Re: zeroes read back more often than appended

[Pat LaVarre <p.lavarre@ieee.org>]

Ahhhhh eventually I did stumble into a context for `dd` that lets me
control its write buffer lengths while writing nonzero bytes ... and yes
that reproduces this issue too.  No kernel patch required.  No .c app
required.  And, curiously, no dmesg complaints from "fs/udf/balloc.c"
"193" "udf_bitmap_free_blocks".

Pat LaVarre

### example tty log

$ time ddudfwh mkudffs 65535 3072
Linux 2.6.0-test7 i686
/mnt/hda11
dd if=/dev/zero of=dd.bin bs=1M seek=0 count=256 2>/dev/null
-rw-rw-r--    1 pat      pat      268435456 Oct 10 12:11 dd.bin
mkudffs 1.0.0b2 for UDF FS 1.0.0-cvs, 2002/02/09
3072+0 records in
3072+0 records out
3072+0 records in
3072+0 records out
00000000  5e 40 5e 40 5e 40 5e 40  5e 40 5e 40 5e 40 5e 40  |^@^@^@^@^@^@^@^@|
*
002bd000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
 
real    0m41.294s
user    0m5.032s
sys     0m6.528s
$ 

### script udfddwh

#!/bin/bash

uname -msr
pwd
rm dd.bin
cmd='dd if=/dev/zero of=dd.bin bs=1M seek=255 count=1'
cmd='dd if=/dev/zero of=dd.bin bs=1M seek=0 count=256'
echo "$cmd 2>/dev/null"
$cmd 2>/dev/null
ls -l dd.bin

sudo losetup /dev/loop0 dd.bin
sudo $1 2>&1 | head -1
sudo $1 /dev/loop0 >/dev/null

sudo mount /dev/loop0 /mnt/loop0
sudo chown `id -u`:`id -g` /mnt/loop0/.
dmesg >1.dmesg
cd /mnt/loop0
dd bs=$2 count=$3 if=/dev/zero | cat -e >$OLDPWD/nz.bin
dd if=$OLDPWD/nz.bin of=wh.bin bs=$2 count=$3
# rm $OLDPWD/nz.bin
hexdump -C wh.bin | head -3
cd -
dmesg >2.dmesg

diff -u 1.dmesg 2.dmesg | grep '^\+' | head -5
rm 1.dmesg 2.dmesg
sudo umount /mnt/loop0
sudo losetup -d /dev/loop0

###