Garbage collection from archival of storage snapshots

A technique improves storage efficiency of an object store configured to maintain numerous snapshots for long-term storage in an archival storage system by efficiently determining data that is exclusively owned by an expiring snapshot to allow deletion of the expiring snapshot from the object store. The technique involves managing index data structures to enable efficient garbage collection across a very large number of data objects. When a snapshot expires, the technique obviates the need to scan the numerous snapshot data objects to determine which index structures are no longer needed and can be reclaimed (garbage collected). The technique is directed to management of underlying storage based on different sets of policies. When certain snapshots expire and are ready for deletion, the technique is directed to finding those data blocks that are no longer referenced (used) by any valid snapshots.