Principles

What's a Live CD / Live USB

A live CD or live DVD is a CD or DVD containing a bootable computer operating system. Live CDs are unique in that they have the ability to run a complete, modern operating system on a computer lacking mutable secondary storage, such as a hard disk drive. Live USB flash drives are similar to live CDs, but often have the added functionality of automatically and transparently writing changes back to their bootable medium.

The term "live" derives from the fact that these CDs each contain a complete, functioning and operational operating system on the distribution medium.

While a live CD typically does not alter the operating system or files already installed on a computer's hard drive, many live CDs include mechanisms and utilities for altering the host computer's hard drive, including permanent installation. This is important for the system management aspect of live CDs, such as removing viruses, drive imaging, and system recovery.

The default option, however, is to allow the user to return the computer to its previous state when the live CD is ejected and the computer is rebooted. It is able to run without permanent installation by placing the files that typically would be stored on a hard drive into RAM, typically in a RAM disk. However, this does cut down on the RAM available to applications, reducing performance somewhat. As of 2007, certain live CDs run a graphical user interface in as little as 32MB RAM.

Boot steps of a Live CD / Live USB

Step 1: Boot into bootloader (Grub, syslinux)

You can choose the environment to boot at this step.

Step 2: Boot kernel with micro environment (Vmlinuz, initrd.gz)

At this step the Linux kernel of your environment is booted with a micro environment (filesystem) to set up the early stages of boot

Step 3: Squashfs filesystem is mounted

At this step the complete and compressed read only filesystem is uncompress and mounted. It will be used instead of the micro environment

Step 4: Live CD / Live USB scripts are called

At this step some scripts are called to make specific custom tasks fo Live CD / Live USB. For example persistent partition is mounted for Live USB. In Ubuntu scripts are called casper-initramfs, in Debian scripts are called live-initramfs. Live-initramfs will supersede casper-initramfs in the future.

Step 5: Text only boot stages

At this step operating system start all background services: servers, schedulers, ...

Step 6: Booting into graphical environment

This is the last step that call the X window system from text mode to boot into graphical mode (Gnome, Kde, Xfce, E17, ...)

Documentation

Squashfs, Wikipedia EN: http://en.wikipedia.org/wiki/SquashFS

Live CD, Wikipedia EN: http://en.wikipedia.org/wiki/Live_CD

Booting, Wikipedia EN: http://en.wikipedia.org/wiki/Booting

Live CD, Wikipedia FR: http://fr.wikipedia.org/wiki/LiveCD