Re: Issues with cdrsin and USB devices on RHEL5

["Thomas Schmitt" <scdbackup@gmx.net>]

Hi,

me:
> > This DVD-ROM drive is too stupid to recognize multiple
> > sessions on DVD-R or DVD+R. (One can scan for ISO 9660
> > headers, though, and successfully mount the sessions
> > via option sbsector= .) 

Bill Davidsen:
> Wow, is that actually useful for anything? Amazing!

I am running incremental updates of my $HOME.
Each session constitutes a complete snapshot of the
tree:
  $ xorriso -dev /dev/sr0 -toc
  ...
  Media current: DVD+R
  Media status : is written , is appendable
  TOC layout   : Idx ,  sbsector ,       Size , Volume Id
  ISO session  :   1 ,         0 ,    739194s , UPDATE_HOME_2008_06_22_191945
  ISO session  :   2 ,    741392 ,     20333s , UPDATE_HOME_2008_06_25_190436
  ISO session  :   3 ,    763936 ,     19565s , UPDATE_HOME_2008_06_29_001832
  ...
  ISO session  :  11 ,    950400 ,     27435s , UPDATE_HOME_2008_07_15_084804
  ISO session  :  12 ,    980048 ,     32535s , UPDATE_HOME_2008_07_16_143936
  Media summary: 12 sessions, 990208 data blocks, 1934m data, 2501m free

So i get 12 backups, 1.5 GB each, for the price
of only 2 GB on media. The media will be full after
about 40 of them.

If you insert such a media into a DVD burner drive then
it will report at least the first and the last
session and thus Linux can automatically mount
the superblock of the last session.

Not so with my DVD-ROM drive. It announces all DVD media as
"DVD-ROM", shows only the start of the first session
and sometimes a second one at a fantasy address.
I.e DVD multi-session is totally broken on MMC level.
In that case you have to give mount the superblock address
  mount -o sbsector=980048 ...

To learn about the sessions on a non-multi-session drive,
xorriso has an option
  -rom_toc_scan on
which allows to hop along the chain of sessions and find
out their superblocks. This implies several MB of reading
and eventually ugly noises when entering unwritten area.
But afterwards xorriso might tell the sbsectors and be able
to extract files from the particular sessions.

Another multi-session feature of xorriso is that it can
maintain a session list on DVD+RW and other overwriteable
media. This is achieved by writing the first session
to address 32 and to copy its head to address 0 as if it
already was a follow-up session.
Due to the smaller waste area between sessions, a DVD+RW
can take about 60 of my incremental $HOME backups.


Have a nice day :)

Thomas