1. Apparatus for processing data comprising: processing circuitry to execute a sequence of program instructions including memory access instructions to generate non-volatile memory access requests to non-volatile memory; a persist buffer to store persist buffer data indicative of a sequence of non-volatile writes to said non-volatile memory; and a write controller responsive to said persist buffer data to control writes to said non-volatile memory; wherein a persist barrier indicates for a set of writes that any writes of said set to said non-volatile memory preceding said persist barrier in said sequence of program instructions should be made before any of said set writes to said non-volatile memory following said persist barrier in said sequence of program instructions; said persist buffer data is indicative of an order of any persist barrier relative to said sequence of non-volatile writes a relative order of any persist barrier within said sequence of non-volatile writes; and said write controller is responsive to said persist buffer data to prevent writing to said non-volatile memory by writes to said non-volatile memory following a given persist barrier within said sequence of program instructions before writes to said non-volatile memory preceding said given persist barrier within said sequence of program instructions.