Index files are much smaller than the collection of data files.
Index files are read sequentially multiple times, and based on the result of searching the indices, some groups of data files are read. Each group of data files are always read sequentially in the same order every time. 3-D volume rendering software working on a 220x220x220 data volume, rendering 22 slices with stride 10, along the X axis, then Y axis, then Z axis. Both read and write intensive. This happens, for example, when a readahead was issued for this block in the previous read system call.