Re: Patch for the CD FAQ

[George Danchev <danchev@spnet.net>]

On Wednesday 14 December 2011 13:33:37 Steve McIntyre wrote:
> Hey folks,
> 
> I've got a patch from George and Thomas that they've been pestering me
> to do something with for a while. I've made minor tweaks and added
> some extra stuff. Please review/commit.

Hey guys,

Here is another patch to the CD FAQ which corrects the information about the 
verfication of the written media as given at [1]. It explains the problem of 
reading surplus amount of bytes from the written media, and how to properly 
deal with it. It also removes the link to an outdated and unmaintained 
checkiso script, suggesting another one instead. Comments and review welcome.

[1] http://www.debian.org/CD/faq/#verify

-- 
pub 4096R/0E4BD0AB <people.fccf.net/danchev/key pgp.mit.edu>

Index: index.wml
===================================================================
RCS file: /cvs/webwml/webwml/english/CD/faq/index.wml,v
retrieving revision 1.101
diff -u -r1.101 index.wml
--- index.wml	14 Dec 2011 16:49:30 -0000	1.101
+++ index.wml	15 Dec 2011 08:57:34 -0000
@@ -446,11 +446,41 @@
 (install the <q>coreutils</q> package), or you may be able to find standalone
 binaries compiled for <a href="http://www.mingw.org/";>MinGW</a>.</p>
 
-<p>If you want to check many CD image files or burned CDs easily
-on Unix/Linux systems, the
-<a href="http://fly.cnuce.cnr.it/software/#checkiso";>checkiso</a>
+<p>The problem with the verification of the written optical media is,
+that some of them will possibly deliver more bytes than those found in
+the ISO image. The trailing garbage is impossible to avoid with CD
+written in TAO mode, incremental recorded DVD-R[W], formatted DVD-RW,
+DVD+RW, BD-RE, and also with USB keys. Therefore we need to read exactly
+that many sectors of bytes from the media as they are found in the ISO
+image itself, since reading any more bytes from the media would alter
+the checksum result.
+</p>
+
+<p><ul>
+<li>Finding out the <q>sector count</q> and the <q>sector size</q> from the ISO image:
+  <pre>
+  $ /sbin/isosize -x debian-x.y.z-arch-MEDIUM.iso
+       sector count: 25600, sector size: 2048
+  </pre>
+</li>
+<li>Reading back the stream of bytes from the already written media:
+  <pre>
+  $ dd if=&lt;device&gt; count=&lt;sector count&gt; bs=&lt;sector size&gt; | md5sum
+  </pre>
+</li>
+</ul></p>
+
+<p>There is an exhaustive script called
+<a href="http://people.debian.org/~danchev/debian-iso/check_debian_iso";>
+check_debian_iso</a> which can verify the written media reading the
+appropriate amount of bytes from it.
+</p>
+
+#<p>If you want to check many CD image files or burned CDs easily
+#on Unix/Linux systems, the
+#<a href="http://fly.cnuce.cnr.it/software/#checkiso";>checkiso</a>
 #<a href="ftp://fly.cnuce.cnr.it/pub/software/unix/checkiso";>checkiso</a>
-script can be a great help.</p>
+#script can be a great help.</p>
 
 # ============================================================