[GLLUG] Re: cheap cd-rw drives

Marr marr at flex.com
Fri Jun 25 18:27:15 EDT 2004


On Thursday 24 June 2004 09:23pm, Scott Harrison wrote:
> > I run a checksum after burning
> > to be certain.
>
> How? (i.e. can you be marr specific?...cymbal crash)

Nyuk, nyuk, nyuk... </Curly>  :^)

Sorry for the delayed reply....

My nightly backup consists of a few TGZ and a few TXT files that first get 
created on the hard disk drive in preparation for a 'mkisofs' command. 
(Without compression via 'tar', I'd never fit everything on a single CD-RW 
disc.)

When the backup to CD-RW media ('cdrecord') is complete, my backup script 
merely does a 'cat file1.tgz file2.tgz file3.txt | md5sum - | cut -f 1 -d -' 
on both the hard disk drive and the CD-RW disc and compares the two results. 
I've never had a mismatch yet.

> I vaguely recall in the past building an iso, checksumming it,
> burning cd, and then dumping contents of cd, and
> the-formatting-disk-layout-or-something-while-I-was-using-dd-cat-or-somethi
>ng (cat /dev/cdrom blah blah)
> returned a bunch of data similar but not exactly the same in
> size and of course the checksum didn't work out.
> Can't remember mkisofs (rockridge?) invocation details.

On Friday 25 June 2004 09:39am, Scott Henry Harrison wrote:
> Okay... found all I needed to know at:
> http://twiki.org/cgi-bin/view/Wikilearn/CdromMd5sumsAfterBurning

Hmm... This is all interesting. Thanks for bringing it to my attention. I've 
never heard of an issue with the checksumming of ISO-9660 media. My method of 
backup/compare clearly circumvents this issue (since I don't checksum the ISO 
file that 'mkisofs' creates), but it's nice to know about nonetheless.

I suspect that even if I didn't create TGZ files, I could use a scheme similar 
to the one I'm currently using to just 'cat' the contents (file by file) of 
the ISO-9660 image through 'md5sum', both on the mounted CD-RW disc and on a 
loopback mount of the ISO image file created by 'mkisofs' on the hard disk 
drive. Hmmm, maybe not -- I'd run into command-line length limitations that 
way. Well, there's probably still a way to do that, if someone was 
sufficiently clever/motivated (that is, more than I am at the moment). :^)

Bill Marr



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