1. Data processing apparatus comprising: a chunk store having a plurality of chunk sections, each operable to store specimen data chunks, the apparatus being operable to: process an input data set into one or more input data chunks; identify a specimen data chunk in one of said chunk sections which corresponds to a first input data chunk; identify a second input data chunk not corresponding to a specimen data chunk in the chunk store; and store the second input data chunk as a specimen data chunk in deliberate proximity to the identified specimen data chunk corresponding to the first input data chunk.