One embodiment is a data processing apparatus that has a chunk store containing specimen data chunks, a manifest store containing a plurality of manifests, each of which represents at least a part of previously processed data and includes at least one reference to at least one of the specimen data chunks, and a sparse chunk index containing information on only some specimen data chunks. Input data is processed into a plurality of input data segments. Each manifest of the first set has at least one reference to one of said specimen data chunks that corresponds to one of the input data chunks of a first input data segment. Specimen data chunks corresponding to other input data chunks of the first input data segment are identified by using the identified first set of manifests and at least one manifest identified when processing previous data.