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szmaszmo

Member

Registered: 2022-09-11

Posts: 2

K3b/cdrecord BD-R burning broken on kernel 6.x (worked on 5.x)

Hi all,

I am experiencing a consistent BD-R burning failure with K3b/cdrecord on kernel 6.x
that used to work fine on kernel 5.x. I have reproduced it on two completely different
machines, which rules out a hardware-specific issue.

== Hardware ==

Machine 1: Samsung NP505P with hp HLDS BDRE BU50N BW51 optical drive (/dev/sr0)
Machine 2: Dell 7140 tablet with the same usb external optical drive

== Software ==

K3b Version:  26.4.2
KDE Version:  6.26.0
Qt Version:   6.11.1
mkisofs:      3.2a09
cdrecord:     3.2a09

== Kernels Tested ==

- Kernel 6.12.94-1-MANJARO → FAILS
- Kernel 6.18              → FAILS (on Dell 7140)
- Kernel 5.x               → WORKED fine (approximately one year ago)

== What Happens ==

K3b starts a SAO BD-R burn session and cdrecord immediately fails at sector 0
with the following critical error:

  Sense Key: 0x5 Illegal Request
  Sense Code: 0x21 Qual 0x02 (invalid address for write)

Additionally, the following warnings appear before the write attempt:

  WARNING: Phys disk size 4452354 differs from rzone size 24438784! Prerecorded disk?
  WARNING: Phys start: 25088 Phys end 4477441

Note: the disc is new and blank. The "Phys disk size" warning appears to be a known
cdrecord misinterpretation of the BD-R disc type identifier bytes ("BDR"), not a real
problem with the disc itself.

The error message "buffer underrun" printed by cdrecord at the end of the log is
misleading — it is a secondary consequence of the write failure, not the root cause.

Notably, mkisofs itself did NOT crash - it only terminated because cdrecord exited
early and closed the pipe.

== Root Cause 1: cdrecord kernel minor version >= 6 bug ==

There is a known bug in cdrecord/wodim where, when the kernel minor version is >= 6,
it attempts to access obsoleted device files (/dev/scd<n>), causing the burn to fail.
This was referenced in Debian bug #833346 and a related Thomas Schmitt post on the
debian-user mailing list (November 2025). Since both kernels I tested have a minor
version >= 6 (6.12 and 6.18), this strongly correlates with my observations.

== Root Cause 2: /tmp space silently causing failures ==

On the Dell 7140, insufficient free space on /tmp (mounted as tmpfs) was also
causing silent failures in K3b with no clear warning message. For BD-R DL (50GB),
/tmp needs to have sufficient free space.

Fix: redirect K3b's temp directory to a volume with enough free space:
- In K3b: Settings → Configure K3b → Advanced → Temporary Files
- Or launch K3b with: TMPDIR=/path/to/big/volume k3b

== Workaround 1: cdrskin as drop-in cdrecord replacement inside K3b ==

K3b's program backend list is hardcoded to cdrecord/wodim, cdrdao and growisofs —
xorriso is not supported as a K3b backend (Debian bug #1113906 requests this but
it has not been implemented yet).

However, cdrskin (from the libburn package) is a true drop-in replacement for
cdrecord with identical syntax, so K3b will use it transparently:

    sudo pacman -S libburn

Then in K3b: Settings → Configure K3b → Programs → point the cdrecord
entry to /usr/bin/cdrskin

cdrskin supports BD-R and BD-R DL (50GB dual layer) media and is actively maintained.

== Workaround 2: Xfburn as GUI alternative ==

Xfburn successfully burns BD-R where K3b fails. This is because Xfburn uses
libburn and libisofs directly as its sole backend, completely bypassing
cdrecord/wodim and all associated kernel 6.x bugs.

    sudo pacman -S xfburn

Important caveat: Xfburn currently has no multi-session support.
For multi-session BD-R, use xorriso directly from the terminal:

    xorriso -as cdrecord -v dev=/dev/sr0 -dao your_image.iso

== Full cdrecord Log ==

[paste your full log here]

== Questions for the Community ==

1. Can anyone else reproduce this with kernel 6.x and BD-R media?
2. Is cdrskin confirmed as a working drop-in fix inside K3b?
3. Should this be escalated to upstream cdrecord/wodim maintainers
   or to the linux-scsi mailing list?

Thanks in advance.

Last edited by szmaszmo (2026-06-27 15:01:58)

johan.cb

Member

From: Leipzig, Germany

Registered: 2016-07-10

Posts: 69

Re: K3b/cdrecord BD-R burning broken on kernel 6.x (worked on 5.x)

Hi there!

I came across your post about burning Blu-rays on Linux and wanted to chime in, especially since I use them for backups too, and our setups seem quite similar.

I just tested your steps (currently burning a disc as I type this), and everything works smoothly for me with the 6.18.37-1-lts (64-bit) kernel and k3b 1:26.04.2-1. I’m using 25 GB blank Blu-rays from Verbatim and a Buffalo external USB burner.

While I wasn't able to help, I will definitly monitor your answer to support if I can.

y8v

Member

Registered: 2026-05-28

Posts: 50

Re: K3b/cdrecord BD-R burning broken on kernel 6.x (worked on 5.x)

Can you be a little more specific on the version number of the kernel that you're using?

---

"I hate snaps because they are snaps and snaps suck"

laszlo

Member

Registered: 2016-04-16

Posts: 15

Re: K3b/cdrecord BD-R burning broken on kernel 6.x (worked on 5.x)

Have you tried using Brasero? I also burn discs from time to time, though I do have an external CD/DVD burner. However, I remember that years ago, back when I was still using a desktop computer, I had a Blu-ray burner and K3B wasn’t able to burn a disc. I don’t remember which distribution it was, let alone the kernel version. In any case, Brasero supports multisession burning.