Some surprise while "dd-ing" a video DVD...

[Noah Dain]

On 1/18/06, dan <hentaidan at gmail.com> wrote:
> On 18/01/06, Andrea Giuliano <a.giuliano at iccu.sbn.it> wrote:
> > dan wrote:
> >
> > >
> > > Never had any problems myself with dd on DVD's, though i've only tried
> > > it with  Mac OS X disc. I use:
> > >
> > > umount /dev/hdc
> > > dd if=/dev/cdrom of=cd.iso
> > >
> >
> > I guess you mean the disk used to install the operating system. I also
> > guess it's not a video DVD, that's right? If so, I think I could do the
> > same too. I tried to dd Debian DVD, Fedora DVD and a couple of other
> > data DVDs I burned, and they all work perfectly.
> >
> > Again, the problem only seems to arise with video DVD.
>
> Yes, sorry it was the install disc (I guess) a data disc. I also tried
> to dd a video dvd (Pirates of the Cari.) and it seemed to be fine for
> ~200Mb (I Ctrl-C'd it). Though it might have been Nauty unlocking it
> before hand as Philip suggested. I killed all nautilus processes and
> unmounting etc. and it still seemed to work.
>
> --
> http://www.danicity.co.uk
>
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I work a lot with cd/dvd/floppy images and dd aborting is so common, I
have to mark it as "normal".

when dd is run in it's default form (dd if=xxx of=xxx), it will abort
on the first error it encounters resulting in 'input/output' error. 
This is to avoid copying bad data.

resolution:  everytime i've run into this problem, baring a bad disk,
backing off on the drive speed has solved the problem.  You can set
this with "hdparm -E <cdrom speed>".  Note that this is not dvd speed.
 I usually start with something like 24 and back down from there if
necessary (20, 16, 12).

Also note that this process is NOT good for attempting to get an image
of a damaged cd/dvd.  As soon as dd hits an error, it still exits. 
But, there are arguments to use to alter this behaviour:

basic recovery syntax:
  dd if=/dev/hdc of=file.txt conv=noerror,sync

  conv=noerror,sync - do not fail on errors and write zeros to
destination sectors with cannot be read

I don't know what this has to do with udf as all the dvd's i've seen
are iso9660 filesystem, or at least compatible with it.  The linux
kernel hasn't even had udf support for all that long now, and write
support is still experimental.  I know udf is used for packet writing,
where an optical disk is used read and write like one would a normal
harddisk.  I don't too much about the specifics, but this guy seems to
have done his homework:
http://www.mrichter.com/cdr/primer/udf.htm

As far as encryption on dvds go, the dvd itself isn't encrypted, just
the .vob files on it.  So, if you can't see the VIDEO_TS directory
with .vob (mpeg) files in it, then the problem is not with normal dvd
encryption.

and if all else fails, escalate to dd_rescue.


--
Noah Dain
"Single failures can occur for a variety of reasons that have nothing
to do with a hardware defect, such as cosmic radiation ..." - IBM
Thinkpad R40 maintenance manual, page 25