Data processing apparatus comprising: a chunk store containing specimen data chunks, a manifest store containing a plurality of manifests, each of which represents at least a part of a data set and each of which comprises at least one reference to at least one of said specimen data chunks, a sparse chunk index containing information on only some specimen data chunks, the processor being operable to: process input data into input data chunks; identify manifests having at least one reference to one of said specimen data chunks that corresponds to one of said input data chunks and on which there is information contained in the sparse chunk index; and prioritize the identified manifests for subsequent operation.