1. A backup system comprising: a backup engine; a backup protocol configured to couple a client backup module to the backup engine; a backup database coupled to the backup engine, the backup database comprising a first set of clusters, the first set of clusters forming one or more backups, wherein each cluster of the first set of clusters is unique such that single-instance storage across clients is achieved; a processor; and a memory operatively connected to the processor, the memory having computer-executable instructions that when executed by the processor provide: the backup engine configured to: receive from the client backup module a set of current cluster hashes, wherein each one of the set of current cluster hashes is calculated based on a corresponding one of a plurality of clusters of one or more storage devices, and wherein each one of the set of current cluster hashes uniquely identifies the corresponding one of the plurality of clusters; reduce the set of current cluster hashes to include only those not already stored in the backup database, resulting in a reduced set of cluster hashes; receive from the client backup module a reduced set of clusters of the plurality of clusters identified by the reduced set of cluster hashes; and store in the backup database the reduced set of clusters, wherein each cluster stored in the backup database is unique, wherein single instance storage in the backup database is provided.