Hybrid storage systems

Abstract

In recent years, both industry and academia have increased their research effort in the hybrid memory management space, developing a wide variety of systems. It is worth mentioning that “hybrid” is a generic term and it can have different meanings depending on the context. For instance, a storage system can be hybrid because it combines HDD and SSD; an SSD can be hybrid because it combines SLC, MLC and TLC Flash memories, or it combines NAND with Storage Class Memories (SCMs), which are non-volatile memories like ReRAM, PCM or MRAM. In this chapter we look at all these different meanings. The last section covers over-provisioning and the Write Amplification Factor (WAF): these parameters have a great impact on SSD performances and reliability, as well as on the available storage capacity.