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split
¶Splits a chunk in a sharded cluster into two chunks. The mongos
instance splits and manages chunks automatically, but for exceptional circumstances the split
command does allow administrators to manually create splits. See Split Chunks in a Sharded Cluster for information on these circumstances, and on the MongoDB shell commands that wrap split
.
The split
command must be run in the admin
database and uses the following form:
The split
command takes a document with the following fields:
split |
string | The name of the collection where the chunk exists. Specify the collection’s full namespace, including the database name. |
find |
document | An query statement that specifies an equality match on the shard key. The match selects the chunk that contains the specified document. You must specify only one of the following: You cannot use the |
bounds |
array | New in version 2.4: The bounds of a chunk to split. You cannot use the |
middle |
document | The document to use as the split point to create two chunks. split requires one of the following options: find , bounds , or middle . |
When used with either the find
or the bounds
option, the split
command splits the chunk along the median. As such, the command cannot use the find
or the bounds
option to split an empty chunk since an empty chunk has no median.
To create splits in empty chunks, use either the middle
option with the split
command or use the splitAt
command.
To create a chunk split, connect to a mongos
instance, and issue the following command to the admin
database:
Or:
Or:
To create a split for a collection that uses a hashed shard key, use the bounds
parameter. Do not use the middle
parameter for this purpose.
Warning
Be careful when splitting data in a sharded collection to create new chunks. When you shard a collection that has existing data, MongoDB automatically creates chunks to evenly distribute the collection. To split data effectively in a sharded cluster you must consider the number of documents in a chunk and the average document size to create a uniform chunk size. When chunks have irregular sizes, shards may have an equal number of chunks but have very different data sizes. Avoid creating splits that lead to a collection with differently sized chunks.
See also参阅
moveChunk
, sh.moveChunk()
, sh.splitAt()
, and sh.splitFind()
, which wrap the functionality of split
.
The following sections provide examples of the split
command.
The split
command identifies the chunk in the people
collection of the test
database, that holds documents that match {
_id : 99 }
. split
does not require that a match exist, in order to identify the appropriate chunk. Then the command splits it into two chunks of equal size.
Note
split
creates two equal chunks by range as opposed to size, and does not use the selected point as a boundary for the new chunks
To define an arbitrary split point, use the following form:
The split
command identifies the chunk in the people
collection of the test
database, that would hold documents matching the query { _id : 99 }
. split
does not require that a match exist, in order to identify the appropriate chunk. Then the command splits it into two chunks, with the matching document as the lower bound of one of the split chunks.
This form is typically used when pre-splitting data in a collection.
This example uses the hashed shard key userid
in a people
collection of a test
database. The following command uses an array holding two single-field documents to represent the minimum and maximum values of the hashed shard key to split the chunk:
Note
MongoDB uses the 64-bit NumberLong type to represent the hashed value.
Use sh.status()
to see the existing bounds of the shard keys.