Background on live CD/DVD
Note: This section is a clarification of how live CD works. You don't have to read it. You can skip it if you want.
A live CD/DVD is basically a normal linux installation just like an ordinary harddrive installation. However, simply copying the harddirve installation over to a CD/DVD is not enough to produce a working system. Why? because there are still minor differences between a live CD/DVD and on ordinary harddrive installation. So in addition to copying our harddirve installation to the CD/DVD we must address those differences as well.
So what are these differences?
- The CD or DVD is read only media. Linux needs to have write access to certain parts of the system to be able to operate properly (like "/dev" "/proc" "/var" "/tmp"). There are a lot of approaches to address this problem. All of which utilize the system RAM. Some of these approaches enable write access only to essential directories and files, and hence, they do not allow the user to modify the system or install new packages while in the live CD. Other approaches, like unionfs which is what is used in this guide, allows the user to write to any part of the system. This is done by merging part of the RAM with the read-only filesystem of the live CD/DVD and making the look like one filesystem that have read-write access. Unionfs has to be mounted at boot in a certain manner.
- With the harddrive installation the location of the root filesystem is fixed. So it is passed to the kernel at boot time using the root=/dev/... parameter. With a live CD/DVD, the location of the root device is not fixed as the user might have multiple cdrom drives, these drives can be ide, scsi ... etc. So for the root filesystem to be mounted, there must be a way to identify the root device, and then we have to load the suitable kernel modules (to be able to access the cdrom controller as well as the CD filesystem). All this has to be done even before we have a root filesystem mounted.
- To fit on a CD, the filesystem is usually compressed using squashfs. So we need to autodetect the filesystem type. We also need to have the proper modules for mounting it.
These considerations require special preparation at boot time, some of which must be performed even before mounting the actual filesystem. How can we do this?
Linux introduced a mechanism that allow for such preparations at boot time before the actual root filesystem is mounted. It is called the initial root filesystem or initramfs. This mechanism is used also in mounting normal harddirve installations, as it adds flexibilty to the boot process.
initramfs is virtual filesystem. It is a compressed cpio (cpio is an archive format similar to tar) archive that contains a minimal shell, kernel modules necessary for mounting the root filesystem and number of scripts that perform some tasks at boot time. The most important of these scripts is a script called init located at the root of the initramfs.
How does initramfs work?
The boot loader loads both the kernel and the initramfs into memory and starts the kernel. The kernel then unpacks the initramfs and mount it as initial root filesystem, and then looks for the init program within the initial filesystem, and once it finds it, it executes it and hand the boot process over to it. This init scirpt is responsible for finding the real root filesystem and mounting it. It is also responsible for any special preparations required at boot time.
So any special operations required for booting the system from live media can be coded into the initramfs boot scripts.
How is initramfs is created?
We do not have to create initramfs manually (although it can be done). There are tools for creating and updating initramfs like the command update-initramfs. Moreover, these tools can include custom scripts into the initramfs if they are placed in a certain preset locations (/usr/share/initramfs/scripts). So all we have to do is dump our custom scripts (which will do all the required preparation for booting the live CD/DVD) into these preset locations, and then create a custom initramfs by running update-initramfs.
We don't even have to write these scripts. Why? becuase there are packages that have scripts tailored for booting form live CDs. One of these packages is called casper (this is the package used in this howto). By installing casper into the system, it places the scripts in there proper locations (where they can be spotted by update-initrfamfs). The only thing we need to do after installing casper is running update-initramfs to create an initramfs suitable for live CD/DVD.
The live CD/DVD structure:
The directory tree of the live CD/DVD we are going to create is going to look like this:
Code:
(CD ROOT)
|-------+casper
| |-------filesystem.${FORMAT}
| |-------filesystem.manifest
| |-------filesystem.manifest-desktop
|
|-------+boot
| |--------+grub
| | |--------menu.lst
| | |--------stage2_eltorito
| |
| |-------vmlinuz
| |-------initrd.gz
| |-------memtest86+
|
|--------md5sum.txt
- /casper/filesystem.${FORMAT}: This is the container of the linux filesystem we are going to copy from our harddisk. It is usually a compressed filesystem like squahsfs.
- /casper/filesystem.manifest: This file is optional. You only need it if you decide to include the Ubuntu installer in the CD. The purpose of this file will be explained later.
- /casper/filesystem.manifest-desktop: This file is optional. You only need it if you decide to include the Ubuntu installer in the CD. The purpose of this file will be explained later.
- /boot/grub/menu.lst: File containing boot options for the live CD/DVD.
- /boot/grub/stage2_eltorito: The boot loader of the CD. (stage2 of grub).
- /boot/vmlinuz: The linux kernel. This is copied form the linux filesystem.
- /boot/initrd.gz: the initramfs that contain the customizations necessary for the live CD/DVD.
- /boot/memtest86+: Optional file used to test the RAM of the machine form the live CD/DVD.
- /md5sum.txt: Optional file containing checksums for all the files in the CD.
