warning to delta ISO users: rawread script can no longer be reliably used to extract Fedora images from disc

Andre Robatino robatino at fedoraproject.org
Tue Aug 2 21:11:27 UTC 2011

I removed all references to the rawread script from
https://fedoraproject.org/wiki/Delta_ISOs . This script can be used to extract
ISO files from burned media, using the Volume Space Size in the ISO header to
read off the correct amount. Unfortunately, many of the recent Fedora images,
including the 16-Alpha.TC1 x86_64 DVD, are not true ISOs and include extra
padding beyond the size indicated by the ISO header. This means that if such an
image is read off a burned optical disc using rawread, it will be truncated.
This both prevents the checksum from matching and prevents applydeltaiso from
working if the delta ISO was made using the untruncated oldiso file.

I prefer to continue making delta ISOs using the official checksummed "ISO"
files for both oldiso and newiso. This means that in general the rawread script
should no longer be used to read oldiso off a burned optical disc into a file.
For oldiso, one should either use the "ISO" file matching the official checksum,
or use the disc directly ("applydeltaiso /dev/dvd diso newiso").

It's still possible to read an "ISO" file off a burned optical disc by
specifying the size directly. For example, Fedora-16-Alpha.TC1-x86_64-DVD.iso is
3589275648 bytes = 1752576 2048-byte blocks, although the Volume Space Size is
1752162 blocks. The rawread script would only read off the latter amount,
truncating the file. Using the explicit command

dd if=/dev/dvd bs=2048 count=1752576 conv=notrunc,noerror > oldiso

will read off the file correctly. This requires knowing the exact size of the
file, which may not be available (especially for TCs/RCs which get deleted
often). I filed https://bugzilla.redhat.com/show_bug.cgi?id=727387 in the hope
of having file sizes included in the official checksum files. (Note that I filed
the bug before finding this issue - there are probably others lurking.)

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