Making the Most Out of Direct-Access Network Attached Storage

The performance of high-speed network-attached storage applications is often limited by end-system overhead, caused primarily by memory copying and network protocol processing. In this paper, we examine alternative strategies for reducing overhead in such systems. We consider optimizations to remote procedure call (RPC)-based data transfer using either remote direct memory access (RDMA) or network interface support for pre-posting of application receive buffers. We demonstrate that both mechanisms enable file access throughput that saturates a 2Gb/s network link when performing large I/Os on relatively slow, commodity PCs. However, for multi-client workloads dominated by small I/Os, throughput is limited by the per-I/O overhead of processing RPCs in the server. For such workloads, we propose the use of a new network I/O mechanism, Optimistic RDMA (ORDMA). ORDMA is an alternative to RPC that aims to improve server throughput and response time for small I/Os. We measured performance improvements of up to 32% in server throughput and 36% in response time with use of ORDMA in our prototype.