A data processing apparatus is provided that comprises volatile storage circuitry to store data while power is provided. Persistent storage circuitry stores data in the absence of power and transfer circuitry transfers data from the volatile storage circuitry to the persistent storage circuitry. The transfer circuitry is adapted to transfer the data from the volatile storage circuitry to the persistent storage circuitry in response to a primary power supply becoming unavailable to the volatile storage circuitry. The transfer circuitry is adapted to transfer a subset of the data from the volatile storage circuitry to the persistent storage circuitry in response to an explicit request comprising an indication of the subset of the data.