Persistent Memory File Research Articles

PM-AIO: An Effective Asynchronous I/O System for Persistent Memory

Due to the expected near DRAM performance, popular async I/O systems are disabled on local persistent memory (PM) file systems, which instead use the pseudo-async I/O path (namely the sync one), to serve async I/O (AIO) requests of applications. This paper first identifies the performance shortcomings of this kind of I/O method and argues the necessity of applying the real async I/O on PM device. Then, this paper proposes PM-AIO, a general method to create an effective async I/O path on PM file systems. PM-AIO leverages kernel-level threads to achieve real asynchronism and concurrency. We implement PM-AIO in the Native-AIO of PMFS and NOVA respectively. Extensive experiments are conducted to verify the advantages of PM-AIO on a real PM platform. Compared with the original I/O methods of PM file systems, the results show that PM-AIO can reduce the latencies of AIO requests up to 3 orders of magnitude while reaping up to 2.11× IOPS in realistic workloads which contain relatively large I/O operations (often <inline-formula><tex-math notation="LaTeX">$\geq$</tex-math></inline-formula> 4 KB). Meanwhile, PM-AIO incurs up to 4% overhead when handling small I/O operations (often <inline-formula><tex-math notation="LaTeX">$&lt;$</tex-math></inline-formula> 4 KB) because of the inherent overhead.

CFFS: A Persistent Memory File System for Contiguous File Allocation With Fine-Grained Metadata

Extensive research on persistent memory (PM)-aware file systems has led to the development of numerous methods for improving read/write throughput. In particular, accessing or modifying file contents in a similar manner to the memory operations through <i>mmap</i> is a common approach. We designed a file system, CFFS (Contiguous File Allocation with Fine-Grained Metadata File System), to rapidly allocate PM pages to upper layer applications for <i>mmap</i> and to alleviate page fault overheads due to <i>mmap</i>. We optimized the physical contiguity of files in PM to reduce file fragmentation and increase fragment alignment with the goal of reducing software overhead. To achieve this goal, we implemented greedy-based buddy systems and implicit preallocation with a not-most-recently-used (NMRU) policy based on our overall page allocation strategy of considering not only the spatial but also the temporal locality of file access patterns. Furthermore, for efficient and atomic metadata operations, we fully leveraged the byte-addressable property of PM to design fine-grained metadata. CFFS adopts persistent doubly linked lists for directory operations to identify and recover from inconsistencies caused by system failures, doing so without using traditional log mechanisms. In experiments, CFFS showed superior page allocation performance to EXT4-DAX and NOVA did under different PM fragmentation levels. Our allocation algorithm also reduced the cost of page faults for frequently appended files. Finally, CFFS&#x2019;s lightweight directory operations performed excellently in creating and deleting files of various quantities. In summary, the main contribution of the paper is proposing an efficient page allocation algorithm to improve the performance of subsequent <i>mmaps</i>, based on the strategy of considering not only the spatial but also the temporal locality of file access patterns in PM file systems. Another contribution was the fine-grained and log-free method for atomic directory operations.

LosPem

New and emerging types of Persistent Memory (PM) technologies boost the opportunity to improve the performance of storage systems. PM can unify the main memory and secondary storage by incorporating it into legacy computer systems through the memory bus. In recent years, innovative results have been presented that exploit the byte-addressability, low latency, and non-volatility of PM; these have included local PM file systems and PM systems. However, the high overhead of ensuring data consistency has limited the performance of these systems. In this article, we propose LosPem, a novel log-structured framework for persistent memory to address the performance challenge. LosPem utilizes two techniques to accomplish this. Firstly, LosPem deploys efficient hash-indexed linked lists to maintain the log contents to reduce the significant overhead of log content retrieval. Secondly, LosPem improves the transaction throughput by decoupling a transaction into two asynchronous steps and creating a write buffer on Dynamic Random Access Memory (DRAM) write buffer for processing the frequent data writes. The experimental results show that LosPem outperforms Non-volatile Memory Library (NVML), Mnemosyne and Log-structured Non-volatile Main Memory (LSNVMM) by 27%, 1.2x, and 1.0x on a read-intensive workload. On a write-intensive workload, LosPem outperforms NVML, Mnemosyne, and LSNVMM by 1.8x, 1.2x, and 34%, respectively.