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Berkeley DB Reference Guide:
  Access Methods

# Page fill factor

The density, or page fill factor, 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 data set, setting the fill factor can enhance performance. A reasonable rule to use to compute fill factor is:

>     (pagesize - 32) / (average_key_size + average_data_size + 8)

The desired density within the hash table can be specified by calling the DB->set_h_ffactor function. If no density is specified, one will be selected dynamically as pages are filled.

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