A New File System I/O Mode for Efficient User-Level Caching

摘要

A large number of cloud datastores have been developed to handle the cloud OLTP workload. Double caching problem where the same data resides both at the user buffer and the kernel buffer has been identified as one of the problems and has been largely solved by using direct I/O mode to bypass the kernel buffer. However, maintaining the caching layer only in user-level has the disadvantage that the user process may monopolize memory resources and that it is difficult to fully utilize the system memory due to the risks of the forced termination of the process or the unpredictable performance degradation in case of memory pressure. In this paper, we propose a new I/O mode, DBIO, to efficiently exploit OS kernel buffer as a victim cache for user-level file content cache, enjoying the strengths of kernel-level cache rather than just skipping it. DBIO provides the new file read/write function calls, which enable user programs to dynamically choose the right I/O behavior based on their context when issuing I/Os instead of when opening the file. On the cloud key-value store workloads and the traditional OLTP workloads with the modified version of MySQL/InnoDB, DBIO improves the in-memory cache hit ratio and the transaction performance compared to both buffered and direct I/O mode, fully utilizing the user buffer and the kernel buffer without double caching.