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4. Dear Winfried,...

This is the section usually known as "frequently asked questions with answers". If you have a problem with your partner, kids or dog, just send it in, as long as it is related to writing CD-Rs or is otherwise entertaining.

4.1 How sensitive is the burning process?

Test it. Use the option -dummy to do an empty run of cdrecord. Do everything you would do otherwise and watch if the burning process survives.

If you feed cdrecord directly from mkisofs, then disk intensive processes such as updating the locate database lower the maximum flow rate and may corrupt the CD. You better check such processes are not started via cron, at or anacron while you burn CD-Rs on older machines.

4.2 Has file fragmentation a bad impact on the throughput?

Fragmentation of files is usually so low that its impact isn't noticed. However, you can easily construct pathological cases of fragmentation, which lower the throughput of your hard disks under 100 kbytes/second. So don't do that. :-) Yes, files on a hard disk get fragmented over the years. The faster, the fuller the filesystem is. Always leave 10% or 20% free space, and you should run fine with respect to writing CD-Rs.

If you're uncertain then look at the messages printed while booting. The percentage of fragmentation is reported while checking the filesystems. You can check for this value with the very dangerous command

shell> e2fsck -n  /dev/sda5        # '-n' is important!
[stuff deleted -- ignore any errors]
/dev/sda5: 73/12288 files (12.3% non-contiguous)

In this example the fragmentation seems to be very high -- but there are only 73 very small files on the filesystem. So the value is not alarming.

There is an experimental utility called e2defrag to defragment extended-2 filesystems. The current version does not work reliable enough to use it even for private environments. If you really want to defragment your filesystem, make a backup copy (better: two copies), practice restoring the data, then create a new filesystem (that will destroy the old) and restore the data. This sketch is currently the safest technique.

4.3 Is it possible to store the CD-image on an UMSDOS-filesystem?

Yes. The only filesystem that isn't reliable and fast enough for writing CD-ROMs from is the network filesystem (NFS). I used UMSDOS myself to share the disk-space between Linux and DOS/Win on a PC (486/66) dedicated for writing CD-ROMs.

4.4 Isn't there some way to get around the ISO-9660 limitations?

Yes. You can put any filesystem you like on the CD. But other operating systems than Linux won't be able to deal with this CD. Here goes the recipe:

  • Create an empty file of 650MB size.
    dd if=/dev/zero of="empty_file" bs=1024k count=650
  • Create an extended-2 filesystem on this file
    shell> /sbin/mke2fs  -b 2048  empty_file
    empty_file is not a block special device.
    Proceed anyway? (y,n) y
  • Mount this empty file through the loopback devices (you need a reasonable new mount for this; read above).
    mount -t ext2 -o loop=/dev/loop1 empty_file /mnt
  • Copy files to /mnt and umount it afterwards.
  • Use cdrecord on empty_file (which is no longer empty) as if it were an ISO-9660-image.

If you want to make an entry in /etc/fstab for such a CD, then disable the checking of the device file on system startup. For example:

/dev/cdrom  /cdrom  ext2  defaults,ro  0 0

The first 0 means "don't include in dumps" (backup), the second (=important) one means "don't check for errors on startup" (fsck would fail to check the CD for errors).

4.5 How to read the tracks from audio CDs?

There are several software packages available. The newest one is "cdpranoia" and can be downloaded from

Or you want to try the combination of "cdda2wav" and "sox", available from sunsite and its mirrors:

cdda2wav enables you to get a specific interval (or a whole track) from your audio CD and converts it into a .wav-file. sox converts the WAV files back into the (audio CD) cdda-format so it can be written to the CD-R using cdrecord. You don't necessarily need sox if you use a recent version of cdrecord, because it has built-in support for .au and .wav files.

4.6 How to probe for SCSI devices after boot?

The file drivers/scsi/scsi.c contains the information

 * Usage: echo "scsi add-single-device 0 1 2 3" >/proc/scsi/scsi
 * with  "0 1 2 3" replaced by your "Host Channel Id Lun".
 * Consider this feature BETA.
 *     CAUTION: This is not for hot plugging your peripherals. As
 *     SCSI was not designed for this you could damage your
 *     hardware !
 * However perhaps it is legal to switch on an
 * already connected device. It is perhaps not
 * guaranteed this device doesn't corrupt an ongoing data transfer.

