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4. Frequently asked questions with answers

4.1 "How sensitive is the burning process?"

Answer: that depends on your writer. Modern ones should have a data-buffer of 1MB or such and can live 1-2 seconds without data. See the manuals or ask your manufacturer if you want to know the details.

Regardless of the size of those data-buffers you must guarantee a constant throughput of 300kb/s or 600kb/s in the long time run.

Disk intensive processes such as updating the locate-database lower the maximum flow-rate will surely corrupt the CD; you better check such processes are not started via cron, at or anacron while you burn CD-Rs.

On the other hand, people reported that they compiled a kernel while burning a CD without a glitch. Of course you need a fast machine for such experiments.

4.2 "Has fragmentation a bad impact on the throughput?"

Fragmentation is usally so low that it's impact isn't noticed.

If you're uncertain than 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

bash> 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 this filesystem (used as /tmp) so the value is _not_ alarming.

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 CDs from is the network filesystem (NFS).

I'm using UMSDOS myself to share the disk-space between Linux and DOS/Win on a PC (486/66) dedicated for writing CDs.

4.4 "Isn't there some way to get around the iso9660 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:

If you want to make an entry in /etc/fstab for such a CD, disable the checking of it, e.g.:

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

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

4.5 "How to read and write audio CDs?"

Please get the packages "cdda2wav" and "sox", available from sunsite and it's 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 cdwrite.

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 hotplugging 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.

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.

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

cdwrite -v -D /dev/sgc --pad -b  $(isosize  /dev/scd0) /dev/scd0
cdrecord -v dev=3,0 speed=2 -isosize /dev/scd0

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

Second case: you don't have a seperate CD-ROM drive. You have to use the writer to read out the CD-ROM in this case:

dd if=/dev/scd0 of=cdimage bs=1c count=`isosize  /dev/scd0`

This command is equivalent to the result of mkisofs, so you should procede as described in chapter 3. Please note that this method will fail on audio CDs!

4.8 "Can Linux read Joliet CDs?"

Yes. But you need to patch the kernel and recompile it. For further 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 CDs for reading. Example-entry for /etc/fstab:

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

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