1. A computer implemented method of maintaining usage metadata nodes about data snapshots of data present in data storage in a data processing system, comprising the steps of: forming a snapshot of data present in units of memory segments of the data processing system; scanning the usage metadata nodes for units of memory segments overlapping with the the units of memory segments of the data in the snapshot resulting from the step of forming a snapshot; and if overlapping units of memory segments are found during the step of scanning, forming an entry in the usage metadata at locations indicating the memory segments found to overlap during the step of scanning; and modifying the metadata node to indicate that the usage metadata contains at least one entry at a location identifying the memory segment to overlap.