Split Chunks in a Sharded Cluster
By default, MongoDB potentially splits a chunk only when migrating data belonging to it. However, you may want to split chunks manually if you have a large amount of data in your cluster and very few chunks, as is the case after deploying a cluster using existing data.
To split chunks manually, use the split command with either
fields middle or find. mongosh provides the
helper methods sh.splitFind() and sh.splitAt().
splitFind() splits the chunk that contains the first
document returned that matches this query into two equally sized chunks.
You must specify the full namespace (i.e. "<database>.<collection>")
of the sharded collection to splitFind(). The query in
splitFind() does not need to use the shard key, though it
nearly always makes sense to do so.
Example
The following command splits the chunk that contains the value of
63109 for the zipcode field in the people collection of
the records database:
sh.splitFind( "records.people", { "zipcode": "63109" } )
Use splitAt() to split a chunk in two, using the queried
document as the lower bound in the new chunk:
Example
The following command splits the chunk that contains the value of
63109 for the zipcode field in the people collection of
the records database.
sh.splitAt( "records.people", { "zipcode": "63109" } )
Note
splitAt() does not necessarily split the chunk
into two equally sized chunks. The split occurs at the location of
the document matching the query, regardless of where that document is
in the chunk.