Round-like behavior in multiple disks on a bus

Abstract

In modern I/O architectures, multiple disk drives are attached to each I/O bus. Under I/O-intensive workloads, the disk latency for a request can be overlapped with the disk latency and data transfers of requests to other disks, potentidly resulting in an aggregate I/O throughput at nearly bus bandwidth. This paper reports on a performance impairment that results from a previously unknown form of convoy behavior in disk I/O, which we call munds. In rounds, independent requests to distinct disks convoy, so that each disk services one request before any disk services its next re quest. We analyze log tiles to describe read performance of multiple Seagate Wren-7 disks that share a SCSI bus under a heavy workload, demonstrating the rounds behavior and quantifying its performance impact.