LRU-C: Parallelizing Database I/Os for Flash SSDs

Abstract

The conventional database buffer managers have two inherent sources of I/O serialization: read stall and mutex conflict. The serialized I/O makes storage and CPU under-utilized, limiting transaction throughput and latency. Such harm stands out on flash SSDs with asymmetric read-write speed and abundant I/O parallelism. To make database I/Os parallel and thus leverage the parallelism in flash SSDs, we propose a novel approach to database buffering, the LRU-C method. It introduces the LRU-C pointer that points to the least-recently-used-clean page in the LRU list. Upon a page miss, LRU-C selects the current LRU-clean page as a victim and adjusts the pointer to the next LRU-clean one in the LRU list. This way, LRU-C can avoid the I/O serialization of read stalls. The LRU-C pointer enables two further optimizations for higher I/O throughput: dynamic-batch-write and parallel LRU-list manipulation. The former allows the background flusher to write more dirty pages at a time, while the latter mitigates mutex-induced I/O serializations. Experiment results from running OLTP workloads using MySQL-based LRU-C prototype on flash SSDs show that it improves transaction throughput compared to the Vanilla MySQL and the state-of-the-art WAR solution by 3x and 1.52x, respectively, and also cuts the tail latency drastically. Though LRU-C might compromise the hit ratio slightly, its increased I/O throughput far offsets the reduced hit ratio.