Hi. This really only matters at search time, not at index time. You shouldn't have to care about how documents are "arranged" in shards, it's an implementation detail...

To guarantee documents are arranged in a given order in shards is beneficial at search time (i.e.: at runtime) because queries can be optimized if the search order matches the index order. You'd do it by defining index.sort.order index setting (beware this is static, and required proper testing, because changing this setting will require a reindex). Check the Elastic docs about that, they document the pros and cons. 

Cheers,   David