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Live CD
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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.
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Encyclopedia
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.
History
Compact Discs, originally developed for storing audio, were adapted for use as media for storing and distributing large amounts of computer data. This data may also include application and operating system software, sometimes packaged and archived in compressed formats. Later, it became convenient and useful to boot the computer directly from compact disc, often with a minimal working system in order to install a full system onto a hard drive.
The first Compact Disc drives on personal computers were generally much too slow for running complex operating systems. Often, the computer could not boot from optical discs. When operating systems were distributed on compact discs, either a boot floppy or the CD itself would boot specifically, and only, in order to install onto a hard drive.
Origin of Linux live CDs
Although early developers and users of distributions built on top of the Linux kernel were able to take advantage of cheap optical disks and rapidly declining prices of CD drives for personal computers, the Linux distribution CDs or "distros" were generally treated as a collection of installation packages that must first be permanently installed to hard disks on the target machine.
However in the case of these distributions built on top of the Linux kernel, the free operating system was meeting resistance in the consumer market because of the perceived difficulty, effort, and risk involved in installing an additional partition on the hard disk, particularly the ext2 filesystem.
The term "live CD" was coined because after typical PC RAM was large enough and 52x speed CD drives and CD burners were widespread among PC owners, it finally became convenient and practical to boot the kernel, run X11, a window manager and GUI applications directly from a CD without disturbing the OS (generally Windows on FAT32 or NTFS) on the hard disk.
This was a new and different situation for Linux than other OSes, because the updates/upgrades were being released so quickly, different distributions and versions were being offered online, and especially because users were burning their own CDs.
Copying Linux from the installation media was also encouraged instead of actively hindered and discouraged with such things as requiring the input of long and elaborate serial numbers and lengthy and complicated installation procedures.
The first Linux-based 'Live CD' was Yggdrasil Linux first released in beta form 1992~1993 (ceased production in 1995), though in practice its functionality was hampered due to the low throughput of then-current CD-ROM drives. Some time later, Knoppix, a Debian-derived Linux distribution was released in 2003, and found popularity as both a rescue disk system and as a primary distribution in its own right. Since 2003, the popularity of live CDs has increased substantially, partly due to and remastersys which made it very easy to build customized live systems.
Most of the popular Linux distributions now include a live CD variant, which in some cases is also the preferred installation medium.
Uses
While some live CDs are designed to "demo" or "test drive" a particular operating system (usually Linux or another free or open source operating system), there are live CDs made for many different uses.
Although some live CDs can load into memory in order to free the optical drive for other uses, loading the data off a CD-ROM is still slower than a typical hard drive boot, so this is rarely the default with large live CD images, but for smaller live CD images loading the filesystem directly into RAM can be highly practical. Loading the filesystem image into RAM can provide a significant performance boost as RAM is several orders of magnitude faster than a hard drive. Also, since RAM has no moving parts, a system running from a live CD loaded into RAM can run with improved power efficiency. Experienced users of the operating system may also use a live CD to determine whether and to what extent a particular operating system or version is compatible with a particular hardware configuration and certain peripherals. Or as a way to know beforehand which computer or peripheral will work before buying. Users may also use a live CD to troubleshoot hardware, especially when a hard drive fails. Some live CDs can save user-created files in a Windows partition, a USB drive, a network drive, or other accessible media.
A few additional uses include:
- installing a Linux distribution to a hard drive
- testing new versions of software
- testing hardware
- system repair and restoration
- high security/non-invasive environment for a guest
- cracking/stealing passwords
- network security testing
- being the primary or backup operating system for any computer
- quick and simple clustering of computers
- computer forensics
- playing video games
- providing a secure server platform where crucial files cannot be permanently altered
- Internet kiosks, which can be brought back to their original state by a reboot
Live CD software appliances
Packaging a software appliance as an installable live CD can often be
beneficial as a single image can run on real hardware in addition to
most types of virtual machines.
