A consistency mechanism for NVM-Based in-memory file systems

Non-Volatile Memory (NVM) has evolved to achieve non-volatility and byte-addressability with latency comparable to DRAM. This inspires the development of a new generation of file systems, namely NVM-based in-memory file systems, which include NVM on memory bus and allow in-NVM data to be directly accessed like DRAM. Meanwhile, an important issue, the consistency problem, arises as a new challenge. That is, the direct modification to the in-NVM data can be interrupted by arbitrary crashes in the system, which results in part of the modification being durable and others being lost. Traditional consistency mechanisms assume the existence of DRAM buffering and hence cannot be applied to this hybrid memory architecture. While several consistency methods have been proposed for NVM-based in-memory file systems, most of them have side-effects including unfriendliness to DRAM and penalties on concurrency control, which degrade the system performance. In this paper, we propose a novel mechanism to guarantee the consistency of NVM-based in-memory file systems. We abstract the storage area as a layered structure and employ a lazy-validated snapshot strategy to achieve a high consistency level. Since every consistency method comes with a cost, we introduce several algorithms to efficiently deal with block-sharing and reduce the overhead of consistency mechanism. The experimental results show that our mechanism incurs negligible consistency overhead and outperforms a state-of-the-art snapshot file system by reducing the latency of snapshot taking and removal by 95% and 60% respectively.