Chunks are marked as jumbo by the balancer when it is unable to split them, since they have many adjacent documents with the same shard key.

Once a chunk is marked as jumbo the server will never check again if the cardinality of the shard key change and if that chunk is no longer jumbo. Additionally jumbo flags are not cleared when the configured maxChunk size is increased.

The balancer does not attempt to move chunks that are jumbo, this is true also when the balancer is draining a shard. In fact if the shard to be drain the balancer will drain all the chunks from that shard except the jumbo ones. Thus, if a draining shard contains jumbo chunks the user will need to manually clear the jumbo flag on that chunk or manually split it.

The goal of this ticket is to clear the jumbo flags from all the chunks belonging to a shard at the beginning of the removeShard operations. In this way we ensure that the balancer will attempt to move at least once all the chunks of the shard to be drained.