This allows developers to avoid the complexities involved in supporting
multiple incompatible virtual machine images formats and
focus on the lowest common denominator instead.
Typically after booting the machine from the live CD, the appliance will
either run in non-persistent demo mode or install itself, at the user's request, to an available storage device.
Mounting without burning
The files on a live CD ISO image can be accessed in Microsoft Windows with a disk image emulator such as Daemon Tools, or in Unix variants by mounting a loop device.
After mounting the live CD's filesystem, software on the live CD can be run directly (I.e., without booting) by chrooting into the live CD's mounted filesystem.
Common traits
Some live CDs come with an installation utility launchable from a desktop icon that can optionally install the system on a hard drive or USB flash drive. Most live CDs can access the information on internal and/or external hard drives, diskettes and USB flash drives.
Generally live CDs are booted from read-only media, requiring either copying to rewriteable media (i.e. a hard drive) or complete remastering to install additional software; however, there are exceptions such as Morphix and Puppy Linux which are one of the few Linux live CD distributions able to save files to the live CD itself or other multisession medium, allowing users to carry data, and more importantly, added programs and customized settings, along with them on optical disc.
Most live CDs are based on Linux, as this was the operating system that had the most to gain by offering free trials and demonstrations without regard to sales or copyright. Now others are using the term live CD for other operating systems, such as OpenSolaris, BeleniX and others based on Solaris. Other "live" operating systems include Mac OS, Microsoft Windows, ReactOS, NetBSD, FreeBSD, OpenBSD, MINIX 3, Plan 9 from Bell Labs, MorphOS and FreeDOS.
The first personal computer operating system on a CD to support "live" operations might have been the AmigaOS, which could be booted from CD on an Amiga CDTV in 1990.. Earlier examples of live OS are of course the operating systems used from floppy, and most widely spread is DOS.
Unlike previous operating systems on optical media, though, Linux "live CDs" were specifically designed to run without installation onto other media like a hard disk drive. The live CD concept was meant to promote Linux and showcase the abilities of the free, open source operating system on conventional personal computers with Microsoft Windows already installed.
On a PC, a bootable Compact Disc generally conforms to the El Torito specification. Many Linux based live CDs use a compressed filesystem image, often with the cloop compressed loopback driver, or squashfs compressed filesystem, generally doubling effective storage capacity, although slowing application start up.
The resulting environment can be quite rich: typical Knoppix systems include around 1,200 separate software packages. Live CDs have a reputation for supporting advanced auto-configuration and plug-and-play functionality. This came out of necessity to avoid requiring the user to configure the system each time it boots and to make it easily usable by those who are new to the operating system.
Technique
A read-only file system, such as on a CD-ROM has the drawback of being unable to save any current working data. For this reason, a read-only file system is often merged with a temporary writable file system in the form of a RAM disk. Often the default Linux directories "/home" (containing users' personal files and configuration files) and "/var" (containing variable data) are kept in ramdisk, because the system updates them frequently. Puppy linux has a savable layer so if you choose to, the next time you boot you can resume (pick right back up again) from where you left off. Each time the CD boots, it looks for the file and then uses it if it has the right name.
In modern live CDs, a read-only file system is merged with ramdisk using transparent techniques such as UnionFS or AuFS. In MS-DOS systems, a DOS utility, ramdrive.sys, can be loaded at boot for this purpose.
Live CDs have to be able to detect a wide variety of hardware (including network cards, graphic cards etc.). This is easily achieved nowadays by udev or hotplug, which is a common part of all distributions based on Linux kernel 2.6.
Cheat code
During live CD initialization, a user typically may resort to using one or more cheat codes to change the booting behavior. These vary from distribution to distribution but can most often be accessed upon first boot screen by one of the function keys.
See also
Screenshots
Here are some screenshots of live CDs:
External links
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- Howto create LiveCDs and build tool resource
- - A BSD perspective on the concept of Live CDs
- Article published on Free Software Magazine about Live CDs
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