Files are huge - hundreds of megabytes, gigabytes, or larger.
Files are accessed in large pieces - hundreds of kilobytes or megabytes at a time. Files are accessed sequentially. That is, every byte in the file is accessed, in order, from beginning to end. To test the validity of that assumption, we traced the workloads of two different parallel file systems, on two different machines, at two different sites, running primarily scientific applications. The tracing involved recording every single access that was made to the parallel file system over a period of weeks.