Anatomy of an Embedded Linux System - Root File System

A file system is a way of representing a hierarchical collection of directories, where each directory can contain either more directories or files. For computer science types, this hierarchy is a tree structure in which the files are always leaf nodes and directories are internal nodes when they contain something and leaf nodes otherwise. The point of making this trip down data-structure memory lane is that the top node in a tree structure is the root node and that, in Linux, the file system mounted at the top node is aptly called the root file system.

On your desktop Linux system, you can see what’s mounted as the root file system by doing the following:

gene@imac-2:~$ mount | head -1
/dev/hda3 on / type ext3 (rw,errors=remount-ro)

Just typing mount shows all the file systems mounted. Most Linux systems have several file systems mounted, but all the file systems are mounted relative to the root file system.

When the Linux kernel boots, it must be able to mount a root file system. During the boot process, the root file system can be replaced with another, but only one root file system can be mounted at a time. Failure to mount a root file system means that the system can’t find something to run, because a file system is a container for your program and the kernel panics and halts.

Depending on the board’s hardware and application requirements, you’re free to select any number of root file system types. A completed device contains a single file system mounted at root but likely uses several different file systems mounted at other directories within the root file system. Only one file system can be mounted at the root (/ directory), but Linux allows for an arbitrary number of file systems to be mounted at other locations in the root file system. For example, a system that uses flash memory for storage mounts a RAM-based file system for temporary storage because it’s faster, and flash memory has a much smaller duty cycle than RAM. In the chapters that follow, this book covers how to select a root file system type and how to build one from the ground up.

Source of Information : Pro Linux Embedded Systems (December 2009)