Tai-Sheng Chang

Two Emerging Serial Storage Interfacesfor Supporting Digital Libraries: Serial Storage Architecture(SSA) and Fiber Channel-Arbitrated Loop (FC-AL)

Digital libraries require not only high storage space capacity but also high performance storage systems which provide the fast accesses to the data. These requirements can not be efficiently supported with the traditional SCSI interfaces. Several serial storage interfaces have been proposed for constructing storage systems with high transfer bandwidth, large storage capacity, and fault tolerance feature. Among them, Serial Storage Architecture (SSA) and Fibre Channel-Arbitrated Loop (FC-AL) are considered as the next generation storage interfaces with broad industry support. Both technologies support simple cabling, long transmission distance, high data bandwidth, large capacity, fault tolerance, and fair sharing of link bandwidth. In this paper, a tutorial and a comparison of these two technologies are presented. The tutorial examines their interface specifications, transport protocols, fairness algorithms, and capabilities of fault tolerance. The comparison focuses on their protocol overhead, flow control, fairness algorithms, and fault tolerance. The paper also summarizes the recently proposed Aaron Proposal which incorporates features from both SSA and FC-AL and targets at merging these two technologies.

Interface comparisons: SSA versus FC-AL

The serial storage architecture and fibre channel-arbitrated loop interfaces offer a simple cabling system, higher bandwidth, the ability to connect more than 100 disks, fault tolerance, and fair accesses on the channel. This article investigates the performance of these two emerging serial storage interfaces for fairness, latency, overhead and aggregate throughput under various traffic loads.

Supporting continuous media: is Serial Storage Architecture (SSA) better than SCSI?

The existing SCSI (Small Computer System Interface) parallel bus has been widely used in supporting multimedia applications. However, it may not fully utilize the aggregate throughput from disks in supporting continuous media because of its unfair bus accesses. The emerging serial interface, Serial Storage Architecture (SSA), provides high data bandwidth, fair accesses and fault tolerance. The fairness algorithm in SSA ensures that a fraction of the data bandwidth is allocated to each disk. In this paper, we study the performance of SSA and SCSI in supporting continuous media.

Performance of a Scalable Multimedia Server with Shared-Storage Clusters

The existing SCSI parallel bus has been widely used in various multimedia applications. However, due to the unfair bus accesses the SCSI bus may not be able to fully utilize the potential aggregate throughput of disks. The number of disks that can be attached to the SCSI bus is limited, and link level fault tolerance is not provided. The serial storage interfaces such as Serial Storage Architecture (SSA) provide high data bandwidth, fair accesses, long transmission distance between adjacent devices (disks or hosts) and link level fault tolerance. The fairness algorithm of SSA ensures a fraction of data bandwidth to be allocated to each device. In this paper we would like to know whether SSA is a better alternative in supporting continuous media than SCSI. The scalability of a multimedia server is very important since the storage requirement may grow incrementally as more contents are created and stored. SSA in a shared-storage cluster environment also supports concurrent accesses by different hosts as long as their access paths are not overlapped. This feature is called spatial reuse. Therefore, the effective bandwidth over an SSA can be higher than the raw data bandwidth and the spatial reuse feature is critical to the scalability of a multimedia server. This feature is also included in FC-AL3 with a new mode called Multiple Circuit Mode (MCM). Using MCM, all devices can transfer data simultaneously without collision. In this paper we have investigated the scalability of shared-stroage clusters over an SSA environment.

The scalability of spatial reuse based serial storage interfaces

Due to the growing popularity of emerging applications such as digital libraries, Video-On Demand, distance learning, and Internet World-Wide Web, multimedia servers with a large capacity and high performance storage subsystem are in high demand. Serial storage interfaces are emerging technologies designed to improve the performance of such storage subsystems. They provide high bandwidth, fault tolerance, fair bandwidth sharing and long distance connection capability. All of these issues are critical in designing a scalable and high performance storage subsystem. Some of the serial storage interfaces provide the spatial reuse feature which allows multiple concurrent transmissions. That is, multiple hosts can access disks concurrently with full link bandwidth if their access paths are disjoint. Spatial reuse provides a way to build a storage subsystem whose aggregate bandwidth may be scaled up with the number of hosts. However, it is not clear how much the performance of a storage subsystem could be improved by the spatial reuse with different configurations and traffic scenarios. Both limitation and capability of this scalability need to be investigated. To understand their fundamental performance characteristics, we derive an analytic model for the serial storage interfaces with the spatial reuse feature. Based on this model, we investigate the maximum aggregate throughput from different system configurations and load distributions. We show how the number of disks needed to saturate a loop varies with different number of hosts and different load scenarios. We also show how the load balancing by uniformly distributing the load to all the disks on a loop may incur high overhead. This is because the accesses to far away disks need to go through many links and consume the bandwidth of each link it goes through. The results show the achievable throughput may be reduced by more than half in some cases.