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.
The splitChunk command takes a document with the following fields:
Field Description ns The complete namespace of the chunk to split. keyPattern The shard key. min The lower bound of the shard key for the chunk to split. max The upper bound of the shard key for the chunk to split. from The shard that owns the chunk to split. splitKeys The split point for the chunk. shardId The shard.