Though there are numerous media options, the classic storage hierarchy is often described as hard disk drives (HDDs) providing slow, persistent storage and dynamic random access memory (DRAM) situated closer to processors providing fast, volatile storage. Flash-based solid state devices (SSDs) became available with latency and price between DRAM and HDD, though with limited endurance.
Traditional computer architectures include primary storage systems and backup (data recovery) storage systems. Typically when a primary system fails, data from storage system is replicated to the primary system. While replicating, it may be desirable to simultaneously access data directly, i.e., random access, from the storage system. Because of long latency of accessing a storage system randomly, it is desirable to integrate SSDs as a caching layer in backup storage systems for improved performances.
A challenge is to build a backup storage system with a SSD cache layer balancing cost and performance. Because SSD has limited number write-erase cycles, another challenge is to limit write-erase cycles (churn) of the SSD cache to improve the useful life span of SSD devices. Yet, another challenge is to improve read access latency when retrieving data from both hard drives and SSD drives simultaneously. Yet another challenge is to improve fault tolerance of a SSD data cache layer.