Phenomenal improvements in the computational performance of multiprocessors have not been matched by comparable gains in I/O system performance. This imbalance has resulted in I/O becoming a signi cant bottleneck for many scienti c applications. One key to overcoming this bottleneck is improving the performance of parallel le systems. The design of a high-performance parallel le system requires a comprehensive understanding of the expected workload. Unfortunately, u n til recently, no general workload studies of parallel le systems have been conducted. The goal of the CHARISMA project was to remedy this problem by c haracterizing the behavior of several production workloads, on di erent machines, at the level of individual reads and writes. The rst set of results from the CHARISMA project describe the workloads observed on an Intel iPSC/860 and a Thinking Machines CM-5. This paper is intended to compare and contrast these two w orkloads for an understanding of their essential similarities and di erences, isolating common trends and platform-dependent v ariances. Using this comparison, we are able to gain more insight i n to the general principles that should guide parallel le-system design.