How serious is spectrum fragmentation in an elastic optical network? It is addressed by the tremendous provisioning gain achieved by eliminating a particular kind of fragmentation, namely, one that could be removed by Hitless Optical Path Shift (HOPS), a non-disruptive defragmentation technology. We devised a series of HOPS-based defragmentation algorithms, ranging from proactive to reactive strategies. Analysis and simulation results show that for typical backbone networks operated with a light load, using our optimal reactive defragmentation scheme, 98% of the requested spectrum bandwidth, that is otherwise blocked, can be provisioned. Moreover, the corresponding defragmentation costs (in terms of impact on existing connections, and defragmentation delay) are kept at minimum.