On Thursday 03 March 2005 11:29, Maciej ?enczykowski wrote:
> the problem can be solved by appending 300kB worth of zeroes (dd
> if=/dev/zero bs=2048 count=150 >> file.iso) to the iso before burning (it
> just simulates padding) - if iso's with this padding were distributed,
> we'd get rid of the problem for good.
No, it wouldn't, I believe. The reason being is that zero pad changes the MD5
sum of the ISO file. As this shows:
[lowen@ditto iso]$ cp ubcd24.iso ubcd24-padded.iso
[lowen@ditto iso]$ dd if=/dev/zero bs=2048 count=150 >>ubcd24-padded.iso
150+0 records in
150+0 records out
[lowen@ditto iso]$ md5sum -b ubcd24.iso
10db8f4df5f44caa0af3ed8c83bad682 *ubcd24.iso
[lowen@ditto iso]$ md5sum -b ubcd24-padded.iso
e6dd60ac3623f2e489f3d3ab3cbf184c *ubcd24-padded.iso
[lowen@ditto iso]$
Now, that doesn't sound bad until you remember that anaconda embeds the MD5
inside the ISO during ISO filesystem creation. This MD5 sum is what
mediacheck checks against, IIRC.
I could easily be wrong, but someone would just have to check and see. I know
that the zero-padded ISO you would download would fail MD5 sum if you use the
automatically generated ISO MD5 sum made by anaconda (which is the only good
sum, in reality, because it is also inside the ISO itself). To pass both sum
checks using the same sum for both checks requires the padding to be done at
burn time. Otherwise Red Hat would have already done it that way; this is
far from the first time this question has come up. It came up in 2002 on
another list of which I am a member, that much I know for sure.
Padding on a CD will not be read by accurate raw reading of the CD. Yes, you
heard me right. Simple dd of a CD is not accurate reading of that CD; you
need the right block count and blocksize to get an accurate ISO from the CD.
Anaconda does the read correctly, and does not see the padding for the
purposes of mediacheck. So, your hand padded ISO will fail both checks.
But, try it and see if you get accurate results. Burn a CentOS disc 1 with a
hand-padded ISO and see if it passes mediacheck.
The padding exists to work around bugs in some CD readers and some IDE
chipsets under Linux, which can cause a 'read-ahead' bug to be triggered.
More information at:
http://www.troubleshooters.com/linux/coasterless.htm
--
Lamar Owen
Director of Information Technology
Pisgah Astronomical Research Institute
1 PARI Drive
Rosman, NC 28772
(828)862-5554
www.pari.edu
