F12-i386-DVD iso won't burn properly -- SOLVED

Tony Nelson tonynelson at georgeanelson.com
Wed Mar 3 20:31:30 UTC 2010

On 10-03-03 05:23:02, Andre Robatino wrote:
> Tony Nelson wrote:
> > Reading will never return more than the requested number of bytes.
> > It may read past the desired end of a CD or DVD.  With that in 
> > mind, read what I wrote above.  If you don't understand it, ask a
> > question.
> I'm not sure what you mean by "requested number of bytes", since your
> command doesn't request a specific number, but relies on the OS to
> detect the correct size of both files (which in the case of /dev/dvd
> is unreliable).

Every read has a requested number of bytes.  That I didn't write such a 
number is not important to the various commands that process whole 
files.  They do that by reading chunks of whatever length they want 
until no data is returned.  That point is determined by the kernel, 
but incorrectly for some CDs and DVDs.

> Your description of the possible outputs of the cmp
> command seems to imply that the readahead bug would cause /dev/dvd to
> be seen as bigger than it actually is, which is the opposite of what
> actually happens - there would be read errors on /dev/dvd _before_
> reaching the true end of the image (making it impossible to read off
> the full ISO, even assuming its exact size is known).

This is completely incorrect.  The data on affected CDs and DVDs is 
always longer than the data written.  You should be able to figure out 
for yourself why you must be wrong, and any experiments will prove it 
to you.  The "readahead bug" causes extra data to be returned after 
the end of the intentionally written data.  That data is the leadout 
added either by the writer software or the DVD writer itself.

TonyN.:'                       <mailto:tonynelson at georgeanelson.com>
      '                              <http://www.georgeanelson.com/>

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