Pmfs中目录项索引的实现

Implementation of directory index for Pmfs

可字节寻址的非易失存储介质,如相变存储器等,使数据可以在内存级别持久化。由于非易失存储器(NVM)本身的读写延时非常低,系统软件开销成为了决定整个持久化内存系统性能的主要因素。Pmfs是一个专门为持久化内存所设计的文件系统,然而,Pmfs下的每个目录操作(打开、创建或删除)都会遍历目录下的所有目录项,导致了随文件数增长而线性增长的目录项查找开销。通过测试发现,在特定类型负载下这种开销成为了整个文件系统的瓶颈。针对该问题,在Pmfs中实现了持久化的目录项索引来加速目录操作。测试结果显示,基于单目录下100 000文件的负载,该优化使得文件创建速度提高了12倍,带宽增加了27.3%。

Emerging non-volatile, byte-addressable memories like phase-change memory ean make data persistent at main memory level instead of storage. Since the read/write latency of Non-Volatile Memory （NVM） is very low, the overhead of software in a NVM system has become the main factor in determining the performance of the entire persistent memory system. Pmfs is a file system specifically designed for NVM. However, it still has an undesirable eharacteristie： each direetory operation （create, open or delete） of Pmfs requires a linear search of the entire directory files, resulting in a cost linearly increased with the number of files in the directory. The performance of Prnfs under various workloads was evaluated and the test showed that the overhead of the direetory operations had become the bottleneck of the whole system in some cireumstanee of particular workloads. To solve this problem, a persistent directory entry index was implemented in Pmfs to speed up directory operations. The experimental results show that under a single directory with 100 000 files, the file creation speed is increased by 12 times, the bandwidth is improved by 27.3%.