Re: Using dd to verify a dvd and avoid the readahead bug.
On Wed, 2007-10-03 at 18:11 +0200, Thomas Schmitt wrote:
> Patrick Ohly:
> > Writing 150 empty blocks after the last block with user data is required
> > by the data CD standard - they are called "post-gap".
>
> Yep. But as the publicly available standard MMC-5 clarifies:
>
> "6.33.3.19 Post-gap
> If a Data track is followed by another kind of track
> (such as an audio track), this Data track ends with a post-gap.
> A post-gap is placed at the end of a Data track, and is part of
> the Data Track. A post-gap does not contain actual user data.
> The minimum length of post-gap is 2 seconds. The Drive does not
> perform any action for a Post-gap.
> "
>
> This is not the situation we have with a single TAO track
> on closed media, unless you call the end of media "another
> kind of track".
The disc layouts that I have seen in vendor documentation seemed to
considered the lead-out as "another kind of track" and I tend to follow
that interpretation.
> Also: i do not have this "readahead" problem with SAO sessions.
> And all my drives can read TAO tracks up to the last 2 non-data
> sectors if i apply SCSI command READ 10.
I don't doubt your observations, I know that the run-out blocks are not
readable. I think that's because at the level below blocks, data on the
CD is spread across a wider range to mitigate the effect of scratches:
the two unreadable run-out blocks contain parts of the data of the
preceding readable blocks, but are not complete by themselves.
In SAO the write does not have to stop after the track and therefore no
run-out blocks are needed.
> All payload and all padding is retrievable up to the last byte !
> It is not an issue of the drives but of the operating system.
I agree that an OS could be improved to handle this better; my point was
that in normal operation (= mounting ISO file system on valid CD) the
read-ahead should not fail because only real data blocks are read and
there is the guaranteed post-gap after them.
--
Bye, Patrick Ohly
--
Patrick.Ohly@gmx.de
http://www.estamos.de/
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