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.