writes: Latency has a direct impact on performance—thus in order to identify performance issues it is absolutely essential to understand latency. With the introduction of DTrace it is now possible to measure latency at arbitrary points—the problem, however, is how to visually present this data in an effective manner. Towards this end heat maps can prove to be a powerful tool. When I/O latency is presented as a visual heat map, some intriguing and beautiful patterns can emerge. These patterns provide insight into how a system is actually performing and what kinds of latency end-user applications experience.