Please note that this should only be used if your add SCSI devices to the end of the chain. Inserting new SCSI devices into an existing chain disturbs the naming of devices (directory /dev) and may destroy the complete content of your hard disk.

Some kernel versions do not like re-scanning the SCSI bus at all and your system may freeze solid when trying out the above. You have been warned.

4.7 Is it possible to make a 1:1 copy of a data CD?

Yes. But you should be aware of the fact that any errors while reading the original (due to dust or scratches) will result in a defective copy. Please note that both methods will fail on audio CDs! You have to use cdrdao or cdda2wav on audio CDs.

First case: you have a CD-writer and a separate CD-ROM drive. By issuing the command

cdrecord -v dev=0,6,0 speed=2 -isosize /dev/scd0

you read the data stream from the CD-ROM drive attached as /dev/scd0 and write it directly to the CD-writer.

Second case: you don't have a separate CD-ROM drive. In this case you have to use the CD-writer to read out the CD-ROM first:

dd if=/dev/scd0 of=cdimage

This command reads the content of the CD-ROM from the device /dev/scd0 and writes it into the file "cdimage". The contents of this file are equivalent to what mkisofs produces, so you can proceed as described earlier in this document (which is to take the file cdimage as input for cdrecord). If you want to see a progress-meter and other fancy stuff, then you can also use Jörg Schillings sdd.

In case you run into errors, then install a recent version of cdrecord, which ships a tool called "readcd" (found under misc/). It gives you the same result as dd, but reads sectors on the CD-ROM several times in case of errors.

4.8 Can Linux read Joliet CD-ROMs? (obsolete answer)

Yes. Newer Kernels (2.0.36 and the upcoming 2.2) have built-in support for the joliet format. Remember you have to use both options in your /etc/fstab: the keywords iso9660 and joliet (later is really an extension). For more details, see

4.9 How do I read/mount CD-ROMs with the CD-writer?

Just as you do with regular CD-ROM drives. No tricks at all. Note that you have to use the scd devices (SCSI CD-ROM) to mount CD-ROMs for reading, even if you have an ATAPI CD-ROM (remember you configured your ATAPI devices to act like SCSI). Example entry for /etc/fstab:

/dev/scd0  /cdrom  iso9660  ro,user,noauto  0  0

4.10 How to put even more data on the CD-R?

Use bzip2 instead of any other compressor like gzip or pkzip. It will save you up to 30% of disk-space for larger (>100kb) files. You can download it from

Instead of writing a true audio CD, you can optionally convert your WAV audio files into MP3 audio files and store them on a ISO-9660 filesystem as regular files. Usually MPEG III give you a compression of 1:10. Of course, most CD-players are not able to read files... this is the drawback. On the other hand, why not running the music for your next party from hard disk? 18 Gbytes are enough for 3000-4000 titles. :-)

A software MPEG III-encoder is available from

A MPEG III-player is available from

For recorded speech, you may want to try to reduce its size using shorten or "GSM lossy speech compression":

4.11 How to make bootable CD-ROMs?

You must have an 1.44 MB bootable floppy-disk. Create an exact image of this floppy-disk by issuing the command

dd if=/dev/fd0 of=boot.img bs=18k

Place this floppy image into the directory holding the collection of your files (or into a subdirectory of it, just as you like). Tell mkisofs about this file with the option '-b' and also use '-c'. For details read the file README.eltorito in the mkisofs-distribution.

An interesting application for a custom bootable CD is as a virus safe DOS- or Windows-system. It saves you the money for the hard disks (if you have a network and use samba to put the user-data on a file server). The German computer magazine c't has a article about this issue in the issue 11/99, page 206 (

Some details about the bootable RedHat CD-ROM is available from

4.12 How to make CD-ROMs writable like a hard disk?

There is an overlay filesystem available for Linux, which is mounted over the CD-ROM and intercepts all writing operations. New and modified files are stored elsewhere, but for the user it looks like the CD-ROM is modified. For more information, see

If that is not enough for your needs: wait for the UDF-filesystem to be supported by Linux or help developing it (see At the moment only reading of CD-media is supported due to a limitation in the CD-ROM drivers of the Linux kernel.

