P2CACHE: Exploring Tiered Memory for In-Kernel File Systems Caching

Fast, byte-addressable persistent memory (PM) is becoming a reality in products. However, porting legacy kernel file systems to fully support PM requires substantial effort and encounters the challenge of bridging the gap between blockbased access granularity and byte-addressability. Moreover, new PM-specific file systems remain far from productionready, preventing them from being widely used. In this paper, we propose P 2 CACHE, a novel in-kernel caching mechanism to explore how legacy kernel file systems can effectively evolve in the face of fast, byte-addressable PM. P 2 CACHE exploits a read/write-distinguishable memory hierarchy upon a tiered memory system involving both PM and DRAM. P 2 CACHE leverages PM to serve all write requests for instant data durability and strong crash consistency while using DRAM to serve most read I/Os for high I/O performance. Further, P 2 CACHE employs a simple yet effective synchronization model between PM and DRAM by leveraging devicelevel parallelism. Our evaluation shows that P 2 CACHE can significantly increase the performance of legacy kernel file systems -e.g., by 200x for RocksDB on Ext4 -meanwhile equipping them with instant data durability and strong crash consistency, similar to PM-specialized file systems.