Achieving Reliable Parallel Performance in a VoD Storage Server Using Randomization and Replication

This paper investigates randomization and replication as strategies to achieve reliable performance in disk arrays targeted for video-on-demand (VoD) workloads. A disk array can provide high aggregate throughput, but only if the server can effectively balance the load on the disks. Such load balance is complicated by two key factors: workload hotspots caused by differences in popularity among media streams, and "fail-stutter" faults that arise when the performance of one or more devices drops below expectations due to manufacturing variations, hardware problems, or geometry-related variations. This paper focuses on the random duplicate assignment (RDA) data allocation policy which places each data block on two disks chosen at random, independent of other blocks in the same media stream or other streams. This strategy is compared to traditional single-disk file allocation, disk striping (RAID-0), disk mirroring (RAID-1), and randomization without duplication. The various allocation schemes are implemented and tested using a prototype VoD server with 2 dual-core Opteron processors, 8 SATA disks, and 4 gigabit Ethernet interfaces running the Linux 2.6 kernel. The results indicate that combining randomization and replication allows RDA to effectively tolerate both workload hotspots and fail-stutter faults better than previous schemes.