Re: Using dd to verify a dvd and avoid the readahead bug.
Dear Thomas,
you are doing an excellent job explaining CD writing problems, my thanks
for that. However, in this case I feel compelled to comment ;-)
On Tue, 2007-10-02 at 22:20 +0200, Thomas Schmitt wrote:
> Our dear kernels seem to get confused by the inability
> to read these announced sectors.
> The answer was to attach some blocks as sacrifice to
> this bug-or-feature.
Writing 150 empty blocks after the last block with user data is required
by the data CD standard - they are called "post-gap". For a long time
the combination of "mkisofs" + "cdrecord" did not add any padding unless
explicitly asked to (which users most likely didn't know and thus didn't
do) and even then, it rounded up instead of adding exactly 150 blocks. I
consider such discs in violation of the standard, which may lead to
non-deterministic behavior like read errors.
> The safe size for padding once used to be 32 kB.
> At some time it grew and an urban legend emerged that
> 150 sectors (= 2 seconds of music) might be the new limit.
> Well, 300 kB seems to be safe, indeed ... up to now.
> Cargo cult.
Not a myth at all ;-) For more information, see my postings in this old
thread here:
http://groups.google.ca/group/mailing.comp.cdwrite/browse_frm/thread/ab41402747ff62af/568f34f142e706bd?tvc=1&q=%22reading+out+data+cds%22&hl=en#568f34f142e706bd
Later (Feb 15 2003) Joerg changed mkisofs so that it always adds 150
blocks.
--
Bye, Patrick Ohly
--
Patrick.Ohly@gmx.de
http://www.estamos.de/
Reply to: