How to burn Fedora DVDs to avoid readahead bug?

[D. Hugh Redelmeier]

| From: Andre Robatino <andre at bwh.harvard.edu>

|  When I burned the Fedora ISOs to CDs, I used cdrecord with padding to avoid
| the readahead bug - see
| 
| http://www.troubleshooters.com/linux/coasterless.htm

Your analysis matches my understanding.

| 3) Find some other tool besides cdrecord that can do the necessary padding for
| DVDs.  Are there any?

I pad the iso file, burn the padded file, unpad the iso file, and then
check to see if the burn worked.

You can use the command isosize(8) to find out how much of an image
size is the actual iso9660 file system.

I wrote a utility that I use to pad (and unpad) an image file.  I call
it "isopad".  It pads by 128k bytes of zero which has been enough for
my systems.

To see if a burn worked, I just use cmp(1) to compare the contents of
the dvd with the image.  I only check the filesystem part:
	cmp --bytes `isosize whatever.ios` whatever.iso /dev/hdc
This uses the cmp(1) --bytes flag which is handy but not in the man
page.

================ isopad ================
#!/bin/sh

# isopad [+] [-] isofile...
#
# The Linux IDE CD driver in 2.6 tries to read ahead, even past the end of the
# CD or DVD.  Even when the program issuing the original read request was only
# trying to read legitimate parts of the disc (albeit near the end).
# The result is spurious I/O errors and read failures.
# It does not seem that this driver bug is going to be fixed soon.
#
# This program is intended to facilitate a workaround.  It can pad (or unpad)
# a .iso file so that, when it is burned, the resulting disc will allow
# reads past the end of the content to succeed.
#
# "+" means pad the following .iso files.
# "-" means remove all padding.
# neither means test file and iso sizes.
#
# To see how much readahead is enabled on a drive: hdparm -a /dev/hdc
#
# Why do the padding in place, rather than on a copy of the file?
# .iso files are usually quite large so copying takes a lot of time and space.
#
# Copyright 2005 D. Hugh Redelmeier
# License: GPL
# Version: Sat Jun 18 02:31:48 EDT 2005

# stop at the least sign of trouble
set -u -e

# op is "", "-", or "+": operation to be performed
op=""

for fn
do
	case "$fn"
	in
	"-h"|"--help")
		echo "Usage: $0 [-|+|] isofile..."
		;;
	"+"|"-")
		op="$fn"
		;;
	*)
		isosize -x "$fn"
		isz=`isosize "$fn"`
		fsz=`stat --format='%s' "$fn"`

		# conventional block size for CDs
		bs=2048

		# my guess at a sufficient amount of padding (in blocks)
		pb=64

		if [ $fsz -lt $isz ]
		then
			echo "$fn is shorter ($fsz) than it should be ($isz)" >&2
			exit 3
		elif [ ` expr $fsz % $bs ` -ne 0 ]
		then
			echo "$fn file size ($fsz) is not a multiple of $bs" >&2
			exit 4
		elif [ ` expr $isz % $bs ` -ne 0 ]
		then
			echo "$fn isosize ($isz) is not a multiple of $bs" >&2
			exit 5
		else
			case "$op" in
			"")
				if [ $fsz -eq $isz ]
				then
					echo "$fn: isosize == file size == $fsz"
				else
					echo "$fn: isosize $isz; file size $fsz"
				fi
				;;
			"+")
				echo "$fn: padding with $pb blocks of $bs zero bytes"
				dd if=/dev/zero bs=$bs count=$pb >>"$fn"
				;;
			"-")
				if [ $fsz -eq $isz ]
				then
					echo "$fn: already $fsz bytes"
				else
					echo "$fn: truncating from $fsz to $isz bytes"
					dd if=/dev/null of="$fn" seek=$isz bs=1
				fi
				;;
			esac
		fi
		;;
	esac
done
================ end isopad ================