ExaGrid provides tiered backup storage with a unique disk-cache Landing Zone, long-term retention repository, and scale-out architecture. The system scales as needed by adding ExaGrid appliances, which virtualize into a single scale-out system automatically, adding capacity and processing power while acting and being managed as one unified system. ExaGrid disk-based backup appliances include scale-out computing software, which virtualizes them into a single pool of long-term capacity. Capacity load balancing of all data across appliances is automatic.