4.13 Is it possible to use several writers at once?

Yes. It is reported to work with at least 3 writers at full speed (6x) on a PC with 233 Mhz and a single SCSI bus running kernel 2.2.12. You need either a recent version of the Linux kernel (2.2.12 or higher).

4.14 What about Solaris, *BSD, AIX, HP-UX, etc.? Is my variant of Unix supported?

Only chapter 2 is Linux-specific. You can apply chapter 3 and 4 even if you run another family of operating systems than Linux. Please see the files README.NetBSD, README.aix, README.hpux,, README.solaris, README.sunos, README.vms or README.xxxBSD from the cdrecord-distribution.

Probably yes. Compile cdrecord for your platform and issue the command "cdrecord -scanbus". Read the README.* file for your Unix distributed with the sources of cdrecord. However, not all variants of Unix can read the RockRidge, Joliet or HFS extensions on your newly written CD-R.

4.15 Where to store the local configuration permanently?

You have two options. Either you use the built-in configuration file for cdrecord, or you use a shell wrapper like the one shown below. This shell script reads a configuration file, which lists the options and parameters for cdrecord line by line. The names are exactly the same as on the command line, but without the leading dash. Comments are allowed. Example:

# be verbose
# set the speed of the writer
# the device-coordinates in the form BUS,ID,LUN

The configuration files for the wrapper belong into /etc/cdrecord/ and must be referenced on the command line. Example: if you want to refer to the configuration /etc/cdrecord/mywriter.cfg, then you can issue the command " mywriter.cfg -audio track1...". Everything after mywrite.cfg is passed to cdrecord.

#! /bin/bash



if [ ! -f "$CFGDIR/$CFG" ]
    echo "Configuration file $CFGDIR/$CFG not found. Exiting."
    exit 1

while read LINE
    case $LINE in
        \#*|"") continue;;
    set -- $LINE
    while shift
        case $1 in
            "") continue;;

    if [ -z "$O_VALUE" ]
done < "$CFGDIR/$CFG"

set -x    #DEBUG
exec cdrecord $O_CDRECORD $ARGS_LEFT
echo "Execution of cdrecord failed."

4.16 How can the CD-info be retrieved?

Somewhere behind the first 32 k on the CD, a block with information about the CD is located. You can extract the information with the following shell script:

#! /bin/bash

for i in 32768,7   32776,32 32808,32 32958,128 33086,128 33214,128 \
         33342,128 33470,32 33581,16 33598,16  33615,16  33632,16
    set -- $i
    echo "*`dd if=$RD bs=1 skip=$OFFSET count=$LENGTH   2> /dev/null`#"

4.17 What about re-writing

When overwriting CD-RW media, specify the parameter blank=fast to cdrecord. That's all. See the man page of cdrecord for details about this parameter.

4.18 How to create a multi-session CD?

First of all, the image for a multi-session CD must be formatted using the ISO-9660 filesystem using RockRidge-extensions. And you must use the option -multi for cdrecord as long as you want to add further sessions. So at least for the first session, you must specify the option -multi.

Some CD-writers have no support for CD-ROM XA mode 2 or for session-at-once (SAO), so you need to specify the switch -data for cdrecord on the command line.

The images for the second and subsequent sessions are a little bit more complicated to generate. Mkisofs must know where the free space on the CD-R begins. That information can be gathered by using the option -msinfo on cdrecord (see example below).

shell> NEXT_TRACK=`cdrecord -msinfo dev=0,6,0`
shell> echo $NEXT_TRACK
shell> mkisofs -R -o cd_image2 -C $NEXT_TRACK -M /dev/scd5
         private_collection/ \

For more information, please read the file README.multi, which is distributed with cdrecord.

4.19 Should I use the SCSI adapter shipped with the writer?

Reported via email: Most CD-writer docs say to use a separate SCSI bus if going from a CD-ROM to a CD-writer and I have seen this myself in the following scenario:

Adaptec 2940UW SCSI card, a 24x SCSI CD-ROM and a 4x4 SCSI CD-writer. When I got the CD-writer, it came with its own ISA SCSI card which could only handle one device. I figured I'd toss this and use my better adaptec card for all the stuff. I noticed it was quite prone to buffer under-runs when recording at the 4x speed, but as soon as I tried hooking up the ISA SCSI card, it had no problems. I know 2 other people (both using adaptec 2940 cards), who have experienced exactely the same symptoms, usually when writing from a CD-ROM to a CD-recorder. Though I've never experienced the problem when going from a hard drive to a CD-writer on the same bus.

