[Debburn-devel] readom error reading last sector on image created by wodim

[Thomas Schmitt]

Hi,

what you experience is normal and your
ISO image seems to be ok.

> # genisoimage -o janani.iso /opt/deploy
> ...
> # wodim dev=/dev/scd0 janani.iso speed=1
> wodim: No write mode specified.
> wodim: Asuming -tao mode.
> ...
> Track 01: Total bytes read/written: 510869504/510869504 (249448
> sectors).
>
> # readom dev=/dev/scd0 f=jananireadom.raw
> ...
> end: 249450
> ...
> readom: Input/output error. Error on sector 249448 not corrected.
> Total of 1 errors.

Maybe readom should in this case issue a
message like:
"Only the last two sectors of track NNN are
 unreadable. This is normal if the track was
 written in TAO mode."

Since you burn a preformatted image file
as first session of the media, you may use
wodim option -sao to prevent those unreadable
extra sectors:

  wodim dev=/dev/scd0 janani.iso speed=1 -sao

(You really have a drive and media which can
 burn as slow as speed 1 ? That must be an
 antiquity.)


> Would I be running into the Linux read-ahead bugs in their filesystem
> I/O? Or does readom expect some "pad" to be added
> by the CD writer

Padding won't help as readom obviously inquires
the end address of the track from the drive.
Padding helps with the notorious read-ahead
error of Linux block device i/o, indeed.

But readom avoids that follow-up error of the
TAO problem by this retry:

> readom: Input/output error. Cannot read source disk
> readom: Retrying from sector 249408.

Now it reads 40 readable sectors and
finally fails on the last two.

Not to retry in small steps at the end
of the track: that's the read-ahead error
of the Linux kernel. It drops the 40 sectors
(and often more).
That's why we all pad like crazy.


It seems not possible to find out by MMC
commands whether a first session with a single
track was burned in mode SAO or in TAO.
So one cannot predict whether the last two
sectors of that track should be readable or
not.

Since genisoimage appends padding by default,
the last two sectors of a track with such an
ISO image do not contain payload data.

Well, you _know_ that it was TAO. So the
unreadble sectors do not indicate a media error.


> # mount -o loop jananireadom.raw /mnt

This does not prove much.
One should at least do a thorough data
read. E.g.:

  cd /mnt ; tar cf - . >/dev/null

If this runs without i/o errors then your
image is most probably ok.
(Your readom test tells the same, nevertheless.)


Have a nice day :)

Thomas