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Create filesystem images containing LsLinux
We're going to learn how to create different types of filesystem images. For each type, I'll give a short definition, describe the step by step procedure to create/build the image using common Unix commands, and what usage can be maid of it.
RamFS
A RamFS image is a compressed cpio archive containing a Linux tree loaded in RAM at boot. Kernel will try to launch any executable file named /init it contains.
RamFS are generally used to load needed storage driver modules, to be able to boot on a full Linux system on a local harddrive. But if your system is small enough, it may be fully loaded in RAM and be run directly from it. The problem is that RAM is not a persistant storage solution, and any changes made on system will be lost on halt/reboot.
Creating RamFS
The easiest way to obtain a working RamFS is to use debootstrap to create the tree, and then use cpio and bzip2 utilities to create the archive. In this example, we'll create a basic RamFS file in /boot/ramfs.bz2, using a debootstrap in /boot/ramfs :
root@lslinux ~ # ./debootstrap.sh -hostname=myramfs /boot/ramfs >> Debootstraping LSL to /boot/ramfs ? /boot/ramfs, directory does not exist, create it? ([y]|n) >> Fetching core packages >> Extracting core packages Installing Busybox v1.20.2 links... ..... ... >> All done! Your LSL environnement is now available in /boot/ramfs (build time : 8s) root@lslinux ~ #
Now we have a LsL tree in /boot/ramfs, now let's create the /boot/ramfs.bz2 archive :
root@lslinux ~ # cd /boot/ramfs root@lslinux /boot/ramfs # find | cpio -o -H newc | bzip2 -9 > /boot/ramfs.bz2 7767 blocks root@lslinux /boot/ramfs # du -sh /boot/ramfs* 4.2M /boot/ramfs 1.5M /boot/ramfs.bz2 root@lslinux /boot/ramfs #
Cpio takes a list of files as input and produces archive on standard output. We use find to build the files list, which is passed to cpio to create (-o option) a 'newc' format archive. Cpio outputs archive to bzip2 utility which compress it using best compression (-9 option), result is redirected to /boot/ramfs.bz2 file.
Editing RamFS
To edit a RamFS content, you have to uncompress and unpack archive, edit needed files in produced tree, and rebuild the RamFS file. Let's say that you have a 'ramfs.bz2' file and that you want to update hostname it contains. First, create a directory to store the tree and unpack files to it :
root@lslinux ~ # ls ramfs.bz2 root@lslinux ~ # mkdir ramfs root@lslinux ~ # ls ramfs ramfs.bz2 root@lslinux ~ # cd ramfs root@lslinux ~/ramfs # bzcat ../ramfs.bz2 | cpio -i 7767 blocks root@lslinux ~/ramfs # ls bin boot data dev etc home init lib lib64 proc root sbin sys tmp usr var root@lslinux ~/ramfs #
You may now edit the needed files/make your modifications. To rebuild the ramfs file :
root@lslinux /boot/ramfs # find | cpio -o -H newc | bzip2 -9 > /boot/ramfs.bz2 7767 blocks root@lslinux /boot/ramfs #