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Re: question about dd reading of CD

On Wednesday 01 September 2004 12:39, Paul E Condon wrote:
> I've purchased a combo CD/DVD reader. I'm checking it. I find a
> problem.
> With my old CD only reader, I was able to check on my CD writing by
> using dd to read back a recently burnt CD and comparing the read back
> file to the original iso using diff.

diff is far too slow because of all of the seeks to the alternate iso images, 
better to do an md5sum on each iso.

> But when I use the new reader with dd and using a recent copy of
> sarge-netinstall for which I know the true file length of the iso, I
> get a file from dd that is 4096 bytes too long.

By default cdrecord plonks 2 sectors (2048 bytes each) of nulls on the tail 
end of the burn. Hence the 4096 bytes extra. There is an option to turn it off

Alternatively, just do the read back capturing only the data sectors.

dd if=yourburner of=test.iso bs=2048 count=the_sectors_cdrecord_reported

or better

dd bs=2048 count=asabove | md5sum

if you lose the sector count, you can retrieve it by mounting the cd and then 
getting the count of 1k blocks on it using 'df'. It will always be an even 
number of course..

> But is it always 4096 bytes?  
Yes unless you change the cdrecord option.

Or is it sometimes half or twice times
> that?
Only if you are using Nero in Windows and then the null sector count depends 
on the wind strength and direction or temperature of the day or whatever.
Actually Nero in a version my son was using dropped 200 sectors of nulls or 
none depending on whatever random factors govern the behaviour of a Windows 
program. He has seen the light and now does all of his burning in Linux. Not 
yet Debian flavour but there is hope.

Bob Parker

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