In the near future, large digital media servers are expected to offer storage capacities in the order of petabytes. Servers made of clusters of PCs connected to jukeboxes may represent an interesting alternative compared with servers made of arrays of magnetic disks. However, due to disk exchange overhead, higher seek times and lower data transfer rates, access to data located on optical disks is significantly slower than access to data located on magnetic disks. In the present contribution, we analyze the benefits and limitations of striping files across multiple optical disks in order to speedup the retrieval of large pieces of information. We also show that server applications requiring both computer power and I/O bandwidth may be distributed over several processing nodes and access files striped over multiple optical disks. For such applications, as long as disk exchanges are not necessary, a predictable speedup can be obtained by ensuring a high enough number of server computers, optical disk drive units, and enough communication bandwidth.

Parallel servers for I/O and compute intensive continuous media applications are difficult to develop. A server application comprises many threads located in different address spaces as well as files striped over multiple disks located on different computers. The present contribution describes the construction of a continuous media server, the 4D beating heart slice server, based on a computer-aided parallelization tool (CAP) and on a library of parallel file system components enabling the combination of pipelined parallel disk access and processing operations. Thanks to CAP, the presented architecture is concisely described as a set of threads, operations located within the threads and flow of data and parameters (tokens) between operations. Continuous media applications are supported by allowing tokens to be suspended during a period of time specified by a user-defined function. Our target application, the 4D beating heart server supports the extraction of freely oriented slices from a 4D beating heart volume (one 3D volume per time sample). This server application requires both a high I/O throughput for accessing from disks the set of 4D sub-volumes (extents) intersecting the desired slices and a large amount of processing power to extract these slices and to resample them into the display grid. With a server configuration of 3 PCs and 24 disks, up to 7.3 slices can be delivered per second, i.e. 43 MB/s are continuously read from disks and 4.1 MB/s of slice parts are extracted, transfered to the client, merged, buffered and displayed. This performance is close to the maximal performance deliverable by the underlying hardware. The observed single stream server delay jitter varies between 0.6s (52% of maximal display rate) and 1.4s (92% of the maximal display rate). For the same resource utilization, the jitter is proportional to the number of streams that are accessed synchronously.