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.