1. A method of defining a layout mapping function for a parity distributed RAID array comprising a plurality of target objects, the layout mapping function defining the mapping of a stripe group-address space to a target-address space in the array and comprising a matrix defining a unit space across the plurality of target objects, the matrix comprising columns defining the plurality of target objects and rows defining equally-offset sequential units on the plurality target objects, the method comprising: specifying, on a computing apparatus, P target objects, where P>1; specifying, on a computing apparatus, A target objects as spare space, where A<P and A>1; defining, on a computing apparatus, a sub-matrix of P′ columns, where P′=P−A; defining, on a computing apparatus, a layout of stripe groups across the P′ target objects in the sub-matrix, each stripe group comprising a plurality of data units and a plurality of parity units; adding, on a computing apparatus, A columns, representative of the A spare space target objects, to the sub-matrix to form a complete matrix defining the layout mapping function; and implementing, on a computing apparatus, the layout mapping function on the target objects of the RAID array.