Understanding Software Dynamics

Chapter 5

Measuring Disk/SSD

The third fundamental hardware resource to measure is disk/SSD latency—how long do real storage-device reads and writes take? In contrast to the previous chapters that do CPU and memory measurements of average latency over thousands of identical operations, we will look at the internal dynamics of a single long disk/SSD read or write, revealing a wealth of substructure and a few surprises.

What really happens when you do a disk read or disk write? As shown in Figure 5.1, there are several layers of software involved, including the operating system and the file system. At least the file system and the drive itself have caches of disk data. The file system in general will prefetch and cache read data and buffer writes, ...