Книга: Embedded Linux Primer: A Practical, Real-World Approach

9.9. Other File Systems

9.9. Other File Systems

Numerous file systems are supported under Linux. Space does not permit coverage of all of them. However, you should be aware of some other important file systems frequently found in embedded systems.

The ramfs file system is best considered from the context of the Linux source code module that implements it. Listing 9-19 reproduces the first several lines of that file.

Listing 9-19. Linux ramfs Source Module Comments

/*
* Resizable simple ram filesystem for Linux.
 *
 * Copyright (C) 2000 Linus Torvalds.
 * 2000 Transmeta Corp.
 *
 * Usage limits added by David Gibson, Linuxcare Australia.
 * This file is released under the GPL.
 */
/*
 * NOTE! This filesystem is probably most useful
 * not as a real filesystem, but as an example of
 * how virtual filesystems can be written.
 *
 * It doesn't get much simpler than this. Consider
 * that this file implements the full semantics of
 * a POSIX-compliant read-write filesystem.

This module was written primarily as an example of how virtual file systems can be written. One of the primary differences between this file system and the ramdisk facility found in modern Linux kernels is its capability to shrink and grow according to its use. A ramdisk does not have this property. This source module is compact and well written. It is presented here for its educational value. You are encouraged to study this good example.

The tmpfs file system is similar to and related to rams. Like ramfs, everything in tmpfs is stored in kernel virtual memory, and the contents of tmpfs are lost on power-down or reboot. The tmpfs file system is useful for fast temporary storage of files. I use tmpfs mounted on /tmp in a midi/audio application to speed up the creation and deletion of temporary objects required by the audio subsystem. This is also a great way to keep your /tmp directory cleanits contents are lost on every reboot. Mounting tmpfs is similar to any other virtual file system:

# mount -t tmpfs /tmpfs /tmp

As with other virtual file systems such as /proc, the first /tmpfs parameter in the previous mount command is a "no-op"that is, it could be the word none and still function. However, it is a good reminder that you are mounting a virtual file system called tmpfs.

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