Pannier: Design and Analysis of a Container-Based Flash Cache for Compound Objects

Classic caching algorithms leverage recency, access count, and/or other properties of cached blocks at per-block granularity. However, for media such as flash which have performance and wear penalties for small overwrites, implementing cache policies at a larger granularity is beneficial. Recent research has focused on buffering small blocks and writing in large granularities, sometimes called containers, but it has not explored the ramifications and best strategies for caching compound blocks consisting of logically distinct, but physically co-located, blocks. Containers may have highly diverse blocks, with mixtures of frequently accessed, infrequently accessed, and invalidated blocks. We propose and evaluate Pannier, a flash cache layer that provides high performance while extending flash lifespan. Pannier uses three main techniques: (1) leveraging block access counts to manage cache containers, (2) incorporating block liveness as a property to improve flash cache space efficiency, and (3) designing a multi-step feedback controller to ensure a flash cache reaches its desired lifespan while maintaining performance. Our evaluation shows that Pannier improves flash cache performance and extends lifespan beyond previous per-block and container-aware caching policies. More fundamentally, our investigation highlights the importance of creating new policies for caching compound blocks in flash.