3. Burning files on CD-Rs

"If to smoke you turn I shall not cease to fiddle while you burn." (Emperor Nero about burning his own classic-CDs, AD64. He misunderstood it completely.)

Usually the writing of a CD under Linux is done in 2 steps:

• packaging the desired software into one big file using the mkisofs/mkhybrid-utility
• writing the big file to the CD-R with cdrecord

It is also possible to combine the 2 steps into one via a pipe.

3.1 Collect software

Usally this takes up longer than one expects. Remember that missing files cannot be added once the CD is written. :-)

Also keep in mind that a certain amount of the free space of a CD is used for storing the information of the iso9660-filesystem (usally a few MB).

3.2 Storing data on a CD.

The term iso9660 refers to the format in which data is organised on the CD. To be more precise: it's the filesystem on the CD.

Of course the appearance of files stored in this format is unified by the Linux-kernel as for every other filesystem, too. So if you mount a CD into the directory tree, you cannot distinguish it's files from other files ... beside the fact that they are not writeable ... even not for root. :-) (The mechanism used to unify the appearance of files is called virtual filesystem, short VFS.)

The features of the iso9660 filesystem are not so rich compared to those of the extended-2 filesystem which is normally used under Linux. On the other hand, the CD is only writable once and some features make no sense anyway. The limitations of the iso9660-filesystem are:

• only 8 levels of sub-directories allowed (counted from the top-level directory of the CD) (use RockRidge Extensions to enlarge this number)
• maximum length for filenames: 32 characters
• 650 MB capacity

3.3 Create an iso9660 filesystem

Before any storage medium (e.g. floppy disk, harddisk or CD) can be used, it must get a filesystem (DOS speak: get formatted). This filesystem is responsible for organising and incorporating the files that should be stored on the medium.

Well, a writable CD is only writable once so if we would write an empty filesystem to it, it would get formated - but remain completely empty forever. :-)

So what we need is a tool that creates the filesystem while copying the files to the CD. This tool is called mkisofs. A sample usage looks as follows:

mkisofs  -r   -o cd_image   private_collection/
              `---------'   `-----------------'
                   |               |
          write output to   take directory as input

The option '-r' sets the permissions of all files to be public readable on the CD and enables Rock Ridge extensions. That is what one usually wants and use of this option is recommended until you know what you're doing (hint: without '-r' the mount-point gets the permissions of private_collection!).

mkisofs will try to map all filenames to the 8.3-format used by DOS to ensure highest possible compatibility. In case of naming conflicts (different files have the same 8.3-name), numbers are used in the filenames and information about the chosen filename is printed via STDERR (usually the screen).

DON'T PANIC:

Under Linux you will never see these 8.3 filenames because Linux makes use of the Rock Ridge extensions which contain the original file-information (permissions, filename, etc.).

Now you may wonder why the output of mkisofs is not directly sent to the writer-device. This has two reasons:

• mkisofs knows nothing about driving CD-writers (see section 2.3.)
• On slow machines it would not be reliable (see section 4.)

Because the timing of the CD-writer is a critical point, we don't feed it directly from mkisofs (remember Linux is not a real-time operating system and tasks can be timed badly). Instead it is recommended to store the output of mkisofs in a separate file on the harddisk. This file is then an 1:1-image of the later CD and is actually written to the CD with the tool cdwrite in a second step.

The 1:1-image gets stored in a huge file so you need the same amount of free disk space that your collected software already eats up. That's a drawback.

One could think of creating an extra partition for that and writing the image to that partition instead to a file. I vote against such a strategy because if you write to the wrong partition (due to a typo), you can lose your complete Linux-system. Furthermore, it's a waste of disk-space because the CD-image is temporary data that can be deleted after writing the CD. However, using raw partitions saves you the time for deleting files of 650 MB size.

3.4 Test the CD-image

Linux has the ability to mount files as if they were disk-partitions. This feature is useful to check that the directory layout of the CD-image is ok. To mount the file cd_image created above on the directory /cdrom, give the command

mount -t iso9660 -o ro,loop=/dev/loop0 cd_image /cdrom

Now you can inspect the files under /cdrom - they appear exactly as they were on a real CD. To umount the CD-image, just say umount /cdrom. (Warning: On Linux-kernels prior to 2.0.31 the last file on /cdrom may not be fully readable. Please use more recent kernels like 2.0.35)

Note:

some ancient versions of mount are not able to deal with loopback-devices. If you have such an old version of mount it is a hint to upgrade your Linux-system.

Several people already suggested to put information about how to get the newest mount-utilities into this mini-HOWTO. I always refuse this. If your Linux-Distribution ships with an ancient mount: report it as a bug. If your Linux-Distribution is not easily upgradable: report it as a bug.

If I included all the information that is necessary to work around bugs in bad designed Linux-Distributions, this HOWTO would be a lot bigger and harder to read.

3.5 Remarks on the blank CD-Recordable discs

The german computer magazine "c't" has a list of tips regarding the blank CDs in their november 1996 issue:

• "no-name" discs are generally not of highest quality and should better not be used
• if a recordable CD is defective, this is likely to apply to the whole batch (if you bought more than one at a time); maybe you are lucky and can at least use the first 500MB of such CDs ...
• don't touch the CDs at their shimmering side before writing

3.6 Write the CD-image to a CD

Not much more left to do. If you didn't already tried, it's a good time for the command

cdrecord -scanbus

This will tell you to which SCSI-device your cd-writer is attached to.

Before showing you the last command, let me warn you that CD-writers want to be fed with a constant stream of data because they have only small data-buffers. So the process of writing the CD-image to the CD mustn't be interupted or a corrupt CD will be the result.

To be sure nothing can interupt this process, throw all users of the system and unplug the ethernet-cable ... Read the Bastard operator from hell to learn about the right attitude to do so. ;-)

If you are mentally prepared, dress up in a black robe, multiply the SCSI-id of the CD-writer with it's SCSI-revision and light as many candles, speak two verses of the ASR-FAQ and finally type

cdrecord -v speed=2 dev=X,Y cd_image

The parameters X,Y are to replace with the numbers of the corresponding SCSI-device (as printed by -scanbus above). If you are using a Linux kernel prior to release 2.0.31, you may want to use the option '-pad' to get along a bug in the Linux filesystem code.

IMPORTANT: If you use cdrecord to write CD-RWs a second time, you must use the option "blank=..." to erase the old content. Please read the man-page to learn more about the various methods to blank the CD-RW.

Please note that no writer can re-position it's laser and can't continue at the original spot on the CD when it gets disturbed. Therefore any strong vibrations or even a shock will completely destroy the CD you are writing.

3.7 If something goes wrong ...

... remember you can still use corrupt CDs as coasters. :-)