MetaData persistence using storage class memory: experiences with flash-backed DRAM

Storage Class Memory (SCM) blends the best properties of main memory and hard disk drives. It offers non-volatility and byte addressability, and promises short access times with low cost per bit. Earlier research in this field explored designs exploiting SCM features and used either simulations or theoretical models for evaluations. In this work, we explore the design challenges for achieving non-volatility using real SCM hardware that is available now: Flash-Backed DRAM. We present performance analysis of flash-backed DRAM and describe the system issues involved in achieving true non-volatility using the system memory hierarchy which was designed assuming that data is volatile. We present software abstractions which allow applications to be redesigned easily using SCM features, without having to worry about system issues. Furthermore, we present case studies using two applications with different characteristics: an SSD-based caching layer used in enterprise storage (Flash Cache) and an in-memory database (SolidDB), and redesign them using software abstractions. Our performance evaluations reveal that SCM aware Flash Cache design could enable persistence with less than 2% degradation in performance. Similarly, redesigning SolidDB persistence layer using SCM improved the performance by a factor of two. To the best of our knowledge, this is the first work that evaluates SCM performance and demonstrates application redesign using real SCM hardware.