Precursors of failure are dislocation mechanisms at the nanoscale and dislocation organization at the mesoscale responsible for long-range internal stresses and lattice rotation. Detailed information on the link between both scales is missing, computationally and experimentally. Here we present a method based on x-ray Laue diffraction scanning providing time and sub -micron spatially resolved evolution of geometrical necessary dislocations in volumes that are similar to what advanced computational models can achieve. The approach is used to follow dislocation patterning during accumulation of fatigue cycles using a newly developed miniaturized shear device. Performed on Cu during cyclic shear, it reveals early dislocation patterning influenced by pre-existing dislocation structures. The quantitative information on non -homogeneous structure formation and its evolution corresponds to the need for synergies with continuum dislocation plasticity simulations of fatigue or any other type of plastic deformation. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. This is an open access article under the CC BY -NC ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).