Persistence

The third major theme of the course is persistence. In system memory, data can be easily lost, as devices such as DRAM store values in a volatile manner; when the power goes away or the system crashes, any data in memory is lost. Thus, we need hardware and software to be able to store data persistently; such storage is thus critical to any system as users care a great deal about their data.

The hardware comes in the form of some kind of input/output or I/O device; in modern systems, a hard drive is a common repository for long-lived information, although solid-state drives (SSDs) are making headway in this arena as well.

The software in the operating system that usually manages the disk is called the file system; it is thus responsible for storing any files the user creates in a reliable and efficient manner on the disks of the system.

Unlike the abstractions provided by the OS for the CPU and memory, the OS does not create a private, virtualized disk for each application. Rather, it is assumed that oftentimes, users will want to share information that is in files. For example, when writing a C program, you might first use an editor (e.g., Emacs) to create and edit the C file (emacs -nw main.c). Once done, you might use the compiler to turn the source code into an executable (e.g., gcc -o main main.c). When you’re finished, you might run the new executable (e.g., ./main). Thus, you can see how files are shared across different processes. First, Emacs creates a file that serves as input to the compiler; the compiler uses that input file to create a new executable file (in many steps — take a compiler course for details); finally, the new executable is then run. And thus, a new program is born!