DB->set_h_ffactor(DB *db, u_int32_t h_ffactor);
Set the desired density within the hash table.
The density is an approximation of the number of keys allowed to
accumulate in any one bucket, determining when the hash table grows or
shrinks. If you know the average sizes of the keys and data in your
dataset, setting the fill factor can enhance performance. A reasonable
rule computing fill factor is to set it to: