Approche analytique pour l'étude des performances de serveurs multimédias multidisques

Future multimedia applications requiring servers able to provide video sequences on-demand to simultaneous geographically distributed users are numerous and varied. Clustered multi-disk architectures seem suitable for designing such servers. They consist in a set of storage nodes and a set of delivery nodes interconnected by a switch. Each storage node is composed of one or several disks. Video files are shared into blocks and distributed over the disks. This paper presents an analytical model for the performance study of clustered multi-disk multimedia servers. The modeling of the different component of the system is detailed and the final simplified model derived. The model allows a quick dimensioning of the operating parameters. It allows to save expensive simulations, to avoid rare event problems and then to explore values of operation parameters for which performance is not easily evaluated by simulations. The performance evaluation study focuses on the quality of transmission characterized by the probability of occurrence of break in the delivery of blocks. A block that is not loaded in time in the delivery node that manages its transmission on the external network, will not be transmitted. This will result in a short break at the user end. The performance criterion considered is then the probability of a block to be late. This probability depends on the distribution of the loading time of block; response time of the disk plus response time of the switch. In the final simplified model, the disks are modeled by M/D/I queues and the switch is modeled by a deterministic delay. The proposed model is validated through extensive simulations of an accurrate queuing network model. The dimensioning of the system is then derived.