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