Application Performance on the Direct Access File System

Abstract

The Direct Access File System (DAFS) is a distributed file system built on top of direct-access transports (DAT). Direct-access transports are characterized by using remote direct memory access (RDMA) for data transfer and user-level networking. The motivation behind the DAT-enabled distributed file system architecture is the reduction of the CPU overhead on the I/O data path. In collaboration with Duke University we have created and made available an open-source implementation of DAFS for the FreeBSD platform. In this paper we describe a performance evaluation study of DAFS that was performed with this software. The goal of this work is to determine whether the architecture of DAFS brings any fundamental performance benefits to applications compared to traditional distributed file systems. In our study we compare DAFS to a version of NFS optimized to reduce the I/O overhead. We conclude that DAFS can accomplish superior performance for latency-sensitive applications, outperforming NFS by up to a factor of 2. Bandwidth-sensitive applications do equally well on both systems, unless they are CPU-intensive, in which case they perform better on DAFS. We also found that RDMA is a less restrictive mechanism to achieve copy avoidance than that used by the optimized NFS.