size discrepancy

[Gregory Woodbury]

On Sat, Oct 2, 2010 at 4:40 AM, Andre Robatino
<robatino at fedoraproject.org>wrote:

> JD <jd1008 <at> gmail.com> writes:
>
> > I burned Fedora-14-Beta-i386-DVD.iso to blank dvd.
> > One time I used wodim, and 2nd time I used growisofs.
> > After each burn, I would dd the dvd back in:
> >
> > dd if=/dev/sr0  of=f14.iso bs=2k
> >
> > In both cases the size of f14.iso was 10240 bytes larger than
> > Fedora-14-Beta-i386-DVD.iso.
>
> In general this won't work unless dd is told exactly how much to read off.
> Unfortunately, it works often enough that people think it's supposed to.
> Use the
> rawread script from
>
> http://www.troubleshooters.com/linux/coasterless.htm#rawread
>
> instead. This determines the ISO size from the ISO header and then
> automatically
> reads off the right amount.
>
> There is a defect in the Live and netinst images (which started in F12
> Alpha,
> and only affected i386 and x86_64, not ppc), in that they are larger than
> the
> ISO header indicates:
>
> https://bugzilla.redhat.com/show_bug.cgi?id=585006
>
> To read these off properly, the rawread script won't work, since it assumes
> that
> the ISO header size is correct. Instead, you have to manually compose a dd
> command that reads off the size corresponding to the actual ISO file (not
> the
> size in the ISO header).
>

One can also tell dd to read by single physical layer sectors using the
"bs=2048"
option. This may increase the read time a bit, but generally results in s
more consistent
chance of getting the correct size.

-- 
G.Wolfe Woodbury
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