Embedded Transaction Support Inside SSD With Small-Capacity Non-Volatile Disk Cache

Flash-based Solid State Drives (SSDs) have proved to be ideal devices that support embedded transaction protocols inside SSDs. Existing embedded transaction protocols in SSDs effectively improve transaction throughput, but still incur high transaction overhead and long recovery time. While it is reasonable to provide a small-capacity non-volatile (NVM-based) disk cache in the SSDs, in this paper, we propose a new embedded transaction protocol called Non-volatile Cache Transaction (NVCTX). NVCTX reduces transaction overhead and provides fast recovery by leveraging the small-capacity NVM-based disk cache from two aspects. First, we store transactional metadata, which is of small amount but is frequently accessed, in the NVM-based disk cache rather than in the flash memory. Second, we introduce two techniques, i.e., a dynamic allocation algorithm and a hybrid storing method, to improve the performance when the capacity of the NVM-based disk cache is very limited. We have implemented NVCTX on a real hardware board called Cosmos+ FPGA platform, and modified ext4 file system and NVMe (Non-Volatile Memory express) driver to be compatible with the transactional interfaces provided by NVCTX. For comparison, we also implement SCC, BPCC, WAL, and X-FTL protocols in the firmware of Cosmos+ FPGA platform. Evaluations using DBMS (Database Management System) and file system workloads show that, compared to four typical transaction protocols (SCC, BPCC, WAL, and X-FTL), NVCTX improves transaction throughput by up to 136.5, 9.4, 131.6 and 29.9 percent, reduces write traffic to flash memory by up to 42.8, 4.1, 62.4, 31.2 percent, lowers garbage collection overhead by up to 93.2, 63, 66.5, 22.1 percent, and shortens recovery time to 1/2574, 1/2559, 1/95 and 1/2 respectively compared with SCC, BPCC, WAL, and X-FTL.