Storage Management

Computer systems must provide mass storage for permanently storing files and data. Modern computers implementmass storage as secondary storage, using both hard disks and nonvolatile memory devices.

Secondary storage devices vary in many aspects. Some transfer a character at a time, and some a block of characters. Some can be accessedonly sequentially, and others randomly. Some transfer data syn- chronously, and others asynchronously. Some are dedicated, and some shared. They can be read-only or read–write. And although they vary greatly in speed, they are in many ways the slowest major component of the computer.

Because of all this device variation, the operating system needs to provide a wide range of functionality so that applications can control all aspects of the devices. One key goal of an operating system’s I/O subsystem is to provide the simplest interface possible to the rest of the system. Because devices are a performance bottleneck, another key is to optimize I/O for maximum concurrency.