File systems are typically observed as a layer between an application and the I/O services providing the underlying storage. When you look at file system performance, you should focus on the latencies observed at the application level. Historically, however, we have focused on techniques that look at the latency and throughput characteristics of the underlying storage and have been flying in the dark about the real latencies seen at the application level.
With the advent of DTrace, we now have end-to-end observability, from the application all the way through to the underlying storage. This makes it possible to do the following:
Observe the latency and performance impact of file-level requests at the application level.