4.20 How to burn over the network?

Usally a file transfer with FTP is fast enough to feed a CD-recorder at quadruple (4x) speed even over a 10 Mbit ethernet. You can couple the ftp-client and cdrecord via a fifo. First create a fifo named cdimage:

mkfifo cdimage
get cdimg cdimage

Then treat cdimage like a regular file, i.e. issue the following command:

cdrecord dev=0,1,0 speed=2 cdimage

Your ftp-client will notice when cdrecord wants to read from the file and will start transfering data from the ftp-host.

4.21 I hear a crack or click sound at the end of the each track.

You have to use disk-at-once mode (DAO) to get rid of the cracks.

4.22 How can this be set up so that a user can burn CDs instead of always being root?

You can add the setuid-bit to the cdrecord-executable. However, this might be a security risk. Just setting the permissions on the device files does not help as cdrecord issues privileged commands via the SCSI generic interfaces.

which cdrecord
chown root.root /usr/bin/cdrecord
chmod 4111 /usr/bin/cdrecord

4.23 Where do I get the "Yellow Book" and "Orange Book" standards?

You get the printed specifications from Philips and they are expensive.

4.24 I've been searching for information on burning Video-CD under Linux.

Here you can find utilities for creation of MPEG videos and Video-CDs:

Political correct people mention the Berkeley-tools and other YUV-strategies. Their usage is complicated, takes much time and harddisk-space and gives you no audio track. I recommend to use conceptually higher level applications like the ones mentioned above.

4.25 Which is easier to set up, IDE or SCSI?

SCSI CD-writers are slightly easier to set up with regard to CD-writing under Linux. And they are reported to have better error recovery. If that outweights the higher price cannot be answered generally.

4.26 How can I overburn a CD using {cdrecord,cdrdao}?

Overburning a CD-R is nothing special. It is at your own risk that the data fits on the CD-ROM, but thats all. There are no 650 Mbytes-limits in the software under Linux.

4.27 What will cdrecord do when it stops getting input from the pipe?

It will finish writing. So you can just couple your favourite backup tool with cdrecord using a pipe, like in "bru -size=640m -f - | cdrecord dev=0,1,0 speed=2 -". You have to take special care within the backup utility if the backup spans multiple CD-Rs.

4.28 Is there an equivalent to ignore=hdX for the ide-scsi emulation?

I know of no way, but anybody is welcome to add this feature to the Linux kernel sources.

4.29 How many times can you re-use CD-RW before they become faulty?

Good question.

4.30 Which format to choose for a platform independant CD-ROM?

A CD-ROM to be read by all systems can only use the plain ISO 9660 format. That means stupid 8+3 filenames from old MS-DOS and without any HFS (Macintosh), Joliet (Microsoft) or RockRidge (newer Unices) extensions. There is no extension for longer filenames, which could be read by all operating systems.

4.31 Is multi-session for audio tracks possible?

Audio CD-players are only able to deal with audio tracks stored in the first session. In other words, you cannot add audio tracks using subsequent sessions. However, writing data tracks into the second session effectively hides them from audio CD-players. This way you prevent having an silent track on your mixed mode CD (audio and data mixed).

4.32 What hardware resources do I need? Is an old Pentium enough?

An answer depends on your wishes. If you need a reason to buy a new computer, here is the answer from the international association of computer manufactures: Whatever you plan to do, you need a processor with 800 Mhz. Because it won't fit into your existing motherboard, you need a new motherboard, too. The easiest solution is just to buy the typical complete offer as seen in advertisements on TV. Please ignore the rest of this section.

Now the case if you just want a rational answer: I wrote several CD-ROMs sucessfully using a "486" with 66 Mhz. Although MS does not consider them to be PCs anymore, but recommends to run a CE version on them (seen on the CeBIT), Linux runs even fine on the predecessors of the Pentium and is even able to write CDs. You can easily find out if the power of your hardware is sufficient for writing CD-ROMs by testing it out. Just add the command line switch -dummy when executing cdrecord and the laser will be kept off. Watch the burning process.

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