NVWAL: Exploiting NVRAM in Write-Ahead Logging

摘要

Emerging byte-addressable non-volatile memory is considered an alternative storage device for database logs that require persistency and high performance. In this work, we develop NVWAL (NVRAM Write-Ahead Logging) for SQLite. The proposed NVWAL is designed to exploit byte-addressable NVRAM to maintain the write-ahead log and to guarantee the failure atomicity and the durability of a database transaction. The contribution of NVWAL consists of three elements: (ⅰ) byte-granularity differential logging that effectively eliminates the excessive I/O overhead of filesystem-based logging or joumaling, (ⅱ) transaction-aware lazy synchronization that reduces cache synchronization overhead by two-thirds, and (ⅲ) user-level heap management of the NVRAM persistent WAL structure, which reduces the overhead of managing persistent objects. We implemented NVWAL in SQLite and measured the performance on a Nexus 5 smartphone and an NVRAM emulation board-Tuna. Our performance study shows the following: (ⅰ) the overhead of enforcing strict ordering of NVRAM writes can be reduced via NVRAM-aware transaction management. (ⅱ) From the application performance point of view, the overhead of guaranteeing failure atomicity is negligible; the cache line flush overhead accounts for only 0.8～4.6% of transaction execution time. Therefore, application performance is much less sensitive to the NVRAM performance than we expected. Decreasing the NVRAM latency by one-fifth (from 1942 nsec to 437 nsec), SQLite achieves a mere 4% performance gain (from 2517 ins/sec to 2621 ins/sec). (ⅲ) Overall, when the write latency of NVRAM is 2 usec, NVWAL increases SQLite performance by at least 10x compared to that of WAL on flash memory (from 541 ins/sec to 5812 ins/sec).