Abstract: A garbage collection process for a data deduplication storage system is disclosed. In one implementation, a method is disclosed to perform garbage collection that works effectively across a scale-out cluster and across very large amounts of data. The method includes compacting data in an object store in the scale-out cluster by examining data in a reference map of data blocks in the object store to determine which of the locations within a back-end object in an object store are referenced, and which locations are no longer referenced by a process. The back-end object in an Object Store are altered to remove block data from locations which are no longer referenced, and a hash-to-location table is updated to remove the entries for the removed block data.