk may or may not be labeled as jumbo. If true, the command can move the chunk. If false, the command cannot move the chunk. The default is false. Warning When you run moveChunk with forceJumbo: true, write operations to the collection may block for a long period of time during the migration. To migrate these large chunks without this long blocking period, see Balance Chunks that Exceed Size Limit instead. New in version 4.4.版本4.4中的新功能。
_secondaryThrottle	boolean	Optional.可选。Starting in MongoDB 3.4, for WiredTiger, defaults to false. If true, then by default, each document move during chunk migration propagates to at least one secondary before the balancer proceeds with the next document. This is equivalent to a write concern of { w: 2 }. Use the writeConcern option to specify a different write concern. If false, the balancer does not wait for replication to a secondary and instead continues with the next document. For more information, see Secondary Throttle.
writeConcern	document	Optional.可选。A document that expresses the write concern that the _secondaryThrottle will use to wait for secondaries during the chunk migration. writeConcern requires _secondaryThrottle: true.
_waitForDelete	boolean	Optional.可选。Internal option for testing purposes. The default is false. If set to true, the delete phase of a moveChunk operation blocks.

The value of bounds takes the form:

[ { hashedField : <minValue> } ,
  { hashedField : <maxValue> } ]

The chunk migration section describes how chunks move between shards on MongoDB.

Considerations考虑事项¶

Only use the moveChunk in special circumstances such as preparing your sharded cluster for an initial ingestion of data, or a large bulk import operation. In most cases allow the balancer to create and balance chunks in sharded clusters. See Create Chunks in a Sharded Cluster for more information.

Behavior行为¶

errmsg: "The collection's metadata lock is already taken."